In article , Nightjar
writes
On 05-Sep-16 7:18 PM, misterroy wrote:
On my way to work a bridge is being repaired. Today I was in the
stream of traffic heading towards town, no cars going the other way.
Mid stream the lights went red, just as I reached them. I went
through. I reckon I travelled 100m between the lights and then about
600m up the hill before the lights changed.
I realise there has to be a margin to be sure a car gets through, but
how big should the margin be?
Why when the lights have sensors do they never seem to be switched
on? It just looks like a crude timer is used.
Under what circumstances are the sensors on the lights used?


Modern temporary traffic lights always use traffic sensors. However,
they also have a maximum length of red signal, which varies from 2-5
minutes, depending upon normal traffic flow on the road. When that
maximum is reached, the lights will change, even if no traffic has been
detected from that direction and there is still traffic coming from the
opposite direction. It is a precaution against sensor failure leaving
one direction permanently stopped. The time before the lights change
red in one direction and green in the opposite direction depends upon
traffic flow, speed limit through the works and the length of the works.

Contrary to urban legend, temporary traffic lights are compulsory
lights and passing the 'If Red Light Shows Wait Here' sign, or the
lights if there is no sign, when the red light is showing is an offence.

Unless it is snowing when it seems all such rules are suspended.
--
bert