Chapter 1 – Introduction
Page 2 of 2
1.5 Definitions and Concepts
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.
Figure 1-4: Freeway Interchange (I81 / I690 in Syracuse,
(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
||Capacity / Design Element
- Degree of curvature
- Length of grade
- Vertical curves – sag and crest
- Number of lanes
- Lane width
- Lateral Clearance
- Shoulder type and width
- Median type and width
- Clearance to obstructions
- 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.
Figure 1-5: Toll Plaza
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.
220.127.116.11 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
- 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
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
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
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.
18.104.22.168 Quantifying Congestion
NCHRP Report 398 entitled "Quantifying Congestion" (Reference 16) identifies
the following four components that interact in a congested roadway or
- 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
- 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".
Measures of congestion are discussed in Chapter
22.214.171.124 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
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."
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.
126.96.36.199 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
- 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.
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
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;
- 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.
188.8.131.52 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.
184.108.40.206 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.
220.127.116.11 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
- 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.
18.104.22.168 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.
22.214.171.124 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.
126.96.36.199 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.
188.8.131.52 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
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.
184.108.40.206 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.
220.127.116.11 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 5 – 17 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,
- 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
- 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
- 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
- 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
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
11. ITS America 10-year Plan
12. Sussman, Joseph; "Transportation Operations:
An Organizational and Institutional Perspective"; ITE Journal, December
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