The Invisible Referee

21st Century Traffic Control: The Invisible Referee University of Southampton

Highway Control Game

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Be the Referee!

In this game you will learn about the world of phantom traffic jams, and you will apply that knowledge to help control a busy motorway junction. What's the best way to prevent flow breakdown, and keep the traffic flowing freely?

The Science

In a phantom traffic jam, you are driving along the motorway when all of a sudden you come to a halt. When you reach the front of the queue, there is no accident or obstruction to explain the hold-up. The explanation is that the traffic jam is actually a "wave" that rolls up the highway in the opposite direction to the flow. The chances are that the "wave" was formed far in front of you — which is why you don't see what caused it.

So what did cause the wave? Well, when traffic is very busy, for example at a congested motorway junction, it becomes "unstable". This means even tiny effects such as a couple of bad lane changes get magnified and cause phantom jams.

The consequence is that if too much traffic is forced down the road, flow breaks down and the capacity of the road is decreased!

The Engineering Solutions

We know that reducing the speed limit can help stabilise traffic flow and prevent breakdown. This is what happens in "Managed Motorways" like the M25 orbital around London, or the M42 motorway near Birmingham. Ramp metering (traffic lights on the merge road) also helps.