## Rule breakers make traffic jams less likely

Rules are a good thing when it comes to road traffic: drive on the wrong side of the highway and you’ll cause chaos, if you live.  If that seems forehead-smackingly obvious, then an analysis by Seung Ki Baek at Umea University in Sweden and pals my come as a surprise.

They say that a small proportion of lunatics driving on the wrong side of the road actually reduces the chances of a jam rather than increasing it and they have an interesting model to prove it.

Their model is a 100 lane highway in which cars can drive in either direction in any lane.  When two cars collide, that lane becomes blocked and other vehicles have to move to one side or the other to get round them.

In theory, it’s easy to imagine that the best strategy is for everyone to agree to move to their left (or right, the model is symmetrical)  when they meet.

The question is what happens when there are two kinds of drivers: rule-followers and rule-breakers who move either way.

Ki Baek and co considered the two obvious extremes.  When everybody is a rule-breaker, the result is chaos and the road jams up quickly as collisions ensue. Equally, when everybody is a rule-follower, the likelihood of jam is much lower and road users travelling in the same direction tend to end up driving on the left (or right), just as they do on real roads.

But here’s the strange thing: the probability of a jam reaches a minimum somewhere in between, when the number of rule-breakers is between 10 and 40 per cent.

That’s kinda counterintuitive but Ki Baek and co say several factors explain what is going on.

First, a small number of collisions disperses the rule-followers to their respective side of the roads more quickly,  making jamming less  likely.

And second, rule-followers tend to form convoys which can lead to pile ups that jam the road. A few collisions here and there helps to break up these convoys into smaller groups, making large pile ups and the jams they cause, less likely .

“Our result suggests that there are situations when abiding too strictly by a traffic rule could lead to a jamming disaster which would be avoided if some people just ignored the traffic rule altogether,” say the team.

Might be fun to try it on the San Diego Freeway one of these days. Dare ya!

Ref:  arxiv.org/abs/0901.3513: Flow Improvement Caused by Traffic-Rule Ignorers

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