Photos of cars on freeway, speeding sign

Freeway Management and Operations Handbook

Chapter 1 – Introduction
Page 2 of 2

1.5 Definitions and Concepts

1.5.1 Freeways

The Highway Capacity Manual (HCM) (7) defines a freeway as a divided highway with full control of access and two or more lanes for the exclusive use of traffic in each direction. Freeways provide uninterrupted flow (Note: “Uninterrupted” is used to describe the type of facility, not the quality of the traffic flow at any given time. A freeway experiencing extreme congestion, for example, is still an uninterrupted-flow facility because the causes of congestion are internal.) Opposing directions of flow are continuously separated by a raised barrier, an at-grade median, or a continuous raised median (Figure 1-4). Operating conditions on a freeway primarily result from interactions among vehicles and drivers in the traffic stream and among vehicles, drivers, and the geometric characteristics of the freeway.

aerial photo of a freeway-to-freeway interchange in an urban setting, showing multiple ramps and infrastructure elevations. One freeway is shown aligned vertically and the other horizontally, with two levels of flyovers above urban streets

Figure 1-4: Freeway Interchange (I81 / I690 in Syracuse, NY)
(Courtesy of NYS DOT)

The AASHTO "Green Book" (13) defines freeways as "arterial highways with full control of access. They are intended to provide for high levels of safety and efficiency in the movement of large volumes of traffic at high speeds. With full control of access, preference is given to through traffic by providing access connections with selected public roads only and by prohibiting crossings at grade and private driveway connections".

Several physical attributes of the freeway facility impact its capacity and operational characteristics as summarized in Table 1-3. Additional factors include level of enforcement, lighting conditions, pavement conditions, pavement markings and signing, and weather.

Table 1-3: Physical Factors Affecting Roadway Capacity and Operations
Category Capacity / Design Element
Horizontal Alignment
  • Degree of curvature
  • Superelevation
Vertical Alignment
  • Grade
  • Length of grade
  • Vertical curves – sag and crest
Cross Section
  • Number of lanes
  • Lane width
  • Lateral Clearance
    • Shoulder type and width
    • Median type and width
    • Clearance to obstructions
Other
  • Interchange density
  • Ramps & ramp junctions
  • Weaving sections

A tollway or toll road is similar to a freeway, except that tolls are collected at designated points along the facility, either electronically, manually, or some combination. Although the collection of tolls may involve interruptions of traffic flow (Figure 1-5), these facilities should generally be treated as "freeways", particularly with respect to strategies and technologies for management and operations. Special attention should be given to the unique characteristics, lane management opportunities, and constraints associated with toll collection facilities. Accordingly, the term "freeway" as used in this Handbook refers to any limited access facility, including the interstate system, expressways, toll roads, and connecting bridges and tunnels.

photo of a toll plaza from the point of view of a driver approaching the plaza; both cash lanes and electronic toll collection (ETC) lanes are available among the 10 lanes under one roof stretching across both directions of a highway

Figure 1-5: Toll Plaza

1.5.2 Congestion

The FHWA publication "Managing our Congested Streets & Highways" (10) documents the results of several surveys, with delays caused by traffic congestion topping the list of transportation issues that people reported as affecting their communities. But what is actually meant by the term "congestion"? To truly comprehend freeway management and operations strategies and supporting technologies, and to fully appreciate their potential to deal with congestion problems, it is important to understand both the nature of congestion and the events that occur in the traffic stream as congestion forms. There are multiple definitions and measures of congestion – both quantitative and qualitative, as discussed below.

1.5.2.1 Traffic Flow Theory
The generalized relationships between speed, density and flow rate are shown in Figure 1-6, with these parameters defined as follows:

  • Flow Rate – the equivalent hourly rate (i.e., vehicles per hour) at which vehicles pass over a given point or section of a lane or roadway during a given time interval of less than one hour. (This is different from "volume", which is the number of vehicles observed or predicted to pass a point during a specified time interval, such as annual or average daily traffic.) (7)
  • Speed – defined as a rate of motion expressed as distance per unit of time, generally as miles per hour (mi/h). In characterizing the speed of a traffic stream, a representative value must be used, because a broad distribution of individual speeds is observable in the traffic stream. The curves in Figure 1-6 utilize "average travel speed", which is computed by dividing the length of the highway segment under consideration by the average travel time of the vehicles traversing it (7).
  • Density – the number of vehicles occupying a given length of a lane or roadway at a particular instant. For the curves shown in Figure 1-6, density is averaged over time and is usually expressed as vehicles per mile (veh/mi) (7).
three graphs showing generalized relationships between speed and density, speed and flow rate, and flow rate and density

Figure 1-6: Generalized Relationships Between Speed, Density, and Flow Rate on Freeways
(Reference 7) D

The form of these curves depends on the prevailing traffic and roadway conditions on the segment under study. Moreover, while the diagrams in Figure 1-6 show continuous curves; in reality there are likely discontinuities, with part of these curves not present. The curves illustrate the following significant points.

  • A zero flow rate occurs under two different conditions. One is when there are no vehicles on the facility—density is zero, and flow rate is zero. The second is when density becomes so high that all vehicles must stop—the speed is zero, and the flow rate is zero, because there is no movement and vehicles cannot pass a point on the roadway (7).
  • Between these two extreme points, the dynamics of traffic flow produce a maximizing effect. As flow increases from zero, density also increases, since more vehicles are on the roadway. When this happens, speed declines because of the interaction of vehicles. This decline is negligible at low and medium densities and flow rates. As the density further increases, these generalized curves suggest that speed decreases significantly just before capacity is achieved, with capacity being defined as the product of density and speed resulting in the maximum flow rate. This condition is shown as optimum speed So (often called critical speed), optimum density Do (sometimes referred to as critical density), and maximum flow Vm. (7). In general, this maximum flow (i.e. capacity) occurs at a speed between 35 and 50 mph.

Efficient freeway operation depends on the balance between capacity and demand. In the simplest terms, highway congestion results when traffic demand approaches or exceeds the available capacity of the highway system. As vehicle demand approaches highway capacity, traffic flow begins to deteriorate. Flow is interrupted by spots of turbulence and shock waves, which disrupt efficiency. Then, traffic flow begins to break down rapidly, followed by further deterioration of operational efficiency. The result of this spiraling inefficiency can be observed during every weekday commute in almost every metropolitan area: Drivers push their way onto already crowded freeways to join thousands of others already caught in seemingly endless traffic jams. Unfortunately, by joining the already impeded traffic flow, drivers become part of the problem, creating even greater inefficiencies: more stop-and-go traffic conditions, longer delays, and greater potential for accidents.

While this is a simple concept, traffic demand is not constant. It can vary significantly depending on the season of the year, the day of the week, and even the time of day. Also, the capacity is not a constant – it can change (sometimes rapidly) because of weather, work zones, traffic incidents, or other events. It is not necessarily simple, either. The physical fact of finiteness and the principle of conservation underlies traffic stream behavior, as reflected in the smooth curves in Figure 1-6. However, the actual performance of a particular section of freeway at a particular point in time is more ambiguous, resulting from variations in individual human behavior and the mix of vehicle types using the facility. It may be possible to predict the average behavior and average capacity, and the variances about these averages for a traffic stream, but never the precise behavior.

1.5.2.2 Quantifying Congestion

NCHRP Report 398 entitled "Quantifying Congestion" (Reference 16) identifies the following four components that interact in a congested roadway or system:

  • Duration – the amount of time congestion affects the travel system.
  • Extent – the number of people or vehicles affected by the congestion, and the geographic distribution of the congestion.
  • Intensity – the severity of congestion (with concomitant measures such as person-hours of delay, average speed, etc.).
  • Reliability – the variation of the other three elements.

The report states: "Any definition of congestion, and the congestion measures derived therefrom, should rely on concepts that are understandable by the intended audience. Travel time and its related quantities are widely understood and fundamentally useful in the definition and measurement of congestion. However, the congested reflected in travel times and delays that are acceptable to travelers can vary by city size, location in the urban (or rural) area, and time of day or year. One method that may be used to resolve this issue is to define two quantities, congestion and unacceptable congestion.

  • Congestion is travel time or delay in excess of that normally incurred under light or free-flow travel conditions.
  • Unacceptable congestion is travel time or delay in excess of an agreed-upon norm. The agreed-upon norm may vary by type of transportation facility, travel mode, geographic location, and time of day, and should be derived taking into account the expectations for each portion of the transportation system as influenced by community input and technical considerations". (16)

Measures of congestion are discussed in Chapter 4.

1.5.2.3 Other Considerations

Perhaps "congestion" lies in the eye (and experience) of the beholder (Figure 1-7). As an example, in 2002, the ITS Forum of the "National Associations Working Group for ITS" (http://www.nawgits.com/itsforum) posted as the question of the month: "What should be our common definition of Congestion?" There were numerous replies and opinions, including:

  • "Any definition needs to be understandable to the general public or to elected officials to be useful. Congestion occurs and is caused by bottlenecks. All other mechanisms for describing traffic flow are related to measures of effectiveness."
  • "The speed-flow relationship is fundamental to traffic theory. In the Highway Capacity Manual this relation is given by a smooth curve, which yields a maximum flow at a speed between 35 and 50 mph. We tested this hypothesis using cross-sectional data. The test rejects the hypothesis that maximum flow occurs between 35 and 50 mph. The finding has some important implications. Congestion delay should be measured as the time spent driving below 60 mph, both because it is the most efficient speed and because drivers experience congestion below this speed."
  • "A metric for congestion – total trip time at posted speeds compared to total trip time at operating speed and an accompanying index for setting service standards."
  • "Caltrans today measures congestion as the time spent driving under 35 mph continuously for 15 minutes."
  • "Transportation congestion has local / cultural and time-of-duration components to it that defy strict terminology."
  • "Congestion will always be relative."
photo of a freeway operating at capacity: four lanes in each direction with long queues of closely spaced vehicles in all lanes

Figure 1-7: Another View of Congestion

With respect to the last point, congestion is typically viewed by travelers relative to their normal day-to-day experiences. Travelers accustomed to low speeds and congestion delays for 12 hours each day may not consider 10 minutes of delay per trip a problem. These travelers have learned to budget extra time or find other ways to cope with the delay. Travelers accustomed to light traffic and reliable trips might consider 5 minutes of delay per trip unacceptable and a problem worth noting at the next City Council meeting. A key aspect of a congestion management strategy is identifying the level of "acceptable" congestion and developing plans and programs to achieve that target. (2)

A major goal of freeway management and operations is to keep freeway capacity and the vehicular demand on a freeway in balance. The most effective way to combat congestion is to take action before traffic flow deteriorates and congestion forms. It would be ideal to manage the demand on the freeway to prevent traffic flow from ever breaking down and congestion from forming. This is usually not possible, and the best result is to delay the onset of congestion and speed the recovery from congestion, therefore minimizing the inefficiencies that congestion causes.

1.5.2.4 Recurrent and Non-Recurrent Congestion

Congestion is often classified as either recurrent or non-recurrent. The type of congestion depends on whether the capacity or the demand factor is out of balance.

  • Recurrent congestion occurs when demand increases beyond the available capacity. It usually is associated with the morning and afternoon work commutes, when demand reaches such a level that the freeway is overwhelmed and traffic flow deteriorates to unstable stop-and-go conditions.
  • Non-recurrent congestion results from a decrease in capacity, while the demand remains the same. This kind of congestion usually results when one or more lanes are temporarily blocked. A stopped vehicle, for example, can take a lane out of service, but the same number of vehicles expects to travel through. Speed and throughput drop until the lane is reopened, and then they return to full capacity. Capacity can also be decreased by weather events and events near the travelway (i.e., "rubber necking"), leading to non-recurrent congestion and reduced reliability of the entire transportation system.

Whereas recurrent and non-recurrent congestion have different causes, their solutions have many elements in common.

1.5.3 Safety

An individual highway crash is a rare, random, multifactor event, preceded by a situation in which one or more persons failed to cope with their environment. In the aggregate, however, traffic crashes are quite numerous and often follow certain patterns that can be identified. Crashes reflect a shortcoming in one or more components of the driver-vehicle-roadway system. It is therefore very important for freeway practitioners to monitor traffic collision experience, and to use this information to identify, plan, implement, and evaluate corrective actions. Numerous approaches exist for improving safety and reducing crashes on highways. Many of these are beyond the scope of freeway management and operations, per se (e.g., enforcing seat belt laws, in-vehicle crash-avoidance technologies, geometric realignment); but others – such as improved signing and lighting, skid resistance pavement, adding shoulders and auxiliary lanes, and removing obstacles – are well within the realm of "operations".

As previously noted, a major goal of freeway management and operations is to reduce congestion; and a reduction in congestion may also enhance safety. But how does congestion affect highway safety? The basic theory behind the interaction is that congestion leads to higher vehicle densities (i.e., more closely spaced vehicles on a roadway), which provides more opportunities for conflict. Congestion also reduces vehicle speeds, which implies that when vehicles are engaged in a crash, the collision forces are lower, thus reducing the injury to occupants. Another aspect of the model is the concept of "secondary" crashes—crashes that occur due to conditions produced by an existing crash. Some of these conditions—which wouldn't exist without the occurrence of the first crash—include rapid backward queue formation (as vehicles suddenly stop to avoid the first crash), rubbernecking by drivers, and the maneuvers of emergency vehicles. Finally, the flow restrictions produced by crashes worsen existing congestion (2).

The details of the relationship between congestion and safety are not well understood (with the exception of lower crash severities, which have been documented in a general way for congested conditions, and the associated lower speeds). Based on the limited work that has been performed, a few tentative conclusions may be drawn:

  • Crash potential (e.g., crashes per vehicle-mile traveled) probably increases as congestion increases.
  • There is a lower proportion of single vehicle crashes (e.g., run-off-road, rollover, collision with fixed object) during congested conditions and a higher proportion of multiple vehicle crashes.
  • Crash severities (extent and nature of personal injuries) are lower during congested conditions, due to lower vehicle speeds at the moment of crash impact.

In general, it can be assumed that any operational improvement that reduces congestion will lead to fewer crashes. The severity of crashes that occur will be higher, however, and it is likely that a greater proportion will be single vehicle crashes. Knowing these facts can target mitigation strategies to single vehicle crashes and higher severities—such as wider roadside recovery zones, protection of highway "furniture," and coordination with emergency medical services. Moreover, an operations philosophy must take a systems-oriented view, where the consequences of a specific action (e.g., flow improvements) consider linked impacts such as safety (2).

1.5.4 Freeway Management and Operations

Freeway traffic management and operations is the implementation of policies, strategies and technologies to improve freeway performance. The over-riding objectives of freeway management programs include minimizing congestion (and its side effects), improving safety, and enhancing overall mobility. The TRB Freeway Operations Committee's Millennium Paper (3) states: "Freeway operations, in its broadest context, entails a program to combat congestion and its damaging effects: driver delay, incon and frustration, reduced safety, and deteriorated air quality." Freeway traffic management entails:

  • Understanding both the nature and magnitude of a particular congestion and / or safety problem, including current issues (i.e., reactive), and potential future ones (i.e., proactive);
  • Combining various operational strategies, policies, and systems into a comprehensive program;
  • Using technology, detection and verification systems, communication links, traffic operations centers, motorist information systems, and information sharing among systems;
  • Implementing a high degree of interagency coordination and cooperation to provide emergency services and to restore accident scenes to normal operation in the shortest possible time;
  • Deploying and implementing highly sensitive and sometimes controversial management strategies, such as ramp meters and high-occupancy lanes; and
  • Managing extremely popular services such as tow trucks and patrols to rapidly remove disabled vehicles from freeways (3).

Freeway traffic management is all of this and more. Its components are aimed at providing some level of relief from congestion, improving safety and mobility for the traveling public, and meeting other related objectives. Strategies and technologies associated with freeway management and operations are summarized below.

1.5.4.1 Infrastructure Improvements

Freeway management and operations include "low-cost" (relative to constructing new facilities) improvements to the freeway infrastructure. Examples of such activities include adding auxiliary lanes, ramp widening, restriping to add an additional lane within the existing pavement, and similar improvements to eliminate bottlenecks. Enhancements to other attributes of the freeway infrastructure (e.g., signing, pavement markings, illumination) to increase safety and driver convenience / comfort are other possible improvements.

1.5.4.2 Traffic Incident Management

Traffic incident management is an operational approach designed to quickly detect, respond, and clear disabled vehicles and other "events" (such as debris) from the roadway that would detract from facility performance. Traffic incidents, such as crashes and disabled vehicles, reduce capacity (i.e., non-recurrent congestion) and decrease safety (along with the life-safety issues associated with responding to a crash scene with the proper medical personnel and equipment as soon as possible). Such incidents are the cause of 40 to 60 percent of all congestion in urban areas.

The primary intent of traffic incident management is to prevent incidents from reducing capacity; but when they do, the focus is to restore capacity as quickly as possible. This prevents backups and significantly decreases the occurrence and severity of congestion, and the possibility of secondary crashes. Traffic incident management programs vary from location to location. They can include: technologies and communications for rapidly detecting incidents and identifying their location and the appropriate response needs (e.g., networks of closed circuit television cameras, vehicle detection sensors, incoming 911 reports, incoming media reports, and mobile reports), systems and procedures for dispatching the appropriate emergency response personnel and equipment; service patrols (vehicles specifically intended to look for and help disabled motorists), incident management teams (interagency working groups formed to develop faster and more efficient responses to accidents and other major incidents) traveler information systems.

1.5.4.3 Lane Management

The term "managed lanes" covers a variety of strategies and techniques, many of which have been used for years. The basic concept behind lane management is to employ operational tools to maximize the productivity of the available roadway. Lane management concepts can include the following:

  • HOV lanes – to encourage more people to use high capacity modes of travel, thus moving more people in a single roadway lane
  • HOT lanes – which allow vehicles to purchase access (through tolls) to underutilized HOV lane capacity, thus 1) maximizing vehicle use of HOV lanes without sacrificing HOV speed and reliability, and 2) providing revenue to help pay for HOV lane construction, maintenance, and operation, or other transportation needs.
  • Reversible and contra-flow roadways – which allow increased use of underutilized lanes when traffic volumes in one direction far exceed traffic volumes in the opposite direction
  • Congestion pricing – which uses tolls or other fees to shift travel demand from congested times and locations to other times of the day or to other facilities (or lanes) that are not congested
  • Truck-only facilities – which both improve freight movement and limit truck/car interaction, thus increasing safety and decreasing the effect that heavy trucks have on passenger car performance. Lanes may also have truck restrictions.
  • Work zones – providing efficient movement of traffic through areas of a highway construction, maintenance, or utility work activities; while protecting the safety of both workers and motorists.
1.5.4.4 Ramp Management

Ramp management, is the application of control devices, such as traffic signals, signing, and gates to regulate the number of vehicles entering or leaving the freeway, in order to achieve operational objectives.  Most ramp management strategies lead to improved traffic flow and safety by 1) regulating the number of vehicles entering or exiting a freeway, or 2) smoothing out the rate at which vehicles enter the freeway facility.  By employing either of these approaches, ramp management helps to balance freeway demand and capacity and maintain optimum freeway operation by reducing incidents that produce traffic delays, improve safety on adjacent freeways or arterial streets, or giving special treatment to a specific class of vehicles.  Ramp management strategies are also often implemented with elements of other freeway management programs to create operational efficiencies and to assist in the delivery of overall transportation management goals and objectives.

Ramp management strategies may be used to control access to selected ramps, thus limiting the periods when vehicles may access the ramp or possibly restricting access to the ramp permanently.  This significantly reduces, or may even eliminate, the potential for collisions that occur as a result of traffic entering or exiting the ramp facility and in turn smoothes the flow of traffic on segments of roadway where these collisions have occurred in the past. 

Ramp management may also control the manner in which vehicles enter and exit a freeway.  For instance, vehicles that enter the freeway in platoons introduce turbulence, which causes vehicles on both the mainline and ramp to slow down to safely merge.  This causes congestion around and upstream of ramp/freeway merge points.  Ramp management strategies may be used to control the flow of vehicles entering a freeway, thus smoothing the rate at which vehicles are allowed to enter the freeway.  Similarly, ramp management strategies and treatments may be implemented at the ramp-arterial intersection to better manage the flow of traffic exiting the freeway.  Such treatments may reduce queues on exit ramps that extend out onto the freeway, helping to improve safety and mobility on the freeway.

1.5.4.5 Information Dissemination

This provides the information travelers need to effectively plan their trip prior to departure; and when en-route, the information may be used by drivers to avoid congestion and / or to adjust their driving behavior (e.g., in response to unsafe conditions). Traveler information provides choices for travelers – a key attribute of mobility. They can select different routes, different modes, different times, or even different destinations. They can avoid congested or unsafe routes (e.g., due to adverse weather conditions). Their ability to choose improves their trip. Such knowledge also reduces stress and limits risk taking behavior, thus producing better travel conditions. Pre-trip information is typically disseminated to the public via websites, media broadcasts, and mobile communication devices (e.g., personal digital assistants, pagers, and cell phones). En-route traveler information has traditionally been disseminated via commercial radio, changeable message signs (CMS) and highway advisory radio (HAR). With the emergence of wireless communication technologies, en-route traveler information can also be disseminated through wireless phones, web-enabled wireless phones, and a variety of personal digital assistants (PDA) equipped with wireless communication capabilities.

1.5.4.6 Intelligent Transportation Systems

Intelligent Transportation Systems (ITS) is the application of advanced electronics, computer, communications, and sensor technologies – in an integrated manner – to increase the increase the efficiency and safety of the surface transportation network. ITS encompasses technologies that can lead to:

  • Better management and operations of the existing highway, public transportation and railroad infrastructure to ease congestion and respond to crises.
  • Safer and more convenient travel for people.
  • More efficient and secure freight movements (14).

Used effectively, ITS opens the door to new ways of managing, operating, expanding, refining, reconfiguring and using the transportation system. ITS has proven itself as a significant enabler of freeway management and operations. Combined with ITS technologies, a Freeway Management System (FMS) consists of a set of resources (e.g., electronic systems, people, objects, and strategies) that are used to accomplish a set of goals to improve the operation of the freeway network. Several of these potential ITS components have already been mentioned above (e.g., CMS, HAR, ramp meters). Other elements of an ITS-based freeway management system that support these functions include:

  • Surveillance and Detection Systems – These devices collect information on traffic flow and roadway performance, and allow operators to monitor conditions (in real time) on the freeway system. The sensors may collect data (volume, speeds, travel times), or provide video images via cameras. The data collected feeds the control and information dissemination functions, and allow operators to intervene when appropriate in those functions. The data may also be stored (i.e., warehoused) for future analysis and performance evaluation. Sensor technology also allows the system to monitor roadway and environmental conditions, such as pavement temperature and weather. Roadway and environmental condition information are often used in deciding how best to allocate resources for functions such as snow and ice control.
  • Computer Systems and Systems Integration – Freeway management systems are dependent upon the computer systems on which they operate. Selecting the appropriate hardware platforms and software programs for the desired functions, integrating the software and hardware subsystems into a complete system, and maintaining the complete system are critical elements of providing effective freeway management systems.
  • Transportation Management Center (TMC) – The computer systems and associated software, the user interfaces and operators themselves, and associated resources are typically housed and operated from a traffic management center, although they can also be operated over a communication network without the need a formal center. In systems that include a TMC, it serves as the information "nerve center" for regional operations.
  • Communication Systems – The effective operation of all of the functions mentioned above require communication of data, video, and voice. Communication systems transmit data and video from the field to a TMC or central location, transmit commands from the TMC or central location to the field equipment, transmit information among agencies, distribute traveler information to the systems that disseminate it, and allow personnel at any distance to communicate with one another.

1.5.5 Institutional Considerations

All of the attributes of a freeway management program – policies, funding mechanisms, strategies, systems and technologies, operational activities, etc. – take place within the institutional framework. This institutional fabric is multi-agency, multi-functional, and multi-modal. Moreover, the authority for transportation decision-making is dispersed among several levels, or "tiers", of government (e.g., national, statewide, regional, agency, individual systems), and often between several agencies and departments within each governmental level. This institutional situation can lead to a fragmented delivery system for transportation services, resulting in an agency-specific and mode-specific focus, rather than an area-wide focus that considers the entire trip. After all, the customer's perspective is that there is only one surface transportation system. The public generally does not care which jurisdiction or agency is responsible for the road or mode on which they are currently traveling. As taxpayers (and in some cases fare / toll payers), they want and deserve a safe, reliable, and predictable trip – one that is safe from physical and mental harm, provides a consistent level-of-service with minimal congestion, and is predictable in terms of travel time.

Achieving the safe, reliable, and secure operation of our Nation's transportation network depends on collaboration and coordination across traditional jurisdictional and organizational boundaries. In other words, there must be "integration" (i.e., a term defined in the dictionary as making into a whole by bringing all parts together). In essence, integration is a bridging function between the various components of the surface transportation, and involves processes that focus on the sharing of information and the combining of resources in a manner that facilitates a more seamless operation. In addition to institutional integration (i.e., coordination between various agencies and jurisdictions to achieve seamless operations), the surface transportation network needs to be integrated in other ways, including:

  • Technical Integration (e.g., enabling different agencies and their TMCs to readily exchange information).
  • Operational Integration (e.g., the exchanged information is combined to create a regional database for traveler information; or where signal timing is changed to accommodate increased traffic flow diverted from an adjacent freeway; or where plans are developed jointly in advance by all affected agencies (and then utilized) to manage traffic during special events and emergencies).
  • Procedural Integration (e.g., combining planning and programming processes that address the surface transportation network as a whole and consider all possible improvements in a consistent manner).

Freeway management and operations must be addressed within this larger institutional context. Moreover, it is important that the practitioner have an understanding of the many external factors and dependencies that can impact the performance of the freeway network and influence how it should be managed.

1.5.6 Other Considerations

In addition to the broad concepts discussed above, the following items also warrant a brief discussion. As with most of the other items addressed in this introductory chapter, additional information and detail is provided in subsequent chapters.

1.5.6.1 Programs, Projects, and Process

A "program" is a coordinated, inter-related set of strategies, procedures, and activities, all intended to meet the goals and objectives articulated in vision statements and policies. "Projects" are well-defined, individual actions and activities that make up the program. The implementation of projects is how the program is realized. A program has a long-term temporal view, whereas individual projects generally have a shorter implementation period. Managing a program involves trade-offs between budget and timing, and determinations as to appropriate sequence and scope of the associated projects.

Practically every transportation-related program and project involves some sort of "process" – a series of actions – through which ideals and concepts are brought to fruition, implemented, and managed. There exists in every process an underlying structure that shapes and controls events. This framework consists of formal activities (e.g., written or unwritten policies agreed to in a collaborative fashion) and informal ones (e.g., human relationships), all relating to the ways options are created and decisions are made to improve the performance of the transportation network.

1.5.6.2 Performance Measures

Freeway management programs – consisting of operational strategies, low-cost roadway improvements, and / or ITS-based systems – are funded and implemented as a means to preserve mobility, improve reliability, enhance safety, and meet the public's expectations for efficient and effective travel. It is important to ensure that the funds are spent wisely, that the agency makes the best use of its available resources, and that the full potential of past and current investments is realized. This, in turn, requires that the performance of the freeway be continuously monitored and evaluated, including an assessment of the extent to which the implemented solutions achieve the intended objectives.

Performance measures provide the basis for evaluating the effectiveness of implemented freeway management strategies, as well as for identifying the location and severity of problems (such as congestion and high crash rates). This monitoring information can be used to track changes in system performance over time, identify systems or corridors with poor performance, identify potential causes and associated remedies, identify specific areas of a freeway management program or system that requires improvement, and provide information to decision-makers and the public. In essence, performance measures are used to measure how the transportation system performs with respect to the adopted goals and objectives, both for ongoing management and operations of the system, and for the evaluation of future options.

1.5.6.3 Concept of Operations

Usually associated with the development of a freeway management system, regional architecture, and other ITS deployments, a "Concept of Operations" is a document that defines the environment in which the freeway (and other elements of the transportation network) is to operate, and the needs of the users. This environment includes the relationships between the system and the owning agency's policies, procedures and responsibilities; the interagency working relationships and agreements; the physical environment (i.e., the capabilities of the system); and the expectations (performance measures). The Concept of Operations is a tool by which regions, agencies, and traffic management centers – and the associated practitioners – identify (at a high level with few technical details) the optimal solution based on their preferred approach, their capabilities, and their constraints.

1.6 How to Use This Document

Section 1.3 identified the intended audience of the handbook as "transportation professionals that participate in or responsible for any phase in the life cycle of a freeway network." Persons whose responsibilities are primarily policy development will find this introductory chapter, plus chapter 2, most pertinent. Those with the responsibility for program management and the development of freeway-oriented programs and / or projects should also review chapters 3 and 4.

Chapters 517 address topic-specific aspects of freeway management and operations, and are pertinent to all practitioners with day-to-day responsibilities for managing or operating a freeway facility, depending on their specific interests and needs. The most appropriate strategies, technologies, and services will vary based on the conditions unique to each metropolitan area, including the type and extent of problems, the political structure, the agencies' experience with traffic management, and the level of cooperation between local agencies. Moreover, depending on regional needs, the overall freeway management goals, and the extent to which freeway management and operations are performed, will likely vary from region to region.

It is emphasized that this Freeway Management and Operations Handbook is not a design manual or a detailed technical reference. For many of the technical issues, excellent reference materials exist that provide more detailed information, in more of a "how-to" manner. These include recent handbooks on Traffic Incident Management, Planned Special Event Management, Communications, HOV Lanes, and Detection and Surveillance Sensors. For the chapters covering these areas, the Handbook references these technical documents and provides only a brief summary.

1.6.1 State of the Art and State of the Practice

The FHWA White Paper "Freeway Management and Operations State of the Practice" (6) makes a distinction between the "practice" and the "art" – specifically:

  • State of the Practice is defined as "Proven practices in common use and the effective application of technologies commonly installed and operated in the freeway management and operations disciplines"
  • By comparison, the state-of-the-art is defined as "Innovative and effective practices and the application of leading edge technologies that are ready for deployment in terms of operating accurately and efficiently, but are not fully accepted and deployed by practitioners."

Specific distinctions between the state of the practice and the state of the art are not addressed in this Handbook (as they are in Reference 6). The purpose of this Handbook is to provide the practitioner with a wide range of potentially useful alternatives for implementing freeway management programs and projects, thereby improving freeway operations. Some of these may be considered "practice"; while others may be deemed as "art". Emerging trends will also be identified. Moreover, experience, lessons learned, and examples are provided in many instances.

Many of the items addressed in this Handbook are either technology or program-based, and therefore likely to change and/or become outdated at any time following the release of this Handbook. The reader should check the date of the Handbook. At the time of writing this document, the plan is for it to be more dynamic. The Internet is being used as one of the methods for distributing the Freeway Management and Operations Handbook. As technologies, operating practices, or programs change, as additional experience is gained and new lessons are learned, and / or as new reference documents are developed, the affected chapter(s) of the Freeway Management and Operations Handbook will either be modified and posted on the web, or announcements regarding new reference materials will be posted. For now, additional information on Congestion Mitigation activities can be found at the FHWA Congestion Mitigation web site at http://www.fhwa.dot.gov/congestion, the ITS Joint Program Office web site at http://www.its.dot.gov, or the Office of Operations web site at http://www.ops.fhwa.dot.gov.

1.6.2 Key Themes

Regardless of which chapter(s) the practitioner is perusing, there are a few key concepts that should always be kept in mind. Some of these have already been mentioned, while others will be discussed in subsequent chapters. They nevertheless apply to all aspects and processes involving the management and operation of a freeway facility. These key themes are highlighted in Table 1-4, and discussed in the following bullets.

Table 1-4: Key Themes for Freeway Management & Operations
  • Practitioners must not address freeway management in a singular, isolated manner
  • View the overall performance of the surface transportation network
  • Consider how individual actions complement on another, and how, when combined as a program, contribute to regional goals
  • Implement freeway management systematically on a regional basis, coordinating with all operational activities & organizations
  • ITS is but a subset ("enabler") of management and operations
  • Consider all the available tools (e.g., roadway improvements, traffic control devices, ITS), looking at many potential improvements
  • Freeway management should be an integral part of established process within agencies; with freeway practitioners participating in these processes
  • Even though their primary responsibility may be the freeway network, practitioners must not address freeway management and operations in a singular, isolated manner. All three of the aforementioned legs of the "transportation stool" (i.e., building, preserving, operating), are integral parts of the business of most transportation agencies, and freeways are just one element of the surface transportation network. The same planning, programming, and budgeting processes are applied to all of these facilities and management attributes.
  • Similarly, freeway practitioners must view transportation as a whole, consider a vast array of potential tools. This may mean looking beyond the "typical" freeway management and operation alternatives, and giving consideration to other types of improvements in concert with freeway management systems and strategies. These additional strategies and improvements and may include new construction to increase the roadway capacity, enhancing other attributes of the freeway infrastructure (e.g., signing, pavement markings, illumination) to increase safety and driver convenience / comfort, and strategies to reduce travel demand.
  • Practitioners must carefully consider how individual actions complement one another in the long run and how, when combined into an overall program, they relate to regional and community goals and objectives. Moreover, actions should be selected and implemented that, through sound engineering and planning analysis, are shown to improve problems in a cost effective, multimodal manner. Be realistic in the assessment of what is likely to be accomplished. Set performance targets; identify, collect, and store information for performance measurements; and report to the public and decision makers on system performance.
  • Freeway management and systems are only one part of the many transportation management systems and operations activities that may exist within a metropolitan area, state, or multi state region. Freeway management should be implemented systematically on a regional basis and be coordinated with all the activities typically undertaken to operate the transportation network. This requires cooperation with neighboring governmental jurisdictions, regional transportation agencies, and organizations that provide or are involved with transportation-related services.
  • Many of the later chapters deal with ITS technologies. It must be emphasized, however, that the deployment of Intelligent Transportation Systems is not the same as "operations". ITS can be a significant subset / enabler of improved operations – for example ITS can provide real time information on the traffic flows; but operations is knowing what to do with this information to improve traffic flow, safety, and mobility.
  • Similarly, freeway management and operations extends beyond ITS and electronic systems. Freeway managers must be familiar with all of the tools available to improve the safety and efficiency of the freeway system, including major roadway improvements, minor roadway improvements, and traditional traffic control devices (such as, static signing, pavement marking, and illumination systems). Moreover, practitioners should continuously look for opportunities to improve the performance and safety of freeway facilities with all of the available tools.
  • Several processes have been instituted for developing transportation programs, planning and prioritizing potential improvements, and defining individual projects and strategies. Moreover, transportation agencies have adopted some of these – often with variations – as their formalized approach for making informed decisions regarding the investment of public funds for transportation improvements. Freeway management and operations should be an integral part of the established processes within an agency. Moreover, the freeway management practitioner must be cognizant of and, to the greatest extent possible (commensurate with his/her responsibilities), participate in these processes ensuring that freeway management and operations receives appropriate consideration.

1.7 Document Organization

Each of the 17 chapters that comprise the Freeway Management and Operations Handbook has been developed to "stand alone" within its specific topic area. Not lost on this, though, are the relationships and dependencies between various freeway management activities and other elements that comprise the surface transportation network. Thus, just as the freeway practitioners must view transportation as a whole, this Handbook should be looked at in a similar fashion. The chapters are not completely independent – for example, performance measures (discussed in Chapter 4) should be an integral part of the various processes (discussed in both chapters 2 and 3) by which transportation improvements are planned, developed, and implemented. Moreover, a freeway management program will encompass strategies and technologies from multiple chapters.

It is emphasized that the chapters only provide an "introduction" to their respective subjects. For additional details and design guidelines, the practitioner should consult a variety of references, many of which are identified (including web addresses) in the text of the chapter, along with the references section at the end of each chapter.

Chapters 7 through 17 – the ones that address specific freeway management strategies and technologies – utilize a common basic structure, starting with an introduction of the topic, the purpose of the chapter, and its relationship to other freeway management activities and Handbook chapters. The next section addresses "Current Practices, Methods, Strategies, and Technologies", including an overview of the subject, benefits, key considerations during freeway management program development, the relationship to the National ITS architecture, specific technologies and strategies, and emerging trends. This is followed by a section on "Implementation and Operational Considerations". The chapter concludes with "Examples" and "References". This format is not rigid. Depending on the chapter and its subject matter, these sections may have a different order, additional sections may be included, examples may be included throughout instead of in a separate section, and design considerations may be located in different sections; all to keep an appropriate flow of thought.

A brief summary of the material covered in the remaining chapters is provided below:

  • 2 – Freeway Management and the Surface Transportation Network: This chapter looks at freeway management and operations from the broader view of the entire surface transportation network, addressing the many external factors and dependencies (i.e., "cause & effect" interrelationships) that can impact the performance of the freeway network and how it should be managed. Information is provided regarding the types of constituencies that use and/or impact the surface transportation network, and the various organizational "tiers" where decisions affecting the surface transportation network are made. Several approaches are then discussed for meeting the challenges that these external factors and dependencies often present, thereby further improving the operation of the freeway as well as the entire surface transportation network. This "advice" applies to all potential freeway management activities and supporting technologies, and should therefore be duly considered when reading all subsequent chapters of this Handbook.
  • 3 – Freeway Management Programs: This chapter focuses on processes and activities specific to freeway management and operations. A series of activities are presented for establishing, enhancing, and managing a freeway operations program. These "steps" are not to be viewed as a separate process for developing a freeway management and operations program. Rather, they represent an amalgamation of important activities from other established processes. Intelligent Transportation Systems (ITS) have proven to be a significant enabler of operations, and many freeway management programs will include projects to design and implement ITS-based freeway management systems. Accordingly, this chapter also summarizes a number of published processes that are geared towards ITS deployment (e.g., systems engineering, configuration management, national and regional ITS architectures), with additional information provided in Chapter 14.
  • 4 – Performance Monitoring and Evaluation: Performance measures and the results of evaluations are important inputs to transportation planning and investment decision-making processes of public agencies. Moreover, the day-to-day operation and management of the freeway requires real-time knowledge how well the freeway is performing and the existence of any problems. This chapter addresses several related topics:
    • Performance measures, including discussions of why they are important, their relationship to the decision-making process, and important considerations when selecting performance measures. Several examples of performance measures that may be utilized for freeway management and operations are also provided, along with discussions on information gathering, data archiving, and reporting.
    • Self-assessment tools developed by FHWA for evaluating how well the operations process is organized and administered, and how well it interacts with other agencies and affected stakeholders.
    • Analytical tools (e.g., Highway Capacity Manual, simulation, before-and-after studies, estimating costs and benefits) for evaluating freeway performance and identifying problems, analyzing and prioritizing alternative solutions for correcting these problems, estimating the associated benefits and costs, and determining the actual improvement in performance and its cost effectiveness.
  • 5 – Roadway Improvements: This chapter provides an overview of potential actions that improve freeway performance by modifying the roadway itself, such as adding lanes to increase capacity (and thereby increase operational efficiency) at bottleneck locations, and making changes to the geometric configuration or physical characteristics of the roadway to enhance safety. After a brief overview of the types of problems that can be addressed by roadway improvements (and the potential benefits), and how these potential improvements should be addressed within the freeway management program, the following improvements are discussed: horizontal and vertical alignment; roadway widening (e.g., auxiliary lanes, shoulders); providing additional lanes without widening (e.g., restriping, use of shoulder as travel lane); interchanges (improvements to ramps and weaving sections); and other improvements such as treatment of obstacles and skid resistance.
  • 6 – Roadway Operational Improvements: Starting with this chapter, the rest of this Handbook is devoted to a discussion of operational strategies and enabling technologies. This particular chapter provides an overview of potential actions – specifically static signing, pavement markings, and roadway lighting – that do not modify the roadway footprint or geometry; nor are they usually considered in the context of "real-time" freeway management strategies and enabling technologies. Nevertheless, such improvements can improve the operation of the freeway, particularly in terms of safety and driver convenience and comfort. An overview of Travel Demand Management (TDM) strategies is also provided.
  • 7 – Ramp Management and Control:Ramp management is the application of control devices, such as traffic signals, signing, and gates to regulate the number of vehicles entering or leaving the freeway, in order to achieve operational objectives.  Most ramp management strategies lead to improved traffic flow and safety by 1) regulating the number of vehicles entering or exiting a freeway, or 2) smoothing out the rate at which vehicles enter the freeway facility.  This chapter describes various ramp management strategies, typical benefits, operational and design considerations, and provides case studies demonstrating agency experience with ramp management strategies.
  • 8 – Managed Lanes: A managed lane facility is one that increases freeway efficiency by packaging various operational and design actions. Lane management operations may be adjusted at any time to better match regional goals. This chapter addresses several managed lane concepts, including truck lanes, contraflow and reversible lanes, mainline metering, speed advisories and controls, work zone controls, toll facilities, and congestion pricing.
  • 9 – High Occupancy Vehicle Treatments: A form of managed lane, the preferential treatments for high occupancy vehicles (HOV), as a means for increasing the person-moving capacity of the transportation system, is covered in this chapter. Topics discussed include types of HOV facilities used on freeways (e.g., exclusive HOV lanes, concurrent flow HOV lanes, contraflow HOV lanes; operated either bi-directionally or reversible), park-and-ride facilities, HOV facility access, HOV strategies and operations (e.g., occupancy requirements, hours of operation, enforcement), and public awareness and marketing. High Occupancy Toll (HOT) lanes are also discussed.
  • 10 – Traffic Incident Management: Traffic incident management is the systematic, planned, and coordinated use of human, institutional, mechanical, and technical resources to reduce the duration and impact of traffic incidents, and improve the safety of motorists, crash victims, and traffic incident responders. This chapter provides a summary of the FHWA Traffic Incident Management Handbook, a document that treats traffic incident management in depth and is considered the primary reference on the subject.
  • 11 – Planned Special Event Management: A planned special event is a public attended activity or series of activities, with a scheduled time and location that may increase or disrupt the normal flow of traffic on affected streets or highways. The FHWA technical reference Managing Travel for Planned Special Events presents and recommends various planning initiatives, operations strategies, and technology applications that satisfy the special customer requirements and stakeholder performance requirements driving planned special event travel management. This chapter summarizes that reference, highlighting the essential elements involved in managing traffic during planned special events.
  • 12 – Freeway Management During Emergencies and Operations: Disaster planning, prevention, preparedness, response, and recovery fall into the category of emergency management. Natural disasters (e.g., hurricanes, forest fires, earthquakes, severe winter weather) and terrorist attacks are generally sudden and unexpected. Even those emergencies that can be anticipated have relatively short advance-response times amounting to, at best, a few days. The transportation network – particularly freeways – plays a major role during emergency management, including expediting evacuations from the affected area, and the return following the emergency. In some cases, emergency management also involves the restoration of transportation services. This chapter provides a high-level overview of procedures, institutional arrangements, and supporting documentation that are applicable to emergency management; many of which may be unfamiliar to the freeway practitioner, but nevertheless can have a major impact on the operation and management of the freeway during emergency situations.
  • 13 – Information Dissemination: The traveler information process extends well beyond the freeway, both in terms of where the information is obtained and how it is distributed. Information on freeway conditions and the dissemination of that information to freeway users should therefore be viewed as part of a broader, region-wide, advanced traveler information system (ATIS). This chapter covers the entire spectrum of ATIS, with the greatest emphasis being placed on those technologies and strategies that are directly related to the operation and management of the freeway itself (e.g., Changeable Message Signs (CMS) and Highway Advisory Radio (HAR)). Other topics include the "511" telephone-based system, traveler information dissemination over the Internet, and public-private partnerships.
  • 14 – Transportation Management Centers: The TMC is the hub or nerve center of most freeway management systems. It is here data about the freeway system is collected and processed, fused with other operational and control data, synthesized to produce "information", and distributed to stakeholders such as the media, other agencies, and the traveling public. The information is used by TMC staff to monitor the operation of the freeway, and to initiate control strategies that affect changes in the operation of the freeway network. It is also where agencies can coordinate their responses to traffic situations and incidents. This chapter addresses several aspects of TMCs, including the physical design of the facility, operator workstations and user interfaces, other information displays, local area networks, central hardware and software, and TMC security. The "systems engineering" and "configuration management" processes are also discussed.
  • 15 – Detection and Surveillance: Detection and surveillance technologies provide the information needed to perform nearly all traffic management functions and strategies. This chapter provides a summary of the FHWA document Traffic Detector Handbook, which addresses a number of technologies for measuring traffic flow. Other surveillance options (i.e., not addressed in the FHWA document) are also described, including probe-based surveillance, video surveillance (i.e., the use of real time video images of the freeway), and road-weather information systems (RWIS).
  • 16 – Regional Integration: The integration of multiple systems within a region provides for the real-time sharing of information between ITS based systems and the coordination of management activities between transportation agencies and emergency service providers, thereby enhancing system interoperability and enabling an areawide view of the transportation network. In essence, the goal of regional integration is to bring the operation and management of the surface transportation network into a unified whole, and to incorporate this singular management of the surface transportation network with the management of the broader transportation network. This chapter focuses on "technical integration", including network topologies, database considerations, related elements of the National ITS Architecture, standards for message sets and protocols, etc. that enable different Management Centers (e.g., transportation, emergency services, information providers) to readily (and automatically) exchange, store, and access information from one another – a process known as "center-to-center" communications.
  • 17 – Communications: A communications network provides the means by which information is exchanged between all the entities and components that comprise a freeway management and operations program. There are multiple communications options (e.g., network architectures, technologies, standards, implementation strategies) available for meeting these needs. It is crucial that the most appropriate options be selected to best support the operational requirements of the freeway management program and the associated ITS-based systems. This chapter provides a summary of the FHWA document Communications Handbook for Traffic Control Systems.

1.8 References

1. Meyer, M.D. A Toolbox for Alleviating Traffic Congestion and Enhancing Mobility. Institute of Transportation Engineers, Washington D.C. 1997

2. Lomax, Turner, Hallenback, et al; "Traffic Congestion and Travel Reliability—How Bad is the Situation and What is Being Done About It?"; September 2002

3. Freeway Operations: Year 2000 and Beyond; Committee on Freeway Operations; Chairman: Peter M. Briglia Jr.,

4. ITS National Architecture The ITS National Architecture, Documentation – Version 4.0, April 2002

5. Testimony Before the Subcommittee on Highways and Transit, Committee on Transportation and Infrastructure, House of Representatives; United States General Accounting Office; "Highway Infrastructure – Physical Conditions of the Interstate Highway System Have Improved, but Congestion and Other Pressures Continue"; Statement of Katherine Siggerud, Acting Director, Physical Infrastructure Issues

6. "Freeway Management and Operations: State-of-the-Practice White Paper"; Prepared for Federal Highway Administration, Office of Travel Management; March 2003

7. Highway Capacity Manual, Transportation Research Board, National Research Council, Washington D.C: 2000

8. "Integrated Surface Transportation Systems: The Role of Transportation Management Centers", Jon Obenberger & Walter H. Kraft, October, 2001;

9. FHWA "Operating the Highway System for Safety, Reliability and Security: TEA-21 Reauthorization Proposal".

10. FHWA; "Finding Out What America Thinks", Publication No. FHWA-OP-01-018

11. ITS America 10-year Plan

12. Sussman, Joseph; "Transportation Operations: An Organizational and Institutional Perspective"; ITE Journal, December 2002.

13. A Policy on Geometric Design of Highways and Streets, American Association of State Highway and Transportation Officials, Washington, D.C. 2001

14. ITS America 10 year vision

15. Intelligent Transportation Systems Benefits and Costs – 2003 Update; Mitretek Systems; Washington D.C.; May 2003

16. Lomax, T., Turner, S, & Shunk, G.; NCHRP Report 398 – Quantifying Congestion; National Academy Press; Washington D.C.; 1997