Improving Traffic Signal Management and Operations: A Basic Service Model
II. Literature Review
GAO Report to Congress and the ITE Toolbox
In March, 1994, the General Accounting Office published a report to the Chairman of the House of Representatives Energy and Commerce Committee. That report was titled, Transportation Infrastructure, Benefits of Traffic Control Signal Systems Are Not Being Fully Realized. That report was motivated in part by a 1990 review conducted by the Federal Highway Administration that found that 21 of 24 traffic signal systems did not meet minimum standards of performance. That assessment, titled Operation and Maintenance of Traffic Control Systems, was motivated by the Institute of Transportation Engineers publication A Toolbox for Alleviating Traffic Congestion and Enhancing Mobility, which had documented that many local agencies were installing systems without considering the cost of proper operations and maintenance. In 1992, a panel was convened to review the FHWA Report and make recommendations for improvements. These recommendations led to the establishment of federal funding for operations and maintenance, which was included in the ISTEA legislation.
The following bullets are some of the recommendations that emerged from the ITE panel:
The additional recommendations (there were 34) were related to traffic signal systems, which was the focus of all these efforts.
The GAO report in 1994 summarized the resulting activities and improvements. The report described external issues that contributed to the problem, the main one being jurisdictional cooperation. It also described the emerging ITS technologies and how they should be incorporated, and it described FHWA's activities in responding to the above recommendations.
The main weakness of the report is that it did not describe good operation, or establish a meaning for good basic service. As such, most local agencies still faced the constraints that held them back previously. The Congestion Mitigation and Air Quality Program in the ISTEA funding legislation did, however, provide significant new funding for traffic signal systems, and ISTEA did provide funding for operations and maintenance for some state and local agencies.
The conclusion that the capabilities of new signal systems were being wasted was made without clearly defining what the needs were for basic operation, and thus without knowing whether those features were actually useful in as many situations as the report suggested. Also, the report did not determine whether these advanced features really added to the cost of those systems. The report was correct in noting (and in summarizing the results of those previous studies) that agencies were using capital funds to buy technology rather than focusing on good basic service.
In 1997, ITE updated the Toolbox, with considerable expansion to cover emerging ITS activities. For traffic signals, the Toolbox included signal timing improvements, updated equipment, coordination, removal of unneeded signals, and improved maintenance as tools in the toolbox. They provide an estimate of the benefits and costs of these tools, but the report did not attempt to define good basic service, or to provide examples or information of how agencies to define their own basic service concept.
Traffic Signal Report Card and Self Assessment
In 2005 (and then updated in 2007), the ITE under FHWA sponsorship developed a national report card identifying the overall effectiveness of signal operations and management. The report card was based on a self-assessment process, which itself identifies a range of priorities and objectives for agencies by grading agencies on those objectives.
The contents of the self-assessment include the following categories:
Each of these categories included general questions on overall programming, but more importantly for this review are those questions that reflect specific objectives. The following sections discuss each category, with an attempt to determine the underlying objectives that might be used to guide traffic signal management.
Activities considered important enough to ask about included:
All of these objectives are based on traffic conditions and control responses. Good management was described as including active programs for tracking events, monitoring incidents, and so on. None of the questions, however, determined if the agency had an operational objective, other than being responsive. The self-assessment, however, does not query agencies concerning the importance of these activities in their jurisdiction other than a general question of whether the agency had an overall operational philosophy.
Intersection Signal Timing
Important activities included:
As with the Management category, there is no discussion of whether agencies have an operational objective. It does not question whether agencies have a consistent clearance-interval policy, a consistent pedestrian accommodation policy, etc. It does not make the distinction between design signal timing parameters and operational parameters. Design parameters include clearance intervals, detector timing, minimum greens, and pedestrian intervals which are based on the geometry of the intersection. Assuming a policy was in place for calculating these parameters, well-managed agencies might not see the importance of reviewing them routinely, except to determine that the intersection geometry has not changed, and the signal timings in the controller are the intended settings (which is covered by the database management question).
Underlying measures of effectiveness in this category include:
Again, no attempt is made to determine whether these measures really are relevant to strong operational objectives. For example, if the objective is to minimize travel time, or, in the absence of that, to minimize inequity, then traffic-responsive operation might not be the solution even if traffic volumes are not completely predictable. Rather, an agency may properly respond to this objective by purposely designing signal timings for maximum versatility. A more fruitful approach might be to determine if the agency has identified, implemented, and evaluated a strategy for responding to situations of unpredictable demand. Given that the concepts of signal timing versatility are not well established in the practice, questions may need to be tailored to specific approaches that promote versatility, such as bandwidth optimization in response to unpredictable demand, which a practitioner might prefer because it addresses a wider range of conditions than calculated optimal timings based on a specific, and possibly not representative, set of conditions.
Signal Timing Practices
Underlying objectives in this category are:
These objectives are the most closely related to the general objective of minimizing travel time and providing equitable operation. Several, however, reflect regional practices that are not applied everywhere. For instance, before an agency implements variable phase sequence, which may be an unpopular or unexpected new operation in that region, agency practitioners should determine whether it actually promotes their operational objectives. A key feature in these objectives is the implicit assumption that computer optimization is an important part of the process. This implies a deep requirement for data collection and management that serves no other purpose than feeding the software, and may not be tied directly to agency objectives. Thus, the use of practices such as variable phase sequence does not mark an effective agency as well as the agency's process of considering whether such practices meet their objectives.
Traffic Monitoring and Data Collection
This category implies the objectives of:
The traffic data collected including turning movement counts in 15-minute intervals, and performance data including travel time and number of stops. Again, the data are not directly tied to objectives to determine the true importance of the data.
The underlying objectives implied in this category include:
These objectives allow agencies to score well if they have well-funded and active maintenance programs. Not all agencies have those resources, however, and signal practitioners still must respond to their situation as best they can. There is nothing in the self-assessment that allows an agency to overcome limited resources by more effective design. For example, an agency might achieve more effectively meet their operational goals by minimizing the number of detectors and designing their operation in such a way that those detectors are not needed. This might be done by limiting the cycle length, providing versatile smooth-flow coordination timings, ensuring that clearance intervals are sufficient to minimize the need for dilemma-zone detection, and so on. Highly durable detection construction might be affordable if the detection needs are minimized, further reducing the demands on maintenance. Consistent design standards (rather than high design in some locations that are funded by developers and minimal design in locations funded by the agency) can minimize the need for continued maintenance of the physical plant. Careful selection of technology might prevent expecting maintenance technicians to have skills beyond what the agency can remunerate. All of these strategies for surviving limited maintenance resources flow from a careful relationship between operational objectives and maintenance realities, but the self-assessment does not credit agencies who exercise such care.
The Traffic Signal Report and Self-Assessment was successful at shining the spotlight on the need for greater investment in traffic signal operations and maintenance, and providing a benchmark of common practices within the traffic signal community. Effectively, the Traffic signal report card is a document for policy makers rather than for practitioners, but it does imply some global objectives and standards of performance for local agencies that may not always be consistent with the realities faced by practitioners. The Traffic Signal Audit Guide also developed by NTOC offers an opportunity for practitioners to take a more in depth look at their specific practices and to develop process that more effectively meet local objectives.
When agency professionals are developing and defending their budgets, the Report Card and its associated reports and audit guide are most helpful, and collectively these documents provide the only real synthesis of expected and good practice. In the end, though, it is a diagnostic tool at the screening level, not a clinical approach for a given agency to improve their traffic signal management.previous | next
United States Department of Transportation - Federal Highway Administration