Now You Know

THE QUESTION: How do passenger crossing lights work? You press the button and sometimes the light will turn green right away. More often, there is a wait, sometimes a long one. This puzzles Beryl Young of Vancouver.

THE ANSWER: "The reason for the wait is system co-ordination," writes Chris Neuman from Edmonton's Office of Traffic Safety.

"Traffic signals along a corridor or in an area such as the downtown core are programmed to try and balance the delay that people experience during a trip. Most of this co-ordination favours the direction with the most traffic. "So, for example, Bay Street's lights will be timed to move as much traffic up and down Bay as possible, and people waiting to get onto Bay Street (or go straight across it) may feel as if they're waiting a long time."

However, he says it's not only the traffic lights at intersections that are co-ordinated; pedestrian signals are too. From a traffic-flow perspective, you don't want a pedestrian light to stop traffic at the same time that there is a green light for traffic at some downstream intersection; this interrupts traffic flow.

"So, in most cases, the pedestrian has to wait for the downstream traffic signal to turn red before being allowed to cross." So, if you have the bad luck to arrive at the "wrong" time of the signal cycle, you'll wait up to three minutes before being able to cross.