TTAP Newsletter: RoadTalk
Knowing Your Traffic Signal Operations Objectives
by Dr. Airton Kohls
Let me get started with an exchange during one of our Traffic Signal Academy workshops:
"Are you responsible for traffic signal timing in your agency?"
"Yes, we are actually retiming several intersections currently."
"Please tell me your main objective when timing your traffic signals."
"Ok, good, this is one of the most common objectives for traffic signal timing. Do you know or has anybody measured the amount of delay in the intersections you are currently working on?"
"No, not really..."
So, this brings me to two considerations I would like to address here:
- The need for collecting performance measures;
- Understanding traffic signal operations objectives.
Collecting performance measures in signalized intersections has been historically interpreted as a costly and time consuming effort. More than that, additional personnel is necessary to analyze all the information and implement appropriate strategies to improve operations. Technology has helped change that concept. Today, it is a lot easier and less costly to collect performance measures and make good use of the data.
Recently two documents have been made available addressing the issue:
- Performance Measures for Traffic Signal Systems – an Outcome-Oriented Approach can be found at http://docs.lib.purdue.edu/jtrpaffdocs/3/
- Measures of Effectiveness and Validation Guidance for Adaptive Signal Control Technologies can be found at http://www.ops.fhwa.dot.gov/publications/fhwahop13031/fhwahop13031.pdf
Now, regarding traffic signal operations objectives, minimize delay is by far the most common answer I get during our workshops, but there are other objectives that warrant considerations. We can include maintaining safe operations, smooth flow or minimizing the number of stops, maximize throughput, access equity, manage queues, etc. Let’s see what the second document I listed above tells us about some of these traffic signal operations objectives:
Smooth Flow or Minimizing the Number of Stops
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 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.
So, there you have it... please remember to consider alternative traffic signal objectives and strategies and, start thinking about ways of collecting performance measures (if you have not done so yet) to improve your operations.