Wednesday, February 13, 2013

Three way traffic light analysis

The road-works at Dunley Road in Stourport have switched back to three-way traffic lights; however it appears they're being properly manned this time. The joke in this instance is that the fact they're being manned means the redundancy of having three sets of lights. Allow me to explain.

In a simplistic situation where one lane of a two-way carriageway has been blocked off only two sets of lights are required to allow traffic to pass. One set switches to green to allow traffic through; then switches to red and after a period of time (known as inter-red) to allow that traffic to clear the obstruction the other set switches to green. Introduce a junction along the obstruction and a problem arises.

Traffic at this junction cannot see the lights at either end of the obstruction; they don't know from which direction vehicles will be flowing and if they have the option to turn in either direction; might meet a car heading towards them. In low-traffic situations simply tagging onto the end of a queue might be feasible; but still dangerous and messes up the inter-red times. So a third controlling traffic light is added. However, as I've already stated, this introduces even more of a delay into the system particularly in a 'dumb' or sensored set of lights. If this delay causes a serious problem the lights become controlled manually, to reduce delay.

However control isn't always absolute, the controller may only have the option to press a button to initiate a change, inter-red times may be fixed i.e. the first set switches to red at a button press, followed by the inter-red delay, followed by the second set switching to green. This is still a good thing to allow traffic to clear before junction traffic moves on. So what's the 'joke' I mentioned at the beginning?

Well this works correctly when a junction appears somewhere along the obstruction, in the Dunley Road case the junction is at the very beginning of the obstruction. If the lights were sensor-operated a three-way light system makes sense traffic from the junction would be unable to join the end of the whichever queue they need within the inter-red time and would make it difficult to get out... except this is manually controlled.

Given the traffic flow patterns, there's nothing to stop the controller keeping the lights on the main stretch at green, despite the lack of traffic, simply to allow the junction traffic out. Traffic turning away from the obstruction can do so in two ways (with no traffic up to the obstruction and tagging on to the beginning or end of traffic from the obstruction). Traffic turning towards the obstruction can do so with the one light being on green and tag on to any queue if present. With manual control the inter-red would automatically allow for this traffic to clear.

Instead we have a three-way set of lights with have the main flow switching to red; followed by a completely unnecessary inter-red time to allow for the traffic that is either not impeding the junction traffic or is flowing in exactly the same direction as the junction traffic.

In other words a junction at the entrance to the obstruction combined with a manually controlled set of lights does not require its own controlled system. Anyone observing two light cycles should be able to deduce this.