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Posted: Mon Feb 12, 2018 5:37 pm
I have just returned from an 80 mile round trip to deliver some stuff. The route was urban and country mostly single-carriageway with a comic variety of speed limits. But once again, the vast majority of the traffic was pootling along at 40 mph irrespective. What is it with 40 mph? It seems to be a sort of default comfort speed for a huge number of people. But why?
Posted: Mon Feb 12, 2018 7:00 pm
is there a NVH sweet spot at 40mph, or simply a point at which you can drive comfortably without having to concentrate on driving?
Posted: Mon Feb 12, 2018 7:31 pm
I think that it is the latter. Hence word "pootle". The only trouble is that they are so oblivious to surroundings that if you overtake they die of shock.
Posted: Mon Feb 12, 2018 8:18 pm
akirk wrote:or simply a point at which you can drive comfortably without having to concentrate on driving?
You might have something there. Look at the way the curve roses at 40:
Could be that it's the maximum multi-task (ie do nothing well, just do more) speed, but also the speed at which vision, planning and action have to be exponentially increased.
I remember reading - but don't know whether it's true - that drivers are more likely to keep to 40 limits than 30s. Perhaps this is why.
Posted: Mon Feb 12, 2018 8:30 pm
They keep to 40 alright ...
Posted: Mon Feb 12, 2018 10:26 pm
I have this idea that the non-expert driver's gaze settles somewhere not very far in front of their vehicle, and this distance coincides with the stopping distance from roughly 40mph. In other words, at 40mph they'll be able to cope with anything that happens directly in front of them.
This explains the constant pace that doesn't react to the surroundings, and perhaps why they continue at the same speed in lower limits. The (lack of) thinking that leads to this behaviour doesn't consider the possibility of hazards coming from anywhere other than directly in front.
Posted: Tue Feb 13, 2018 8:54 am
Again, you may be onto something too!
Back in the 1970s, a researcher called Hill did a lot of work on vision and driving, including describing perceptual limitations. IIRC, one of those is that we simple wetware humans (although he probably didn't say that, exactly) can't really guage the speed and distance of approaching vehicle when both are traveling above 50mph.
So if he was right, 50 = 'impossible', therefore 40 = comfortable / reasonable / acceptable.