Credit: Parsons Brinckerhoff

A pilot project by the Los Angeles County Metropolitan Transportation Authority for the Crenshaw Boulevard corridor is looking at signal timing for its buses.

Credit: Parsons Brinckerhoff

This simulation shows how vehicle traffic would be stopped to allow light-rail transit lines to operate more efficiently using the “signal priority” method.

In urban environments across the country, traffic congestion has become a seemingly intractable problem. A major contributing factor to congestion—the lack of appropriate traffic signal timing—is one challenge that can be met head-on with effective solutions that will make a real difference.

Three relatively simple measures have been used by transportation officials to improve traffic flow through intersections, reducing congestion: adaptive signal control, signal preemption/priority for transit and emergency vehicles, and advanced signal timing.


The central goal of signal control is to optimize vehicle flow through a given road network. In most cases, the most powerful and cost-effective approach is to improve the timing of the traffic signals at all intersections in the network. Adaptive signal control uses very recent data to adjust signal timing “on the fly” based on close to real-time traffic conditions. Software systems like MIST (Management Information System for Transportation) and OPAC (Optimized Policies for Adaptive Control) have been used successfully by municipalities, state DOTs, and others for this purpose.

OPAC is a traffic adaptive signal control algorithm that provides for continuous online optimization of phase duration, cycle length, and offsets in immediate response to real-time traffic flows. OPAC eliminates the need to develop and store timing plans for different traffic conditions while controlling both isolated intersections and networks of intersections in coordinated operation.

OPAC uses a predictive type of optimization with a rolling horizon. This congestion control strategy—which attempts to maximize throughput—adjusts splits, offsets, and cycle length but maintains the specified phase order. For uncongested networks, OPAC uses a local level of control at the intersection to determine the phase online and a network level of control for synchronization, which is provided either by fixed-time plans (obtained offline), or a virtual cycle (determined online).

The types of control and levels of local and global influence are flexible. Predictions are based on detectors located approximately 10 to 15 seconds upstream. After the initial 10 to 15 seconds, a model predicts traffic patterns. System monitoring and coordinated control features are provided through MIST, which communicates with individual intersection controllers and monitoring devices, gathering data to feed the OPAC algorithm and transmitting signal timing instructions rendered by OPAC.

The implementation of an adaptive control system such as OPAC provides many advantages over existing fixed-timed plans by minimizing traffic delays through effective signal operation, improving travel time, reducing fuel consumption, and improving air quality while simultaneously reducing maintenance and operation costs and facilitating automated traffic data collection.

Several U.S. cities, including Chicago and Seattle, already have such sophisticated computer-guided traffic management systems in place; Cary, N.C., and Sarasota, Fla., are among those currently implementing such systems; and other cities around the nation are following suit.


Traffic signal preemption and priority systems allow emergency vehicles and transit vehicles to move easily through a grid of signalized intersections by means of a vehicle-mounted transponder that communicates with the signal controller at each intersection.

In the case of emergency vehicles, signal preemption changes the traffic light signal to green in the direction the vehicle is traveling and red for all other directions. For transit vehicles, signal priority enables the vehicle (bus, train, or streetcar) to extend an existing green cycle for an upcoming intersection so that it can pass through the intersection without stopping. This promotes mobility, timeliness, and fuel efficiency, and improves air quality in the surrounding neighborhoods.