Freeway Management and Operations Handbook

Chapter 7 – Ramp Management
Page 1 of 2

Chapter 7 was revised in March 2006. For more information on the extent of these revisions, see the Revision History Table at the beginning of this handbook.

7.1 INTRODUCTION

If designed, operated, and maintained effectively, ramps allow motorists to make connections between different facilities in a safe, convenient, and comfortable fashion with little or no delay or impact on traffic.  However, conditions on ramps seldom represent the conditions for which they were initially designed.  Instead, ramps are often too closely spaced, do not offer adequate acceleration distances for posted speeds, or are simply overwhelmed by the increasing number of motorists that use them on a daily basis.  When conditions like these exist, impacts may develop that affect the efficient and safe operation of traffic on ramps and/or the facilities to which they are connected (i.e., freeways and arterials).  Despite poor conditions, however, agencies can proactively manage traffic on ramps to obtain desired benefits, to satisfy pre-determined goals and objectives, and improve safety and efficiency of freeway operations. 

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, such as ( 1 ):

• Improved safety.
• Improved mobility.
• Improved perception of transportation management agencies and staff.
• Reduced environmental impacts.

Ramp management also helps to achieve these objectives by smoothing the rate at which vehicles enter and exit the freeway. 

Ramp management typically encompasses the following four strategies:

• Ramp metering.
• Ramp closure.
• Special use treatments.
• Ramp terminal treatments.

Other than freeway-to-freeway interchanges, freeway entrance and exit ramps represent the only opportunity for vehicles to legally enter or exit a freeway and, therefore, represents the only location which control can be exercised.

7.1.1 Purpose of Chapter

The primary purpose of this Chapter is to improve the operation of freeways and their associated ramps by providing support, information, guidance, and recommended practice to practitioners responsible for freeway management and operations.  This chapter summarizes the Federal Highway Administration’s (FHWA) Ramp Management and Control Handbook ( 1 ):, by providing an overview of the essential elements of an effective ramp management program.  The use or application of these recommended practices, contained in this Chapter and in the Ramp Management and Control Handbook ( 1 ):, will in time serve to enhance the use and effectiveness of various ramp management strategies and techniques.

The Ramp Management and Control Handbook ( 1 ): describes ramp management in depth and is considered the primary reference on the subject.  This Handbook can be accessed using the following URL: http://ops.fhwa.dot.gov/freewaymgmt/index.htm.

7.1.2 What is 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”( 1 ).  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. 

7.1.3 Relation to Other Freeway Management Activities

There are many freeway management activities that are related to or dependent upon ramp management.  To understand the role of ramp management within the freeway management spectrum, it is important to understand the relationships that exist between these activities.

Ramp management is a single element or function that falls under an agency’s freeway management “umbrella”.  With that said ramp management should be viewed as an element of the overall transportation management system - not as a program that operates in parallel to or separate from it ( 1 ).  In this regard, the relationship between ramp management and freeway management can be likened to puzzle pieces, with ramp management representing just one piece of the entire freeway management program puzzle (see Figure 7‑1).  Ramp management elements must work together with elements of other freeway management programs to deliver freeway management and surface transportation system goals and objectives (e.g., minimize freeway congestion and its side effects, to improve safety, to enhance mobility, etc.).  The freeway management functions that comprise a freeway management program are identified in Figure 7‑1, and their relationship with regard to ramp management is described below.

7‑1:  Freeway Management Programs and Their Relationships with the Surface Transportation Program (Reference 1) D

The freeway management programs presented in Figure 7‑1 are often integrated with other freeway management programs, including ramp management.  Relationships that exist between ramp management and other freeway management programs are briefly summarized in Table 7‑1. 

Although it does not have a direct relationship to ramp management, tolling may indirectly affect operations on ramps, freeways and adjacent arterials in a manner similar to ramp metering.  Toll booths and gates located on entrance ramps can play a similar role to ramp meters because the toll operation can break up platoons as drivers slow or stop to pay tolls.  In this regard, toll booths stagger the release of vehicles, which helps to smooth the flow of vehicles entering the tollway facility.  Additionally, toll booths restrict excess demand to the ramp so vehicle platoons do not form as vehicles enter.  However, with toll booths and gates no attempt is made to control the number of vehicles that are released to reflect traffic conditions.  Therefore, toll booths and gates should be viewed as only a strategy that produces effects similar to that of ramp metering and should not be viewed as a ramp management strategy in itself.  In some cases, the operational effects of tolling can serve as input into the development of operational strategies for locations where toll booths and gates have been deployed or are planned. 

With the advent of automated toll collection systems and the trend toward open road tolling, toll collection acts less like a ramp management strategy than it once did.  In the future, the affect of toll collection on dispersing platoons or regulating entering traffic will be even more limited.

7.2 CURRENT PRACTICES, METHODS, STRATEGIES & TECHNOLOGIES

7.2.1 Overview

Simply speaking, freeway entrance and exit ramps connect high-speed, limited-access freeways to other high-speed, limited-access freeways or lower-speed, principal arterials/highways, and vice versa.  Additionally, ramps are the only facilities motorists may use to legally make connections to and from limited access facilities and as such represent the only locations where traffic entering and exiting a limited access facility can be controlled ( 1 ).

Ramp management strategies can balance freeway demand and capacity, maintain optimum freeway operation by reducing incidents that produce traffic delays, improve safety on adjacent freeways or arterial streets, or give special treatment to a specific class of vehicles.  The balance between freeway demand and capacity is achieved by controlling the manner in which vehicles enter 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 they are allowed to enter the freeway. 

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. 

7.2.2 Benefits

Before and after evaluations of ramp management strategies offer strong evidence that ramps, freeways, and even adjacent arterials operate better once strategies are implemented.  Typical benefits of ramp management strategies include:

• Safety - Ramp management strategies, such as ramp metering, reduce stop-and-go driving behavior, resulting in fewer rear-end collisions.  Ramp metering also breaks up platoons entering a freeway, resulting in fewer side-swipe and merge-related collisions.  During periods of severe weather, ramps may be closed to prevent motorists from accessing freeways that are impassable.  Ramp-arterial treatments (e.g., signal timing improvements, canalization, widening, and striping) may also improve safety by containing vehicle queues to the ramp, preventing queues from spilling back onto the freeway or adjacent arterial.  Safety benefits from ramp metering programs across the county are presented in Table 7‑2.
• Mobility and Productivity - Ramp management strategies may increase travel speeds while reducing travel time and delay.  Freeways that have metered entrance ramps usually carry more traffic than they did before metering began, while attaining the improvements mentioned previously.  A ramp metering study in Minneapolis showed a 25-percent increase in peak period traffic volumes while increasing average speeds by 5 km/h (3 mi/h) ( 2 ).
• Environmental Effects - The improved speeds, reduced stop-and-go traffic, and reduced delays that result from ramp management strategies also result in reduced emissions and fuel consumption.  An evaluation of ramp meters in Minneapolis identified a net annual saving of 1,160 tons of emissions ( 3 ).
• Traveler Perception and Satisfaction - Ramp management and improved operations on freeways demonstrate to the public that agencies responsible for transportation facilities are doing something about congestion and safety problems.  As a result, travelers and the public in general will be more satisfied with transportation agencies and the job they are doing.  Higher public satisfaction makes it easier for agencies to acquire the needed resources to develop, implement, operate and maintain transportation improvements.
• Promotion of Multi-modal Operation - Ramp management promotes the use of transit, carpools, vanpools, and other multi-occupant modes of transportation by giving preferential treatment to these modes.

More information on the benefits of the Minneapolis/St. Paul ramp metering program, as well as a summary of a ramp closure project conducted in Honolulu, Hawaii is provided in Section 7.4.

7.2.3 Key Considerations During Freeway Management Program Development

Ramp management systems and strategies can be an important part of a freeway management and operations program.  Ramp management goals, objectives and strategies should be consistent with regional transportation goals and objectives and must support the mission and vision of the agency responsible for implementing them (1, 4).  Ramp management goals, objectives and strategies should be defined at the regional level and should fit into the context of the broader transportation planning process, including the freeway management program.

Agencies should not implement ramp management strategies if their associated improvements do not support local, regional, and statewide surface transportation system goals and objectives.  If policies are not in place and resources are not available to operate and maintain strategies, deployment may actually result in public contempt for the strategies that are implemented.  This in turn will impede the agency’s ability to effectively manage traffic. 

When properly planned and implemented, ramp management can be an important element of a freeway management program.  However, ramp management strategies are not appropriate for all situations, nor does their implementation eliminate the need to pursue other complimentary strategies. The potential use of ramp management strategies should be examined thoroughly before any such improvements are made.  A comprehensive approach in planning, designing, implementing and operating ramp management strategies can help ensure successful projects.  The Ramp Management and Control Handbook provides a comprehensive overview of the steps involved in planning, designing, and implementing ramp management strategies.  Some of these key considerations for planning and developing a ramp management program are discussed in the following sections:

7.2.3.1 Planning Considerations

To be effective, ramp management strategies often need to take into consideration aspects of ramp management strategy implementation that indirectly affect overall outcomes.  For instance, ramp management implementation and operation may be perfect, but if ramp management strategies are not enforced, the overall objectives of strategies may not be achieved.  Similarly, if issues affecting equity are ignored, strategies may viewed unfavorably by certain groups of individuals, ultimately limiting the positive impacts that strategies offer.  Last but not least, maintenance of strategies must be taken into consideration during the planning process to ensure that strategies can be actively maintained and that malfunctioning equipment does not confuse motorists. 

Enforcement is one key to effective ramp management and control.  This is especially true for ramp metering because compliance is critical to its operational success.  Therefore, enforcement issues associated with ramp management strategies must be taken into consideration during the planning process so appropriate actions are taken to ensure that ramp management strategies are enforceable.  This includes a coordinated effort with law enforcement to ensure that issues such as an appropriate enforcement strategy, safe enforcement areas, adequate staff, and support by the courts are addressed.

Similar to enforcement, equity and environmental justice issues are also key considerations for ramp management projects.  The direct involvement of diverse cultural and economic communities in the development of transportation projects will help to ensure that projects fit harmoniously within their communities without sacrificing safety and mobility.

Planning is also required for maintenance, especially if new systems are implemented.  Therefore, it is important to include the maintenance staff in the planning stages of a project, as they will have recommendations on the types and manufacturers of equipment to procure, equipment location, and other issues that are an integral part of their jobs.

7.2.3.2 Inter- and Intra-Agency Coordination

Practitioners responsible for ramp management should coordinate with individuals inside and outside their respective agencies, first to ensure that ramp management strategies can be supported and secondly, to develop effective procedures to implement and operate these strategies.  The key is to break down barriers that exist within and between agencies and institutionalize working together as a way of doing business among transportation agencies, public safety officials, and other public and private sector interests within the region.  Practitioners responsible for ramp management may choose to use inter- and intra-agency coordination as a means to obtain consensus on how ramp management will operate and how it fits into the overall traffic management program. 

Inter-agency coordination will help the ramp management practitioner identify and exploit possibilities for improving day-to-day operations.  The continual coordination between agencies may foster the development of strong relationships and tactics that, over time, equate to measurable improvements in the safety, efficiency, and quality of service associated with regional transportation facilities, including freeways and ramps.  Inter-agency coordination, however, is not an easy process.  The ramp management practitioner should expect to make several attempts to obtain the level of inter-agency coordination needed to support ramp management activities, especially if this is a first attempt to coordinate with individuals from these outside agencies.  Adding to this difficulty are institutional barriers, such as resource constraints, internal stovepipes in large agencies, and the often narrow jurisdictional perspective of governing boards.  As such, initial attempts to coordinate with outside agencies should begin early in the planning process to allow enough time to ensure that coordination can occur.

Inter-agency coordination covers a broad set of agencies and stakeholders.  The agencies and disciplines that are critical to ramp management include:

• Enforcement agencies - to ensure that motorists obey signing, striping, barricades, and ramp signals to maximize the benefits of these investments.  City or state traffic ordinances may need to be amended to ensure that ramp management strategies can be enforced and to dictate penalties for non-compliance. 
• Local traffic engineering or public works departments - to ensure that ramp management operations fit well with arterial operations.  Coordination with traffic engineering or public works departments may also be needed to define agency roles and responsibilities.  For example, agreements may be needed to distinguish which agency will operate traffic signals located near metered ramps. 
• Transit agencies – to determine if ramp management strategies can support or improve transit operations, and if impacts on transit operations are acceptable.  Coordination with transit agencies should occur throughout the life cycle of ramp management strategies. 
• Local transportation planning agencies - to incorporate regional transportation data into the ramp management decision-making process and to program ramp management projects into the regional transportation planning program.
• Media – to establish solid relations with the media in the initial stages of ramp management.   The media can provide practitioners with a means to gain positive support for ramp management strategies from groups impacted by, and involved in their implementation.  However, the media can also be obstructive if not handled properly.  If the benefits of ramp management strategies are oversold and unrealistic, credibility of the implementing agency may be questioned. 

Similar to inter-agency coordination, intra-agency coordination will help individuals within an agency work together to solve operational problems, improve system performance, and communicate successfully with one another through deliberate collaboration and coordination.  This coordination will allow managers to coordinate their activities with others that comprise the broader traffic management program.  As mentioned earlier, ramp management is one element of the traffic management program and needs to further the goals and objectives of that program.  However, internal coordination goes beyond the traffic management program.  Personnel responsible for ramp management activities must also coordinate with the following set of internal staff:

• Planning staff - to make sure ramp management needs are incorporated into the agency plans.
• Design staff - to make sure that ramp management needs are incorporated in project designs.
• Maintenance staff – to determine if ramp management strategies can be adequately maintained.  If ramp management strategies cannot be adequately maintained, with existing agency resources, the practitioner must work with maintenance staff to determine the resources that are needed to maintain systems and equipment and options for acquiring them.
• Public information staff - who are responsible for informing the public of activities related to ramp management.
• Senior management – to keep these individual abreast of any issues arising from ramp management activities. 

Development of a regional traffic management program Concept of Operations (Con Ops) presents a good opportunity for inter- and intra-agency coordination in defining how ramp management fits into the larger traffic management program.  In short, the process of developing a Con Ops should involve all stakeholders and serve to build consensus in defining the mission, goals, and objectives of ramp management.  It should also provide an initial definitive expression of how functions are performed (thereby supporting resource planning), and identify interactions between organizations ( 5 )

7.2.3.3 Staffing

Successful ramp management requires skilled, well-trained staff in sufficient numbers to operate and maintain the strategies at effective levels.  Staff who operate and maintain ramp management strategies should have knowledge, skills, and abilities (KSAs) similar to staff who operate and maintain other traffic management elements.  The needed KSAs will differ based on the functions that staff are intended to perform – planning/design, operations, or maintenance.

Staff assigned to ramp management must be properly trained and have the knowledge to operate strategies effectively.  Training is available from in-house sources, national transportation organizations (e.g., the National Highway Institute), and educational institutions.

The appropriate staffing level will depend on a number of factors including the size of the system, the system complexity, the hours of operation and the specific ramp management strategies chosen.  For example, ramp metering will usually require more operations staff in a traffic management center, whereas time-of-day ramp closure will take more field staff. 

The ramp management practitioner should determine the impacts that ramp management and control activities will have on existing staff levels.  Practitioners must also assess the effect that changes to staffing have on current funding allocations.  Budgeting for training usually falls into an agency’s overhead budget and is 100-percent agency-funded.  This is often overlooked during the planning stage, yet can be costly if training new staff is required or inadequate training is provided.

7.2.3.4 Performance Monitoring

Effective ramp management relies on the ability to monitor the effects of any action taken to manage the movement of vehicles on ramps, regardless of whether it is on an exit or entrance ramp.  Along with the ability to monitor is the ability to implement actions to change the conditions under which the ramp is operating, such as varying the ramp meter timing, adjusting the time of day a ramp is closed, or adjusting signal timing at the ramp-arterial intersection.  Practitioners must seek to understand the impact (positive or negative) ramp management strategies have on furthering the region’s transportation goals.  Based on this understanding, practitioners must correct problems and seek ways to maximize the benefits of the strategies that have been implemented. 

For each objective associated with the ramp management program, the appropriate measure(s) of effectiveness should be identified, along with the desired threshold level of change that will be used to determine if the strategy has met the objective.  Objectives of a ramp management program typically include ( 6 ):

• Improve traffic flow and travel time reliability for selected corridors.
• Improve safety for selected corridors.
• Improve neighborhood impacts.
• Improve transit operations.

Typical measures of effectiveness that correspond to the above objectives include:

• Change in travel time.
• Change in travel time reliability.
• Change in the total number of crashes.
• Change in the severity of crashes.
• Change in traffic volumes on adjacent surface streets.
• Change in transit travel times.
• Change in transit ridership levels. 

Various types of equipment (e.g., CCTV, detection) can be installed to conduct performance monitoring and streamline the data collection process.  This equipment may be installed before ramp management strategies become operational.  They may also be programmed into other freeway management projects scheduled to be completed before ramp management strategies are planned to be in place. 

7.2.4 Relationship to National ITS Architecture

The National ITS Architecture ( 7 ) “Freeway Control” market package includes ramp metering.  This market package supports ramp meter controls on the freeway, traffic data flow from the freeway to the traffic management center (TMC), the control of ramp meters from the TMC, the control of sensor and controller equipment, and status and performance monitoring. 

7.2.5 Technologies and Strategies

As previously mentioned earlier in this chapter, there are four general classifications of ramp management strategies.  The advantages and disadvantages of each strategy differ, thereby requiring practitioners to closely analyze each to determine their appropriateness for satisfying existing problems and conditions.  Before selecting a strategy, it is important that practitioners understand the purpose of each strategy and its advantages and disadvantages.  This understanding will help narrow the focus and to identify the one strategy, or set of strategies, that is most appropriate given a set of unique issues and characteristics. 

Ramp management strategies are not all mutually exclusive and in certain cases may be combined to maximize their potential advantages.  For example, high-occupancy vehicle (HOV) ramps or bypass lanes - a special use treatment, are often implemented with ramp metering.  Ramp terminal treatments are often combined with any one or combination of the other three.  A brief overview of the available ramp management strategies is provided in the following section.  The four ramp management strategies are discussed in the order they are presented below. 

• Ramp metering.
• Ramp closure.
• Special use treatments.
• Ramp terminal treatments.

Readers should refer to the Ramp Management and Control Handbook ( 1 )for a detailed discussion of ramp management strategies. 

7.2.5.1 Ramp Metering

Ramp metering is the use of a traffic signal(s) deployed on a ramp to control the rate at which vehicles enter a freeway (see Figure 7-3).  By controlling the rate at which vehicles are allowed to enter a freeway, the flow of traffic onto the freeway becomes more consistent, smoothing the flow of traffic on the mainline and allowing more efficient use of existing freeway capacity.  Ramp metering can be an effective tool to address congestion and safety concerns that occur at a specific point or along a stretch of freeway. 

There are several aspects associated with ramp meter operation that practitioners should be aware of prior to making the decision of whether or not to implement ramp meters.  These aspects affect how a ramp meter or system of ramp meters control traffic based on agency goals and objectives and on local conditions.  Aspects of ramp metering that need to be considered are briefly discussed below.

• Metering Strategy - The control decision made that best addresses the specific goals and objectives of the metering system.
• Geographic Extent - the area that will be covered by ramp metering and whether the meters in that area will be operated in an isolated manner or as part of a larger system of meters.
• Metering Approaches - local or system-wide and pre-timed or traffic responsive.
• Metering Algorithms - the specific logic and calculations used to select or determine a metering rate.
• Queue Management - how the metering rate will be affected by ramp queues and how the agency will keep queues at a manageable and acceptable level.
• Flow Control - how traffic will be released from the meter, one at a time or two at a time in one lane or multiple lanes.
• Signing - how drivers will know that a ramp meter is on or off.

Figure 7-2: Ramp Metering Signals D

Figure 7-3: Metered Ramp D

7.2.5.2 Ramp Closure

Ramp closure has the greatest potential impact on existing traffic patterns and is rarely implemented as a long-term strategy.  The potential for significant impact is especially true for full or permanent ramp closures, where access to the ramp is no longer provided, requiring traffic to seek alternative routes to access the freeway.  In many cases, full ramp closure involves the physical removal of the ramp pavement so as not to give the false impression that the ramp will be re-opened.  Other types of ramp closures that affect traffic to a lesser degree include temporary and scheduled closures.  These types of closures usually involve deploying automatic gates or manually placing barriers at the ramp entrance to prevent access to the ramp.  Due to the relatively high impact on existing traffic patterns, ramp closures are seldom considered for deployment if other viable options are available.  Full ramp closure is best applied as a last resort for severe safety problems.  Temporary or scheduled closures may be applicable for reducing potential vehicle conflicts that may result from construction, major incidents, emergencies, or special events.

Figure 7-4: Ramp Closure Gate D

7.2.5.3 Special Use Treatments

Special use treatments for ramp management give “special” consideration to a vehicle class or classes to improve safety, improve traffic conditions, and/or encourage specific types of driving behavior.  Special use treatments require that the necessary policies (e.g., HOV, special events) be in place before strategies are implemented and funding requirements can be met.

Special use treatments include:

• HOV bypass lanes.
• HOV exclusive ramps.
• Construction, delivery, or emergency vehicle exclusive ramps. 

Figure 7-5: HOV Ramp D

7.2.5.4 Ramp Terminal Treatments

Ramp terminal treatments are solutions to specific problems that occur at the ramp/arterial intersection or have the potential to affect operations on the ramp, adjacent arterial, or freeway.  Typically, ramp terminal treatments focus on managing queues that form on the ramp that spill back onto an adjacent arterial or the freeway facility.  Ramp terminal strategies implemented at entrance ramps will provide better flow of arterial traffic not destined for the freeway and will improve the flow and handling of traffic on the ramp.  Ramp terminal treatments implemented at exit ramps may reduce queue spillback from the ramp terminal signal, reducing the potential for collisions on the freeway at the back of the queue.  There are at least six treatments that can be implemented at ramp terminals that can improve traffic conditions (e.g., traffic flow and safety) on or near ramp facilities:

• Ramp widening - to increase capacity, provide additional storage, or to provide a separate lane adjacent to the general-purpose lane so that HOV vehicles can bypass queues at meters.
• Channelization – to delineate and separate traffic movements.
• Signal timing adjustments - to reduce queuing on the ramp and to prevent queues from backing up into the intersection (entrance ramps), onto the freeway facility (exit ramps).
• Turn restrictions - to restrict volumes on the arterial downstream of the interchange, similar to signal “gating” methods.
• Geometric improvements - to improve sight distances, reduce horizontal and vertical curves in the roadway, or fix any other geometric deficiencies that cause motorists to take corrective measures.  
• Signing and pavement marking improvements - to inform or provide guidance to motorists of downstream conditions or to delineate traffic and facilitate vehicle movements. 

Figure 7-6: Advance Ramp Control Warning Sign D

7.2.6 Design and Related Considerations

There are a variety of considerations for the design of the systems and field elements within any capital improvement project that supports a ramp management strategy.  Every project must be reviewed to ensure that it is consistent with the operational objectives and that all pieces of the project are consistent with one another.

The American Association of State Highway and Transportation Officials (AASHTO) Design Guidebook ( 8 ) and the Manual on Uniform Traffic Control Devices (MUTCD)( 9 ) provide design guidelines for freeways.  Some agencies also have their own design guides.  Sections 10.6 through 10.9 of the Ramp Management and Control Handbook( 1 ) addresses the design considerations for ramp closures, special-use ramps, ramp terminal treatments, and ramp metering.  The equipment, signing, and pavement markings needed are outlined for each. 

With many ramp management strategies, ITS elements are typically required.  Section 10.8 of the Ramp Management and Control Handbook ( 1 ) offers guidance in following a systems engineering approach, whereby the agency can guide their ITS procurement through a step-by-step process from the Concept of Operations and detailed requirements and design to implementation and system acceptance.

7.2.7 Emerging Trends

Ramp meter operation is likely to be improved in the future in the following ways:

• Current research into an improved understanding of traffic flow in the region of flow breakdown is likely to result in improved ramp metering control strategies.  For example, different metering strategies might be employed for sustained operation in the free flow region as compared with recovery from the congested flow regions.
• Improved estimation of the tail of the ramp queue will lead to better control of the queue.  Improved estimation is likely to come about through improved estimation algorithms and through the increased use of wider area detection technologies ( 10 , 11).
• Improved recognition of the multi-objective applications of ramp metering (e.g., freeway flow optimization, corridor flow optimization, safety improvement, merge flow smoothing, and limitation of queue waiting time) and the design of systems that accommodate these functions at the required locations.
• Improvement in the ease of tuning ramp metering systems to accomplish their objectives.  The use of fuzzy logic and emphasis on evaluation of performance data are steps in this direction ( 11).
• More frequent use of real-time performance monitoring to gain a better understanding of how ramp meters are performing and to make needed adjustments more quickly ( 1 ).