In cities that have countdowns on their pedestrian walk signs, collisions between cars and pedestrians have decreased since they were installed. You’d expect that. The countdowns make it easier for a pedestrian to judge whether they can safety get across the street in time. But something else happened: the number of collisions between cars actually increased! See, drivers can see the pedestrian countdowns, too, and that gives some of them the idea that they can make it across the intersection before the light changes, and encourages other drivers to wait. And those two kinds of drivers sometimes hit each other. NPR’s David Greene and Shankar Vedantam talked about this phenomenon and what can be done about it.
VEDANTAM: So just think about it, David, in terms of what safety officials do, if there's a bomb in the building or there's a threat to public safety, what you want to do is use a code that alerts the guards and the security officials that something is wrong...
GREENE: You hear that, like code nine, code nine.
VEDANTAM: Exactly or you could say paging David Greene and that's a code to all the security officials that something is wrong, without setting off mass panic. Magesan and Kapoor think we should do the same thing with these traffic countdown timers. 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.