On Coasting and Traffic Signals

Today, when a driver sees a traffic light in the distance, he is likely to immediately make some decisions.  These decisions will be based on general experience with all intersections ever encountered, as well as the specific intersection that he is approaching.

If the light is red, he may begin to break.  If he is familiar with the intersection, he may know that, at this distance, it is better to maintain speed – the light is likely to change before he gets to the intersection. Alternatively, a middle course of action, coasting, may be needed. If the light is yellow, it gives the most precise information about where in a cycle the light is.  This is because the yellow light is the shortest.  In the US, it is always followed by red, as well.  If they are able to stop before the intersection, most drivers will slow upon seeing a yellow light.  If the light is green, the driver will only slow if they are very familiar with the intersection and know the distance to the intersection is such that they are not likely to be able to make the current cycle.

Similar evaluations are likely if other cars are visible at a 4-way stop.  If there are cars approaching the intersection, an attempt may be made to time his approach so that there will be minimal waiting at the intersection.  Of course, this is only down if the driver is aware and considerate.

In a completely autonomous world, it is likely that traffic lights will have some communications capabilities.  I anticipate at least some, if not all, of the following:

  • Indication of light status.  A light should be able to communicate the current state of the light, and where in the cycle it is.  This is likely to be the total cycle (45 seconds green, 15 seconds yellow, 62 seconds red) and the seconds in 72s, for example.
  • a combination of request and acknowledgement for right of way.  This currently exists for emergency vehicles.  It may be warranted to give large platoons a right of way as well.
  •  Command and control from a central location.  This is probably a post in its own right, but suffice to say that the “normal” light timings and responses may need to be modified.
  • General light status made available to navigation/GPS services.

With this communication available to it, I anticipate that an autonomous vehicle will know what the status of a light is as it approaches.  Furthermore, it may be able to ask (presumably a few minutes in advance) to have the light be green when it reaches the intersection.

When the “normal” timing is such that  light will not be green when the autonomous car gets to the intersection, the autonomous vehicle will adjust its speed to arrive when the light is green.  What this will mean is that, in effect, passengers will no longer wait at red lights.  If there is a long red light, the car will slow well in advance, so as to arrive at the light and proceed through it based on optimizing fuel consumption.



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