Measures of Effectiveness and Validation Guidance for Adaptive Signal Control Technologies
Chapter 2. Traffic Signal Operations Objectives that can be applied by ASCT
Validation is a critical phase of the systems engineering process that an agency uses to identify that the system implemented fulfills the objectives and needs it was envisioned to achieve. The FHWA Model Systems Engineering Documents for Adaptive Signal Control Technology (ASCT) Systems (FHWA-HOP-11-27) identifies several operational objectives and strategies that are applicable to coordinated traffic signal systems and adaptive control. This document will offer guidance on how to carry out the validation process by demonstrating measures of effectiveness that are traceable to objectives and then providing a methodology for data collection and analysis. The outcome of the validation process is confirmation of whether or not stated objectives were satisfied.
To be effective, the validation process demands clear articulation of operational objectives. To the greatest extent possible, objectives should be stated in a manner that is Specific, Measurable, Achievable, Realistic and Time-bound (SMART). One method for formulating good objectives is to begin with a statement of goals for traffic signal operations. Goals for traffic signal operations state at a high level what the agency is trying to achieve as an outcome of the commitment of its resources. A logical framework is then used to derive objectives from goals, and then strategies and tactics from objectives. The framework thus provides traceability from goals to tactics. The scope of this report limits the validation to evaluating the achievement of objectives; however it logically follows that validation of objectives also suggests achievement of goals by implementation of specific strategies and tactics.
- Goals – What is trying to be achieved.
- Objective – What needs to be done to achieve the goal.
- Strategy – Capabilities put in place to meet the objective.
- Tactic – Specific methods to achieve the strategy.
Goals for Traffic Signal Operations
The primary purpose of signalized intersections is to safely assign right of way to avoid conflicts. Conflicts are avoided by assigning segments of time when compatible movements of vehicles and pedestrians can safely travel through the intersection. As traffic signal control technology at both the intersection and system level has evolved, goals in addition to safely moving vehicles and pedestrian have become achievable through implementation of more carefully planned strategies and tactics. Goals that are oriented around improving efficiency by keeping traffic moving and minimizing stops and delay can be reasonably pursued and achieved without compromising safety. Well-articulated goals are ensured to be appropriate by considering the context within which they will be pursued. Context for goals of traffic signal operations could include technical capability, surrounding land use, user expectations and traffic demand conditions. Examples of traffic signal operations goals include:
- Minimize Congestion.
- Prevent or Delay Oversaturation.
- Accommodate Long Term Variability.
- Manage Incidents and Special Events.
Congestion in the context of signalized arterial networks is the outcome of demand exceeding capacity at one or more intersections approaches resulting in the formation of destructive queues that propagate, increasing delay and causing impacts to safety and efficiency. The capacity of an intersection is directly related to signal timing and congestion can in fact be induced even in light to moderate demand conditions if signal timing is not appropriate for the demand condition.
Prevent or Delay Oversaturation
In many traffic situations, it is very difficult to prevent a system from becoming oversaturated. When demand vastly exceeds route and phase green time allocations, situations can quickly become unmanageable. ASCT can prevent or delay this situation by adjusting green time allocations for the saturated phases as the demand is increasing.
Accommodate Long-Term Variability
Traditional signal timing plans require maintenance as traffic patterns and land uses evolve. Communities in significant growth cycles have traffic patterns that change slowly over time as new businesses and residential areas are added to the community. Typical signal timing and review cycles (e.g., approximately three years) cannot keep up with the changes that occur in traffic patterns when those patterns are evolving on a monthly basis. ASCT should be able to provide less deterioration in efficiency over time as compared to re-optimization of fixed timings.
Manage Incidents and Events
All traffic systems at times experience incidents due to traffic crashes and a variety of other external influences. Planned special events such as concerts, sporting events, and community activities also cause significant challenges for signal timing plans with fixed parameters. ASCT is particularly suited to accommodate these abnormal situations.
Traffic Signal Operations Objectives
The Model System Engineering Document describes the following five traffic signal operations objectives that can be achieved by Adaptive Signal Control Technology. This list is not comprehensive, and is based on several high level goals including those listed in the previous section. One or more of these objectives may be related to the achievement of a goal. Chapter 3.5 of the Model Systems Engineering Documents describes a set of operational strategies that can be applied by an ASCT. The logical follow on to validation of an objective is that supporting strategies and tactics were successful; however, more rigorous evaluation and measures of effectiveness might be required that are beyond the scope of this report.
- Smooth Flow (FHWA-HOP-11-27, page 25).
- Maximize Throughput (FHWA-HOP-11-27, page 25).
- Access Equity (FHWA-HOP-11-27, page 25).
- Manage Queues (FHWA-HOP-11-27, page 25).
- Variable Objectives (FHWA-HOP-11-27, page 26).
- Changing Objectives under various circumstances – similar to number 5.
- Maximize Intersection efficiency at isolated critical locations – Not addressed in this document.
This objective seeks to provide a green band along an arterial road, in one or both directions, with the relationship between the intersections arranged so that once a platoon starts moving it rarely slows or stops. This may involve holding a platoon at one intersection until it can be released and not experience downstream stops. It may also involve operating non-coordinated phases at a high degree of saturation (by using the shortest possible green), within a constraint of preventing or minimizing phase failures and overflow of turn bays with limited length, and with spare time in each cycle generally reverting to the coordinated phases.
Maximizing throughput is achieved when the highest possible traffic flow is achieved across a cordon line. This is typically achieved by creating smooth flow along a route, but it may also be achieved by maximizing both through and turning movements along a given direction of travel. Satisfactory performance on a throughput maximization objective requires emphasis on maintaining large split times for phases that serve the intended direction of travel and maintaining offset and green-time relationships between adjacent intersections so that downstream queues do not affect the flow of vehicles along the critical route. Non-critical phases may have increased delays in order to provide the best possible level of service for the heaviest travel route.
Traffic signals are often provided so that major traffic generators along a street can have safe and efficient access to and from the arterial. In these cases, the objective is to equitably serve all traffic movements at each intersection. At the same time, coordination is generally provided along the arterial, but not at the expense of accessibility to local land uses. An example is a suburban retail shopping district that generates significant demand for left-turn and side-street movements. Intersections that serve significant traffic volumes on many movements, but are sufficiently isolated from other signals may also benefit from the objective to optimize for access equity. Providing satisfactory performance on such an objective requires appropriate allocation of split time and less emphasis on maintaining opportunities for coordination.
Where there are closely spaced intersections and particularly when a short link is fed by movements from various phases, it can be important is to ensure that queues do not block upstream intersections or movements or that upstream signals do not release traffic downstream when there is nowhere for those vehicles to go. Similarly, a queue management objective can include management of these situations, such as when a left turn bay spills over into adjacent lanes or when through movement queues prevent vehicles from entering a left-turn bay. Providing satisfactory performance on such an objective often requires tight constraints on cycle and phase durations to ensure that a large platoon does not enter a short block if it must be stored within that block and wait for a subsequent green phase. It may also involve “gating” actions, so that vehicles are stored upstream of the critical links because the upstream location has adequate queuing capacity.
It is often the case that different objectives are appropriate at different times of the day and under different traffic conditions. An arterial road that provides access between a freeway and large residential areas, but also has traffic generators such as retail centers and schools, may require an objective of providing smooth flow or maximum throughput during the morning and evening peak periods, but provide access equity during business hours and on weekends.
Changing Objectives by Time of Day
It is often the case that different objectives are appropriate at different times of the day and under different traffic conditions. An arterial road that provides access between a freeway and large residential areas, but also has traffic generators such as retail centers and schools, may require an objective of providing smooth flow during the morning and evening peak periods, but provide access equity during business hours and on weekends. Under these conditions, the ASCT may be required to accommodate switching objectives at different times of day. Most ASCT existing today do not explicitly include features or configurability to address this objective, although many systems will modify their actions to inherently achieve this objective based on detection of the field conditions.
Long Term Variability and Events
All traffic systems at times experience incidents due to traffic crashes and a variety of other external influences. Planned special events such as concerts, sporting events, and community activities also cause significant challenges for signal timing plans with fixed parameters, particularly when the egress from the event cannot be easily predicted (e.g., overtime). Many agencies address these situations with manual approaches (policemen, TMC operators), which can be expensive. ASCT are particularly suited to accommodate these abnormal situations. Figure 2 illustrates the typical shifts that occur in traffic demand due to special events (Bullock et al., 2008). The top graph indicates the directional flow along the routes to the Brickyard 500 on a normal day. The bottom graph indicates the directional flow along the routes to the Brickyard 500 on the day of the event.
Figure 2. Line Graph. Comparison of normal (top) and special event traffic (bottom).
(Source: Wasson et al., 2008.)
Summary of Operational Objectives for ASCT
While the implementation of ASCT will generally be expected to improve the performance of your traffic system, there are a wide range of operational objectives that can be addressed by deploying ASCT. In all cases, the key characteristic of the traffic situation that is addressable by ASCT is variability. Something in the traffic system changes or is changing, either quickly in minutes or slowly in days, months, or years, and the existing signal timings are not appropriate for the new situation. This results in unnecessary degradation to the traffic performance. Identifying appropriate objectives for the deployment of ASCT is important for achieving the results you anticipated. Collecting performance measurements that validate that the system meets those objectives is the second component of effective deployment of ASCT.
In the history of ASCT deployment, it has not always been the case that objectives for ASCT deployment have been explicitly stated. Further, MOEs that have been collected and reported don’t always validate that the system met the objectives. In addition, multijurisdictional deployments often do not have consensus among the stakeholders as to what the operational objectives will be. In the next section, we summarize a review of representative (not comprehensive) literature in evaluation studies for ASCT. This review is intended to identify strengths and weaknesses of past studies with the goal of identifying a recommendation for best practices in evaluation of ASCT and signal timing. From these strengths and weaknesses of past approaches, tools were developed that can help facilitate matching operational objectives with MOEs.