A Conversation for Red Light Bunching

All well and good, but...

Post 1

Is mise Duncan

...the gap method only works if all motorists can see the light and therefore have a simultaneous cue to move off - but if there's an enourmous 18 wheeler at the front only it can get a direct cue from the light and everyone behind it has to rely on it pulling away to alert them that the light has changed.

All well and good, but...

Post 2


Also, there is a reason that cars bunch. If you leave a five foot space between your car and the one in front, some wanker is going to try to jam his car into the gap. It's one thing to be polite and let someone in, another entirely to have some rude bastard shove his way in.

All well and good, but...

Post 3


Some pseudoscience:

Cars are not like carriages in a train; it is more as if they are connected by pieces of elastic. As the front car accelerates the elastic stretches until eventually the car behind it slowly starts to catch up.

This is inevitable because the car behind cannot accelerate faster than the one in front (assuming there is no ability to overtake). The idea of the five- to six- feet gap is simply to remove this constraint (the space acts as a buffer) but obviously it only works if the car in front still accelerates at a pace not much different to the one behind.

In any case, the decision to bunch up is not taken merely on the need to traverse one red light, but because of the general configuration of the road, e.g. the need to prevent blocking side junctions. If traffic is more spaced out then more side junctions are blocked up and traffic flow on surrounding streets is impeded. For this reason, the globally optimal solution (i.e. for the area in general rather than one particular set of lights) is generally to bunch up.

Where traffic bunching or stretching (because of the "elasticity" caused by delays in the acceleration pattern rippling back through the traffic) does cause significant problems, a more optimal global solution can be found by arranging sequences of lights so that cars travel through the problem area in convoy at approximately constant speed. This, of course, assumes that no-one attempts to go much faster or slower than the desired speed -- i.e. usually the maximum speed for the road.

For this reason, if you find yourself screaming away from each traffic light only to find the next set on red, you might do better to stick to the posted limit and sail through all the sets, breezily listening to Patsy Kline. The A34 Kingsway in Manchester is a perfect place to see this in practice.

All well and good, but...

Post 4


Hmmmm.... interesting... altho the blockage of side streets only applies in urban areas...in the rural zones there are less side streets...

All well and good, but...

Post 5


That was just one point... and anyway what are traffic lights doing in rural areas?

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