Traffic Jam – a game theoretic model of an everyday problem
This is how a typical traffic jam builds up. Consider peak hour, or a rainy evening. Traffic from an arterial road enters a narrow road. As the throughput slows, a queue forms and grows at a traffic light, or a temporary obstruction. Traffic jams occur because motorists play their dominant strategies – jump the queue. Their choice can, however, be influenced by one of two ways.
John joining at the end of the queue does not know how long the stack is. He has two choices. He can join the queue, or jump it by driving on the right (wrong, really!) side of the road to get nearer the head. If he joins the queue (and others do, too), the traffic will clear in an orderly fashion after a wait of say, 10 minutes. If he jumps the queue, and others don’t, he will get away in say, 5 minutes.
The problem is others think the same way. Motorists crowding the right lane from both ends of the road will cause a terrible obstruction. Everyone will get stuck, but those who jumped the queue will usually get away quicker than those who stayed back. However, it is clear that they will take longer than if everyone had stayed in their lanes. Let us say the drivers who jumped the queue now take 20 minutes to clear the traffic.
The bottleneck will, however, create problems for Bob who was right behind John but decided to join the queue as a good citizen. He will take even longer, 30 minutes, to clear the traffic and be on his way home. Remember he had a choice: he could have jumped the queue like John did. The following table represents this scenario.
Payoff in minutes
Traffic jams occur because motorists play their dominant strategies – jump the queue. Their choice can, however, be influenced by one of two ways.
- The payoff for jumping the queue can become negative if this behaviour is detected and punished. Vigilant and strict policing can alter the apparent benefit of disorderly behaviour. The time that was saved on the road may now be spent standing in the queue to pay the fine. Or, the penalty may be steep enough to encourage staying in the correct lane. This, though, is a short-term solution. Take away policing and the traffic may return to its disorderly conduct.
- If the difference between payoffs for jumping the queue and obeying rules were to be small, it is likely that most people will stay in their lanes. The rule breaker now saves five minutes by driving on the wrong side of the road if others stay in their lanes. If he saved only a minute or two he might persuade himself to be a law-abiding motorist. Wider roads are thus a more permanent solution.
- The combination of the above two methods is likely to be an even more potent solution. Low traffic density reduces the gain from breaking traffic rules. Getting caught and being slapped with a fine increases the cost of violation.
Perhaps this explains why in India we drive with complete disregard for traffic rules, but play law-abiding motorists in Singapore and London.