Constant Speed and the Disappearing Vehicles
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A recent enquiry presented an interesting issue of cars disappearing down a dead end street. A significantly larger number of vehicles would travel into the dead end than would come out.
We spoke last week about problems I was having with vehicle classifications and you advised it could be due to differing lengths of the tubes. I put the counter out again on the same stretch of road and was very careful to make sure that the tubes were the same length, stretched exactly the same distance (around 15%), and the exact same length of tube was going to the counter however the classifications still appear to be incorrect. This road is a dead end rural road so pretty much all that goes in comes back out and vice versa . I have attached the new count to this email and would be grateful for any advice you can give. I really do not know how I can put the tubes out any more accurately than how I put them out – I triple checked that the tubes were as equal in length as possible. Would the road being pretty much a single track be effecting the results maybe? The pic below is where the counter was placed – just before the bridge.
The key influence on this survey wasn’t the nature of the road or the way the survey was conducted, it was the location of the survey. With an old rickety bridge just ahead of the road tubes, vehicles were rapidly braking as they crossed the tubes. Decelerating vehicles will be classified incorrectly which could lead to a 4 axle truck being identified as 2 cars.
The moral of the story is to always ensure that tubes are installed on a stretch of road that will have a consistent speed.
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