Reducing Speeding by Removing Speeding Opportunities: Field Test of Safe Waves Traffic Signal Timing

The Safe Waves approach to arterial traffic signal timing aims to eliminate, as much as possible, opportunities to speed while still providing good two-way progression for arterial traffic, through measures that include a short cycle length, short coordination zones, a low progression speed, and pedestrian recall (a traffic signal timing function that causes a pedestrian phase to automatically activate every cycle) except where pedestrian demand is very low. Simulation-based studies have shown Safe Waves signal timing can substantially reduce speeding opportunities with little or no increase in arterial delay. Speeding opportunities are defined as the number of events in which a vehicle enters an intersection on a “stale” green—a signal already green as the driver approaches—with no vehicle ahead of it for at least 5 s. However, only field studies can confirm whether timing traffic signals this way will actually lead to less speeding. A before-after field test compared existing signal timing plans with Safe Waves signal timing plans for a stretch of Route 114 in Danvers, MA, U.S., a suburban, 4-lane arterial with six signalized intersections and a 40 mph speed limit. With Safe Waves, the number of vehicles exceeding the speed limit fell by 74% as speeding opportunities fell by 51%. At the same time, average arterial delay increased by only 1.8 s per intersection, and average pedestrian delay crossing the arterial fell by 18 s, or 33%. Signal timing techniques that proved valuable in reducing speeding opportunities included using 30 min peak hour factors and, for phases that serve an infrequently used concurrent pedestrian crossing, programming the phase for less time than needed to serve the pedestrian crossing, a technique sometimes called “oversized ped.”