Concurrent Pedestrian Signals
This page explains the Traffic, Parking & Transportation (TP&T) Department policy on operation of pedestrian traffic signals. It explains the two types of operations and why concurrent operations is preferred in Cambridge.
Types of Pedestrian Operation
The WALK and DON’T WALK signs, also called pedestrian (or ped) signals indicate the best time during the signal cycle for pedestrians to cross the street. We can choose one of two methods of operation, as explained below.
Exclusive Operation
Pedestrians have exclusive use of the intersection. All WALK signals illuminate at the same time, while all the vehicle signals are red. (Some call this a “Barnes Dance,” named for the legendary New York City traffic engineer Henry Barnes.) The obvious advantage of this operation is that pedestrians do not have to share the road with turning vehicles (provided that none are turning right on red, legally or illegally). Second, fast walkers can cross the intersection diagonally.
But there are some less obvious but important disadvantages. First, in an urban environment with congested intersections, we can spare only the minimum time (as defined by engineering standard) of seven seconds for the WALK interval because the lines of vehicle traffic continue to grow while all the intersection lights are red. In addition to providing a short window for people to start crossing the street, pedestrians must wait for an intolerable period before finally getting their turn – sometimes a minute and a half or more. Many pedestrians are not willing to wait nearly this long. Traffic engineers usually install buttons to push to activate the WALK signal, which adds more wait time.
Concurrent Operation
Vehicles are permitted to turn right and/or left across the crosswalk while a WALK sign is showing. Drivers must yield the right of way to anyone in the crosswalk, just like they do when turning onto a side street where no traffic signal exists. When vehicles and pedestrians can use the intersection at the same time, we can leave the WALK sign on for much longer. Further, pedestrians wait far less time for the WALK sign to come on. Finally, the WALK sign comes on every cycle without needing to press a button.
A Head Start
Recognizing that turning vehicles can put pedestrians at a disadvantage, Cambridge intersections give pedestrians a head start. Engineers call this a Leading Pedestrian Interval, or LPI. Our LPIs are a minimum or 3 seconds and are longer for larger intersections. While this may not sound like much, it is plenty of time for pedestrians to walk several feet into the crosswalk and establish control of the space. By the time the first turning car reacts to the green light, several pedestrians are well within view, giving the driver adequate time to react and stop before finishing the turn.
Safety in Numbers
Multiple studies along with our own experience in Cambridge shows that pedestrians are safer in larger groups. Visibility is the most important element of roadway safety in an urban environment and a crowd is easily seen by drivers. In other words, it is best when all pedestrians use the crosswalk at the same time.
Voting With Their Feet
TP&T staff learned long ago that Cambridge pedestrians are clearly comfortable crossing concurrently with the green light no matter what the pedestrian signal indicated, so when traffic signals were set to run exclusive, most pedestrians crossed with the green light. Others pushed the button and were not willing to wait for the WALK sign and crossed with the green light anyway. A few waited for the WALK sign. As a result, pedestrians used the same crosswalk at different times. Drivers did not know when to expect to see pedestrians in the crosswalk.
Since pedestrians want to cross with the green light, we designed our traffic signals to accommodate them. With concurrent pedestrian operation, drivers know when to expect pedestrians to be in the crosswalk. And since they are all crossing at about the same time, the large group of pedestrians will be highly visible.
Department Policy
In 1997, TP&T made concurrent pedestrian operation with LPI official Department policy. Since then, crashes involving pedestrians have declined.
As noted in the Policy, there are some exceptions to this policy. Most notably, at T intersections, where all vehicles are turning either left or right onto the main street, concurrent operation is not feasible. Second, where the number of turning vehicles is very high, we may choose to use exclusive operation.
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