On Mon, 5 Sep 2016 20:13:21 +0100, Nightjar wrote:

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.

Can traffic sensors always detect cycles?

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.