Most of the region's residents and visitors, whether pedestrians, bicyclists, motor vehicle drivers, or transit riders, are impacted by traffic signals in their daily travels. There are over 5,000 signalized intersections in the Washington metropolitan area. These intersections range from simple and isolated to large and complex. Signals are maintained by numerous agencies and jurisdictions, with the goals of safety, efficiency, and meeting community needs. Interagency coordination, especially near jurisdictional boundaries is important. Efficient traffic signal operations are key for getting the most out of the region's transportation system.
Engineers work to determine how a traffic signal (or system of signals) can be timed to cause the least delay for most travelers, while still ensuring safety. Signal timings are based on traffic data, numbers of pedestrians, travel time observations, consideration of intersection geometrics, and computer analysis. The result from the viewpoint of any one driver may not appear to be optimal due to high traffic loads, cross-traffic or other factors, but overall system delay should be reduced.
Signal timing aims to reduce travel times, delays and the frequency of stops. Though variable, improvements are commonly seen in the range of five to twenty percent. An engineering rule of thumb recommends re-checking signal timing at least every three years as traffic patterns evolve. Analysis performed for the Maryland State Highway Administration estimated a benefit of about ten dollars in time and fuel savings for each dollar spent on optimization, at a cost of a few thousand dollars per intersection (just a fraction of roadway construction improvement costs).
The TPB increased its focus on the issue with a 2002-2005 traffic signal timing "Transportation Emissions Reduction Measure" (TERM). The TERM called for an increase in the percentage of the region's signals optimized within the previous three years. Increasing the percentage of signals in the region with optimized timings is beneficial for air quality. At the outset of the TERM in 2002, about 45 percent of the region's signals were classified as optimized. This had increased to 68 percent upon completion of the TERM in 2005. The TPB has called for maintaining and increasing this optimization in future years.
Signal timing occurs within a larger context of traffic engineering activities. On a routine basis, agencies perform systems monitoring and maintenance, respond to public inquiries and perform spot checks. Other notable activities around the region include use of transit signal priority treatments for buses, signal preemption devices for fire trucks and ambulances, red light running enforcement, pedestrian "countdown" signals, use of energy-efficient light-emitting diodes (LEDs) in place of old-fashioned incandescent bulbs, and installation of failure-resistant power backup systems.
