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# Traffic Lights – A Modest Proposal

### Nicholas Denyer shows how to improve traffic management by applying some ideas about induction and following rules.

The Highway Code allows you to proceed when the lights are green, and requires you to stop when they are red. The rule sounds simple enough. But it leads to lots of complexities. Having this rule means that our traffic lights must be very elaborate. They need elaborate mechanisms to change from red to green and back again; and they need even more elaborate mechanisms to ensure that those changes of colour are properly coordinated. Such elaborate mechanisms are bound to be expensive, and cannot always be reliable.

If we were to change the Highway Code, our traffic lights could be much simpler, cheaper and safer. We could simply have some lights permanently on green, and others permanently on red. The change needed in the Highway Code would be this: instead of saying ‘Go on green’, it should say ‘Go on gred’; and instead of ‘Stop on red’, it should say ‘Stop on reen’.

The words ‘gred’ and ‘reen’ are not yet widely understood. Let me therefore explain them. Call by the name ‘odd minute’ any period of sixty seconds which begins an odd number of minutes after the hour. Thus 5.03-5.04, for example, is an odd minute. Likewise, call by the name ‘even’ each of the intervening minutes. Thus 7.00-7.01 and 11.28-11.29, for example, are both even minutes.

We can now explain what it is for a thing to be gred at a moment of time. A thing is gred at a moment if and only if either the thing is green at that moment and that moment occurs during an even minute, or the thing is red at that moment and that moment occurs during an odd minute.

A similar explanation can be given for what it is to be reen. A thing is reen at a moment if and only if either the thing is red at that moment and that moment occurs during an even minute, or the thing is green at that moment and that moment occurs during an odd minute.

Odd and even minutes alternate. Hence a traffic light that is permanently green will switch, every sixty seconds, from gred to reen or from reen to gred. So will a traffic light that is permanently red. The difference between the green light and the red one is this: during even minutes, the green light is gred and the red light is reen; but during odd minutes the opposite happens, for the green one is reen and the red one is gred.

These synchronised changes from gred to reen and back again are all entirely automatic. No costly, elaborate or potentially unreliable mechanism is needed. Just place permanently green lights on one of the roads at a junction and permanently red lights on the other. The lights will automatically change between gred and reen, so that traffic following the ‘Go on gred and stop on reen’ rule can proceed with perfect safety.

It now becomes clear why our present traffic lights need to be such fancy contraptions. Their complexities are needed to make up for the complexities of our present rule telling, us to go on green and stop on red.

Our present rule sounds simple enough. But now that we know about gred and reen, we can see that our present rule is really very complicated. When analysed, it turns out to be saying: during even minutes go on gred but stop on reen; and during odd ones go on reen but stop on gred. Our present rule therefore makes the meanings of gred and reen lights vary according to the time.

It is as if the Highway Code said that O was to mean a roundabout and Y a fork except on Thursdays, when it would be the other way round. If the Highway Code made the significance of O and Y swap about in this way, then we would need devices on our road signs to make the Os and the Ys themselves swap about in compensation. Likewise, since the Highway Code makes the significance of a gred light change every sixty seconds, we need devices on our traffic lights to compensate.

Of course, it would be disastrous for both you and others if you were to change unilaterally to the gred/reen rule. Even though our current green/red rule is only a convention, and a needlessly complex convention at that, you are better off obeying it so long as other people do. But we would all be better off if we collectively changed to the simpler convention that tells us to go on gred and stop on reen.

And what stops us from making that change? Can’t we make any sign mean anything we like? And can’t we follow any rule that we collectively fancy?