Why do traffic jams form at the speed bumps along a highway? Think about it: All cars run over the bumps at the same speed. They only stop when there are pedestrians crossing the road — and that’s rare. I have a hypothesis: I think that if this time interval between cars is less than the reaction time of drivers, then a jam will ensue.
To find out the average reaction time of a driver, I made some measurements on a real road — Kenyatta Avenue, Nairobi. I watched how drivers were reacting to traffic lights.
When the lights go green, it takes quite a while before all the cars in the queue begin to move. The reason is that every driver is waiting for the one in front to get in motion. I timed how long it takes from the moment the lights go green to when the last car in the queue starts to move. I made five observations.
The first queue had eight cars and it took 13 seconds for the eighth car to start moving. The next four queues had 10, 10, 12, and nine cars and the last car in each waited 15s, 16s, 16s, and 16s respectively before starting to move.
From this data, it turns out that the average reaction time of a driver is about 1.5 seconds. The next question is: What distance does this correspond to? The answer depends on the speed of the cars. Now 50km/h is equivalent to about 14 metres per second. In other words, if you maintain a steady 50km/h, you will be covering about 14m every second. Therefore, in 1.5s you will have travelled about 21m.
My hypothesis is that, to avoid a spontaneous traffic jam in a stream of cars travelling at about 50km/h, the distance between them should be less than 21m—about four car-lengths.
Unfortunately, when cars slow down when approaching a bump, those behind start changing lanes trying to pick the one that appears to be moving faster. When a car enters the space between two others, it suddenly cuts the separation from 21m to about 10m.
This leaves the driver behind with just 0.7s in which to react. Since this is too little, he has to slow down to about 25km/h to feel comfortable. Of course, the car behind also has to slow down to 25km/h as well. This separation is now about 10m.
If another driver enters this space, he will have to slow further to just over 10km/h. After a few more such manoeuvres, the flow comes to a stop traffic jam.
The most important thing to realise here is that even the car that crossed over to the “faster lane” also slows in order to maintain the 1.5-second duration between himself and the one in front.
Moral of the story? When you are in a traffic jam, pick your lane and stay in it. Changing lanes slows everyone down, including you!
www.figures.co.ke; Twitter: @MungaiKihanya