Green Light, Red Light, 3-2-1

June 30, 2015
Posted by Jay Livingston

The good news about those countdown timers at pedestrian traffic lights is that they do what they’re supposed to do – save pedestrian lives and limbs. 

The research on this comes from two economists, Arvind Magesam and Sacha Kapoor (here). You may have heard Shankar Vedantam reporting it on NPR a few days ago (here).

The bad news is that while these timers are good for pedestriams, they are bad for cars. They increase car-on-car violence, and of a particular kind – rear-end collisions. 

Economists Magesan and Kapoor think of an intersection as market of walkers and drivers. The purpose of their study was “to evaluate a policy that improves the information of all market participants.” They conclude that giving everyone more information about when the light will change is what’s causing the accidents.

The largest increase is in rear-end accidents and we think it’s because two cars approaching a light, who both see the countdown, the guy behind, he sees the two or three seconds and thinks, oh, the guy in front of me is going to floor it too, I'll floor it and we’ll both get through the intersection. Whereas the guy in front thinks, OK, I only have two or three seconds left, I'm going to slowdown.

It’s like the old joke:

Cop to driver who has run a light: Don’t you know what that yellow light means?
Driver: Yeah, go like hell, the red one’s next.

The problem is not that pedestrians and drivers have the same information but that drivers have two sources of information. My guess is that in these rear-enders, the driver in front is paying more attention to the traffic light. he sees that it’s yellow and might turn red at any moment now. He slows down. The driver behind is focused more on the countdown timer. He sees that he still has a second or two to beat the light. Crash.

The economists have a solution – asymmetric information.  More specifically

Install them so that the pedestrians are aware of the timers but the drivers are not. And one way to do that would be to broadcast the timers via audio so that the pedestrians can hear the countdown clock go down, but drivers cannot.

Would you want the added noise of an audio signal? And if the intersection is already loud with the noise of traffic, the volume on the audio would have to be fairly high for people to hear it.

There’s a different, and cheaper, way. Give the walkers and drivers different information. In New York, some countdown timers for pedestrians are not synched with the traffic lights for cars. At the corner of 79th and Broadway, the light for cars turns red at the 9-second mark and red at 6 seconds. 

At 72nd St., if drivers going downtown on Broadway focus on the timer, rather than racing through the intersection, they will stop while the traffic light is yellow.

 (The poor quality of the video makes it hard to see the timer, but take my word  –
it goes to 0 when the traffic light turns orange.)

As you can see from just these two videos, the time difference between the lights for drivers and walkers varies considerably from one corner to the next. I have no idea whether each timing is based on some logic and evidence about the specific intersection or whether these are different treatments in an experiment.