Thursday, January 15

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.


Eric Orozco said...

A great reading on traffic:

The problem is we design for the comfort of the driver, not anyone else. We get to safer streets when the driver is uncomfortable... My favorite anecdote of Hans Monderman is how he would demonstrate the efficacy of his "squareabout" by walking backwards through the intersection in active traffic with his eyes shut.

It is true that there is a lot we can do to reduce lag time and gridlock. But, lost time represents to me the premium of the city.

We shouldn't take our suburban expectations to the cities, which demand we tie our distances to time. There should always be a time premium for nonresidents to reach downtown. I should be forced to slow down in downtown. An airplane - fastest mode spanning the longest distance - crawls to its gate.

In fact, the harder to reach the center, the more folks will want to get there. As my prof. Julian Beinart put it, cities demonstrate such homeostatic relationships. Because the suburbs are so intensely good at privatizing all our needs, we seek the outlet. But it is not the city that is inconvenient, the crawl has much to give, 5 minutes in the city gives you better than 20 minutes in the suburb. It is the suburb that is inconvenient. It is the suburb that is hard to reach. If it were not so, it would not be the haven people pay a time premium for.

Daniel Nairn said...

Thanks, Eric. You've given me plenty to think about with this comment. I love the airplane analogy.

I'm especially curious about what your prof. noticed, "the harder to reach the center, the more folks will want to get there." In everywhere I've lived, planning for easy automobile access has been justified in terms of economic viability. So, if we don't widen the road or provide free parking downtown to customers they will not come. Therefor, the loss of parking revenues pays for itself in the increased money spent downtown. And so on ...

I've never seen anyone provide evidence for this. It's just an accepted fact.

Eric Orozco said...

Its not the parking that provides economic vitality, but the service and value offered at the center. Humans need outlets from intense privacy. Starbucks realized this and unfortunately capsulated and sent Pikes Place to the voraciously the suburb consume this service.

In the suburban economy, it is true, convenience offered to the automobile is an issue and because it is the only mode available, parking must be provided.

What leads to our woeful environments, with their corridors of dross and noodled centers is this concept that we have to placate the automobile to do anything. Its a divine right to arrive at your cubicle at 45mph. We get the cities we deserve.