How drivers create traffic jams at potholes
The Sunday Nation
Nairobi,
27 May 2012
Last week, motoring columnist Gavin Bennett wrote that it would be better if we drove slowly into and out of a pothole without stopping instead of swerving around it. His argument was that when there are oncoming cars, one must stop in front of the hole for the road to clear and this triggers a traffic jam behind.
After reading that column, Gabriel Manyasi wrote to me to ask whether those arguments are supported by numbers. Well, there is only one way to find out; working it out.
Suppose there is a stream of vehicles flowing smoothly at, say 40km/h on a two-way road. As long as there is no obstruction to their movement, there will never be a jam in the stream.
But now let’s introduce an obstruction in the form of a one-metre round pothole right in the middle of their lane. If there were no oncoming vehicles, cars would slow down a little, swerve around the hole and move on.
Suppose that for this swerving manoeuvre, the cars slow down to, say, 20km/h. Assuming that the cars need two metres before and after the pothole, it turns out that they take about one second to clear the 5m distance around the hole.
Slowing down from 40km/h to 20km/h for one second will NOT trigger a traffic jam, but the knock-on effect will generate a slow region behind the pothole where cars are doing 20km/h instead of the 40km/h of the main stream.
When a vehicle appears on the other lane, our cars have two choices: to drive even slower (say, 10km/h) through the pothole or to stop and wait for the oncoming traffic to clear and then swerve.
The effect of the first choice is similar to that of swerving around the pothole: it creates a “slow region” behind the hole, but this time, the “slow region” will be considerably longer – statistical analysis predicts that it should be about four times longer!
The second choice is more catastrophic. Stopping creates a stationary queue of traffic. Regardless of the speed of the oncoming cars, the drive waiting in front of the pothole gauges whether there is enough time swerve and return to the correct lane.
It turns out that the swerving car must get back to its side at least three seconds before the oncoming one reaches the pothole region. Therefore, the driver who chooses to stop must remain stationary for at least five seconds (two plus three).
In that time the queue of stationary cars behind will grow to about five cars. When the road clears, they begin to swerve round the pothole one by one, each taking about two seconds. Thus it takes at least ten seconds to get the five cars moving again.
Now, if another oncoming vehicle appears before the expiry of the ten seconds, the cars in the potholed lane will stop again for another five seconds and the stationary queue will grow longer…and longer…
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