Traffic control systems - multiple objectives

I posted a comment to Laura McLay's blog on traffic congestion the other day, which reminded me about the way that the traffic flow in Exeter is managed by an adaptive computer program called SCOOT.  SCOOT measures traffic flow and queues via a number of sensors, set under the tarmac around the city.  It is programmed to recognise surrogates for congestion (traffic speed, queues reaching certain thresholds, etc) and then uses a set of criteria to decide when the traffic signals should change.  Because these measures are surrogates, there are times when the information is incomplete and even misleading.

Traffic management presents an everyday example of multiple, conflicting, objectives, as well as illustrating how surrogate measures are sometimes the only ones available to a decision-maker.  And as OR people, we need to remember that the world is messy, and we cannot measure everything!

It is a decade or more since I used SCOOT as a class example, so I may be excused for forgetting it when I was faced with a "I wonder why ...?" thought.  I regularly pass through the junction below on my way home from church.  I travel from north to south from Polsloe Road into Barrack Road crossing the two sets of lights at Heavitree Road and Magdalen Road - which are about 50 metres apart at this point. (Ignore the label saying Newtown Ward - Magdalen Road continues in a straight line, as it has done for at least 2000 years (the Romans used it)).

A junction in the eastern suburbs of Exeter
I had wondered why, when there were half-a-dozen vehicles going north-south, and two or three going south-north, the lights changed before all the north-south traffic had left Polsloe Road.  The lights seemed to be controlled by the south-north queue.  This happened regularly.  Yet, when there were half-a-dozen vehicles going north-south, and a similar number going south-north, then the lights allowed both the north-south and the south-north traffic to clear the intersection.

Then I remembered SCOOT.  Heavitree Road and Fore Street, shown in orange, is a major route into Exeter.  Barrack Road feeds a great deal of traffic which turns left at Chadni (an Indian Restaurant) and continues flowing into the city.  (And there is obviously substantial flow in the other direction.) Therefore these two merging routes have a higher weighting inside the multi-objective algorithm than the flow from the lesser road that I was using to approach the junctions.  Polsloe Road suffers so that flow along the arteries of the city can flow more smoothly.

Multi-objective decision making is a wonderful process - but not for everyone!


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