Benefits of Retiming Traffic Signals, The

Institute of Transportation Engineers. ITE Journal, Apr 2004 by Sunkari, Srinivasa

SIGNAL RETIMING IS ONE OF THE MOST COST EFFECTIVE WAYS TO IMPROVE TRAFFIC FLOW ALONG A CORRIDOR. TRAFFIC SIGNAL RETIMING CAN SIGNIFICANTLY REDUCE DELAYS AND STOPS EXPERIENCED BY MOTORISTS, WHICH CAN IMPROVE SAFETY AND REDUCE FUEL CONSUMPTION AND EMISSIONS. TYPICALLY, THE BENEFIT TO COST RATIO FOR SIGNAL RETIMING IS ABOUT 40:1.

WHAT IS SIGNAL RETIMING?

Signal retiming is a process that optimizes the operation of signalized intersections through a variety of low-cost improvements, including the development and implementation of new signal timing parameters, phasing sequences, improved control strategies and, occasionally, minor roadway improvements.

The signal timing process often includes the training of engineering and maintenance staff to use existing signal control equipment more efficiently and to support new technologies as they become available for implementation.

Each traffic signal operates under a unique set of timing parameters. These parameters include minimum and maximum green durations, pedestrian indication requirements, gap and extension times, overlaps and phase change intervals (yellow change plus red clearance). A fixed-time signal also must have fixed cycle and split lengths that accurately balance average demand over a period of time.

When signals are operated within a coordinated system, additional parameters are used: cycle (the time needed to serve all phases); offset (the time from a reference point, such as the start of green or yellow of the coordinated phase at one intersection, to the same reference point at the other intersections); and split (the time allowed for each movement or phase-their total is the cycle length).

The offset is used to allow vehicles moving at the proper speed to advance from intersection to intersection without stopping. This process is called progressive movement.

As traffic patterns change over days, weeks and times of year (such as holidays), it is appropriate to modify signal timing parameters. This often includes the development of different timing parameters for morning and evening peak periods, midday off-peak periods, weekends and nights.

Signal retiming is oriented toward optimizing a controller unit's response to the demands of roadway users, including all types of motor vehicles, bicycles and pedestrians. Signal timing strategies include the minimization of stops, delays, fuel consumption and air pollution emissions and the maximization of progressive movement through a system.

Occasionally, the signal retiming process includes the reconfiguration of a signal's operation. This can include a change in the sequence of movements used at an intersection or the addition of signal displays and intervals (phases) to accommodate specific demands or movements, such as left-turn signals. Left-turn movements sometimes are programmed to follow the opposing through movement and accommodate the different arrival times of through movements along a coordinated arterial.

As technology continues to change in the traffic control field, it is important to upgrade existing traffic signal control hardware and accommodate enhanced signal operations. It also is possible to associate minor roadway improvements with the signal retiming effort to improve traffic operations.

For example, the provision of an additional left-turn lane, the extension of a right-turn lane, or the addition of rightand left-turn overlaps to allow for the accommodation of additional vehicle demand often are important.

Finally, training activities should be included as part of the signal retiming effort. Advancements in control technology, signal optimization programs and other traffic engineering tools are important.

WHY IS SIGNAL RETIMING CONDUCTED?

Traffic signal retiming is one of the most cost effective ways to improve traffic movement and make streets safer. Signal retiming is needed as much as patching potholes, removing snow and restriping pavement lines and markings. Following is a summary of the various reasons signal retiming is conducted:

* By coordinating or sequencing signals in relation to each other, platoons, queues, or groups of vehicles can travel through a series of signals with minimal or no stopping.

* The delay time on the approach to an intersection can be reduced by balancing the green time to reduce delay at that intersection.

* With developments such as the addition of new homes or stores, traffic increases and creates a need to adjust the timing of affected traffic signals.

* The diversion of traffic off a freeway or interstate due to an incident (an accident or event) associated with a computerized signal system can be accommodated.

* Motorist frustration caused by excessive delays or stops can be reduced by adjusting timing to reduce stops and delays and provide coordinated flow through groups of signals.

* Emissions and fuel consumption can be reduced by optimizing signal timing and coordinating traffic flow.

* Emergency vehicles, buses and commercial vehicles can save time.

* The number of severe collisions on city streets can be reduced by producing smoother traffic flow and fewer stops.