The trouble with yellow lights

Traffic engineers have known this for decades, but I don't know if many people in the general public have given much thought to traffic signals. I know I hadn't until recently.

Consider the moment you approach an intersection and the light turns from green to yellow. You have a split second to make a decision between two entirely opposite courses of action, speeding up to pass before the light turns red or slowing down to a stop. This is a sharp either/or decision with no spectrum of moderation in between, and delaying the decision for seconds could jeopardize the outcome.

So how do you decide between the two options? You mostly need three pieces of information: your current velocity, the distance to the light, and the time before it turns red. The human mind calculates distances fairly well, but velocity is more complex, as it requires an estimation of how far away objects are as they pass through your frame of view. There are also several factors that can distort this judgment. The time of the yellow light is a bit easier to grasp, except for the fact that yellow light timing is not standardized and is up to the whims of local governments. Then you must decide whether an acceleration will allow sufficient time to pass through the light. And what is your risk coefficient? Are you willing to accept a 5% chance of a ticket or an accident to avoid wasting time? 10%? Is this coefficient adjusted if you are running late for a meeting? Remember, you have a split second.

And then there are dozens of other things competing for your attention. Are there other cars approaching from the side? Where does your lane continue after the intersection? Are there pedestrians attempting to cross? Suppose you are unfamiliar with the area, and you're not sure which direction you are going. And maybe you're on a cell phone or thinking about something important in your life. This is the context in which you must make the split second all-or-nothing decision.

We can all see where this is going ...

Stop signs, on the other hand, are unambiguous visual signals. When the meaning of a signal does not change, the human mind can form habits around its recognition. This allows for an immediate pre-cognitive response. Even more importantly, the stop sign indicates a single course of action with a spectrum of variation for correction. For example, if you misjudge the distance to a stop sign, you can correct this mistake by breaking more abruptly as you get closer.

Kenneth Todd, in a wonderful article "Traffic Control: an exercise in self-defeat," wrote,

"Traffic signal control is so unsafe that the official Manual on Uniform Traffic Control Devices already in 1935 recommended a 12-month trial of less restrictive alternatives. Today’s Manual lists 12 alternatives to be considered in preference to signal control, among them all-way stops and roundabouts."

Why do we keep putting them in? To enhance traffic flow, presumably. As a sidenote, the article brings up another crazy inconsistency. Pedestrians are typically required to cross the street at intersections, although by far the safest place to cross is between blocks. Sounds like common sense to me.