This chapter examines benefits at the corridor and system level, reporting on identified or potential synergistic relationships between any of the European- or US-based traffic management techniques. System level applications will examine the following contextual settings:
The intent of this chapter is to document cases where synergistic relationships between traffic management strategies have been found; however, performance monitoring data regarding these strategies to date has been limited. Because this data is rarely stratified by strategy, being able to use it to estimate an individual strategy’s contribution to system level benefits is even more challenging. Nonetheless, research has shown that these strategies are rarely implemented in isolation, and a number of them are clearly complementary. While most of these strategies have some form of relationship with all of the others, there are some that stand out most clearly as complementary and/or supportive to a particular technique. Figure 10 indicates typical complementary and/or supportive strategies for each of the six European traffic management techniques. Note that while one technique may support another, the reverse is not always true. For example, while speed harmonization may be a critical supportive strategy for implementing hard shoulder running; hard shoulder running is not critical to the success of speed harmonization applications.
A brief discussion of how these techniques could be applied in a synergistic manner is provided below. Not all combinations of complementary and/or supporting techniques are discussed, but the write-up provides a general idea of how certain strategies can work together.
Speed harmonization, for example, is often implemented in tandem with queue warning. Queue warning complements speed harmonization because in addition to warning drivers of downstream queues, it communicates the reason why speeds are being lowered. In Germany, a congestion pictograph or icon is displayed on both sides of the overhead gantries to alert motorists of queues or congestion ahead. The value of the system lies in warning drivers of downstream queues so they take appropriate actions (e.g., slow down or change lanes), thereby reducing the occurrence of primary and secondary collisions caused by the congestion. In the Netherlands, motorists are alerted of queues with flashing lights surrounding the variable speed limits, and speed harmonization is activated with variable speed limit signs.
Queue warning is valuable in that it warns drivers of downstream queues so they take appropriate action to slow down or change lanes, reducing the occurrence of primary and secondary collisions caused by congestion. Queue warning can be supported by the implementation of speed harmonization to further enforce the need to reduce speeds.
Hard shoulder running is frequently implemented in conjunction with both speed harmonization and queue warning, as well as with junction control because they share the common need of addressing recurring congestion in bottleneck locations. Limited data from the UK indicates how these techniques can build upon each other. Transportation professionals in the UK believe that using speed harmonization and queue warning is important in order to maintain safety during hard shoulder running operations. Since hard shoulder running removes the shoulder for use as a breakdown lane, having a robust incident management program in place to monitor and respond to incidents as soon as possible is essential. Where hard shoulder running begins or terminates at a ramp junction, junction control is often required to maintain lane continuity and safe operations.
Junction control may be enhanced when supported by ramp metering, particularly when used at freeway on-ramps. If the control strategy has changed a junction from an add-lane to a merge condition, ramp metering would be especially beneficial to break up platooning vehicles from entering a ramp and facilitate the merge condition. Dynamic re-routing can support junction control as well in that if the control strategy reduces capacity of one movement (e.g., the through movement) in favor of another movement (e.g., the exiting movement), then it may be beneficial to re-route some through movement traffic to alternate routes so as to not experience undue congestion.
Dynamic re-routing is highly dependent on traveler information (including queue warning) to not only direct drivers to the desired alternative parallel route, but to also inform them as to why they are being encouraged to reroute. Information could include projected travel times via alternative routes and reasons for the delay on the primary route, e.g., “accident ahead.” Junction control can also support dynamic re-routing by providing additional capacity to a particular movement that has been targeted for re-routing.
Ramp metering can be enhanced by queue warning and traveler information when used to address mainline congestion. These techniques can provide the driver with information on why the ramp meter rate is operating in a given manner, which can lead to more acceptance of the meter and higher compliance.
In many urban roadway networks, overall system capacity is governed by localized constraints, or “bottlenecks,” at a limited number of locations. Addressing these bottlenecks can make a significant improvement in overall system performance and reliability. Often times these bottlenecks cannot be physically expanded due to either prohibitive costs or environmental concerns. These conditions can also be present during construction where detours do not offer the capacity, performance or operational safety of the final roadway configuration.
Because traffic management techniques are generally less expensive than the physical expansion of facilities, it can more feasibly be used as a means of addressing these conditions. For example, various traffic management strategies have been applied in Denmark as a way of maintaining traffic flow during construction.
Hard shoulder running is one of the more effective traffic management techniques that can provide additional capacity on an as-needed basis to address bottleneck issues. For hard shoulder running to be truly effective, it needs to extend through the bottleneck location upstream of typical queues. Otherwise it is simply moving the bottleneck to a new location.
It is important to consider the environment within which potential traffic management techniques will be implemented; for example, are ramp metering, service patrols, existing VMS, managed lanes, or other more US-based techniques already a part of the freeway management system? While these more traditional elements can be integrated into many traffic management techniques or strategies, careful consideration should be given to implementing traffic management techniques that lend themselves to being actively managed when the more basic traffic management systems are not in place. This is particularly relevant when considering the overall operation of the system and the demands it may place on existing traffic management center facilities and staffing requirements.
It is important to consider the overall system and plan for the best combination of traffic management techniques for the region or area under consideration. Many logical and synergistic combinations of techniques lend themselves to incremental implementation that improves effectiveness. The benefit of this is that it allows agencies to implement some applications sooner rather than later, building users’ experience and trust in the general traffic management concept prior to implementation of supporting techniques. Examples of techniques that have been implemented incrementally include speed harmonization, followed by hard shoulder running in the UK on the M42. Speed harmonization produces clear benefits to safety and operations – the inclusion of hard shoulder running in select locations increases the system’s ability to decrease congestion and improve overall operations and throughput. The European experience is filled with examples where an initial corridor was fully instrumented and monitoring demonstrated proven effectiveness before wider system applications occurred. This approach also allowed agencies to ‘learn from their mistakes” in refining the applications in order to improve performance and lower their investment costs.
The deployment of traffic management techniques that will be managed actively should be done considering logical operating segments within the system under consideration. Logical starting and stopping points based on travel patterns, freeway geometrics, observed operations of recurrent congestion or persistent queuing, and areas with higher than expected crash rates provide insight into areas where initial investigation and consideration of active management techniques may be warranted.
Much of the following discussion is based on work conducted by the Washington State Department of Transportation. This dialog is illustrative of similar adaptations occurring in other states as of 2010.
Integration with Current ITS Infrastructure
Current traffic management systems require a variety of instrumentation and communication investments to make the system functional. Some of this equipment is multi-purpose serving a number of traffic management techniques (e.g., in-road or other forms of vehicle detection, CCTV, VMS, communications network/equipment, etc). In many cases new or augmented equipment specific to the desired traffic management technique would be required (e.g., variable speed limit signs for speed harmonization). If existing equipment, detection, or traffic management systems are in place, their functionality should be reviewed to determine if they can be used for both the original intent as well as the proposed traffic management technique(s) to obtain the greatest benefit from existing system investments. The traffic management system will be relying on accurate, reliable and continual input for the various traffic management application algorithms – maintaining a high level of reliability on detection is crucial.
The variable message signs used to support these techniques are similar to sign technology used elsewhere by state DOTs. However, the purpose and application of the VMS signs is different than what has been typically used, due to the over lane positioning of the speed harmonization signs, their size, mounting maintenance and access requirements. Once again, it will be important to assess the existing system capabilities, the requirements of the new system, and their subsequent integration. For example, some transportation management system software can communicate with NTCIP signs, making integration much easier than if proprietary communication protocols are used.
Institutional issues are challenges that can prevent proper implementation of active traffic management techniques, but instead of being attributed to technology or engineering, these issues are caused by regulatory, legal, financial, management, organizational, human, or physical resource concerns and constraints and can even vary by state and locale.
Institutional issues differ between Europe and the US in various respects. European agencies may be subject to less liability due to legal statutes; they are perhaps more attuned to experimenting because there is limited funding and few geometric options to widen or otherwise add capacity through expansion or new routes; and for many applications they have a longer (and successful) track record in active traffic management accepted by their political and public stakeholders.
Because many of these strategies are relatively new to the US, some institutional issues may need to be worked out. Even inter-agency and intra-agency issues can present implementation obstacles. The difficulty with institutional issues is that there are no consistent approaches to resolve them. Some general institutional issues that can impact implementation of a wide variety of actively managed techniques in the US include the following:
Speed harmonization and hard-shoulder running may face additional institutional considerations in the US, such as the following:
Suggestions to Address Institutional Issues
Reaching solutions to a range of institutional issues, which allows for the successful implementation of a variety of traffic management techniques and systems, requires collaborative planning and development. To effectively implement traffic management techniques that are actively managed in the US the following actions are suggested (WSDOT ATM Concept of Operations Report, 2008):
United States Department of Transportation - Federal Highway Administration