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.

One example of such “predictive priority” signal management is the Central Phoenix/East Valley Light Rail Project in Arizona, now under construction. The system will anticipate the approach of a light-rail train and adjusts timing of traffic light signals at 160 intersections to ensure reliable light rail travel times and maintain coordinated signal cycles on all cross streets.

Still in the planning stage is a ground-breaking pilot project by the Los Angeles County Metropolitan Transportation Authority for the Crenshaw Boulevard corridor, a highly congested area of downtown Los Angeles served by two metropolitan bus lines. Key objectives of this pilot are to:

  • Evaluate bus signal priority as a means of enhancing on-time performance
  • Investigate if round-trip running times can be reduced so that a bus can be taken out of the operating schedule or the headway can be shortened
  • Demonstrate the use of smart bus systems, including global positioning system-based automated vehicle location equipment to make the decision to request priority at signalized intersections
  • Evaluate the use of IEEE 802.11b, a wireless local area network, and other technologies to communicate with intersection controllers as a vehicle approaches
  • Demonstrate the application of bus signal priority treatments at intersections operating under intersection controllers running different traffic signal control software systems.
  • One of the overall outcomes of the project will be a plan for other cities to use as a “road map.”


    Signal timing is the implementation of preplanned timing based on time of day. Traffic signal timing is one of the most cost-effective ways to help traffic move and one of the most basic strategies to help mitigate congestion. The Federal Highway Administration (FHWA) estimates that:

  • More than 75% of the more than 260,000 traffic signals in the United States could be improved by updating equipment or simply adjusting and updating the timing plans
  • Poor traffic signal timing accounts for 5% to 10% of all traffic delays (or 295.8 million vehicle-hours of delay) per year on major roadways alone
  • Optimizing traffic signals can result in savings of as much as $120,000 per signalized intersection in terms of fuel efficiency and productivity gains due to increased mobility
  • Interconnecting traffic signals and optimizing the traffic signal timing can result in travel time reductions ranging from 8% to 25% along a corridor or arterial.
  • Three basic issues must be addressed when coordinating and retiming signals.

    First, it is essential to use wireline or wireless interconnection or highly accurate time-based coordinators to provide a common cycle length. Second, it is important to systematically optimize the interval settings for actuated signals and optimize the parameters of pre-timed signals. And third, it is crucial to use a computer system or advanced transportation controller to provide real-time traffic-responsive or traffic-adaptive traffic control.


    The FHWA has found that adaptive control algorithms can decrease travel time and improve traffic volume handling by 3% to 7%. FHWA also estimates that the cost to implement adaptive signal control is between $10,000 and $40,000 per intersection, while the cost of maintaining this infrastructure amounts to about $1000 per intersection per year. These costs must be compared to the expense of retiming traditional signals, which is about $5000 per signal every two years. In addition, adaptive signal control saves users $8 for every reduced vehicle-hour of delay and $20 for each reduced commercial vehicle-hour of delay.

    Proper traditional signal timing is a first step toward more effective traffic management in urban areas. Implementation of adaptive signal control is a next evolutionary step, since its timing reflects and responds to nearly real-time conditions. The benefit of this approach becomes most obvious on those off-days when traffic patterns deviate from historical patterns due to weather, special events, or other unusual conditions.

    — Tiger Harris is an assistant vice president and southeast area manager at PB Farradyne, the intelligent transportation systems division of Parsons Brinckerhoff. He is based in the company's Morrisville, N.C., office.