10:04am Friday 10th July 2009
OUR society is presently cursed by the misuse of models.
Sainsbury’s who are proposing to build a new store on the old Waitrose site in Marlow have been required to provide a traffic impact junction analysis presumably to show that the modest proposed increase in size of the store will not cause unacceptable traffic congestion.
Sainsbury’s use a transport consultant, Mayer Brown, who arranged for simple traffic counts to be made in Marlow during peak periods on 20th and 21st March this year.
Mayer Brown fed the information from the traffic count into two well-reputed computer models Arcady and Picady which are widely used in traffic junction analysis.
Mayer Brown came to the interesting, nay, absolutely startling conclusion that current maximum peak-hour queue lengths at the junctions of Oxford Road and West Street and at West Street/High Street/Spittal Street are only 2.65 vehicles long.
After development, the models predict that the maximum peak-hour queue lengths would increase to 2.83 vehicles.
I pointed out to Bucks County Council that, as every Marlow resident knows, the model output is pure nonsense.
Two years ago, I submitted a DVD to the withdrawn Waitrose appeal, filmed during rush hour, which showed queue lengths frequently extending to 60, 70 or 80 vehicles along Spittal Street, Chapel Street and Dean Street.
Actual queue lengths are several thousand per cent higher than the Mayer Brown models calculate. I received a response from the county council which said: “It must be remembered that Arcady is only a tool and cannot model how people actually drive. Therefore the figures [that] are produced are theoretical and show delays and queue lengths that would occur if all available road space was used.
“The important point is that the increase in queue lengths, no matter how they are derived, is minimal. I accept Mr Post’s comments on the existing queue lengths but I would suggest that this is the way people drive rather than a lack of capacity.”
A more plausible reason for the catastrophic failure of the modelling in this case is not the strange belief that Marlow drivers drive differently but that the consultants who used the model completely failed to take into account all the real obstructions to the free flow of traffic adjacent to the junctions.
These include the numerous light-controlled and simple pedestrian crossings and also the delays caused by vehicles manoeuvring to park legally. Marlow drivers do not in fact drive differently, they merely obey the law!
Perhaps adherents to the Anthropogenic Global Warming religion may reflect that the absurd scenarios propagandised by the likes of Al Gore are not based on science but on bad modelling of a climate system that is infinitely harder to model than two modest traffic junctions in the middle of a small Thames Valley town.
Mike Post, Bisham Road, Marlow.
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