This effect is absolutely true, but is not purely down to the speed that the motorist is travelling, it is about maintaining a sufficient level of stimulation to the brain, and speed is only one of those factors. Risk level is another; hence:
A driver of a modern realtively high-performance car on an almost empty motorway (e.g. 4:00am), that is well lit may need a speed of 90 to 110 mph to be stimulated enough to maintain concentration levels, which are low in the first place because of the early start. 70mph is unlikely to provide that stimulation and the chances of falling asleep are high.
Same driver, same road, but at 5:30 pm in very, very heavy traffic, is still not at optimum levels of alertness due to the long day at work, but because of the extra risk caused by the large numbers of vehicles around will probably be stimulated by 70mph where traffic allows. If the traffic is very heavy or comes to a standstill, then the levels of stress will go through the roof, and driver can become dangerous and lose concentration (high-end of curve) despite being at very low speeds.
Same driver in urban area at 4:00am will probably not maintain alertness in a long period of 30mph limit with no risks around. Chances of losing concentration are high, and driving at 40 or even 50 mph will maintain the necessary stimulation for safe driving. Same road at 8:55am going past the school, the risk level is very high, and 20mph is more than enough.
I suppose that I am agreeing with you, in that the driver needs the correct sort of stimulation to maintain a sufficient level of risk to ensure than concentration is maintained. If the level of risk is percieved to be lower, then the right level of concentration will not be maintained and the driver becomes dangerous. Speed is one of the very few variables that the driver has some control over that can alter the stimulation levels to regain the necessary concentration.
Supporters of speed cameras might think that they raise the risk level and concentration must therefore be maintained. This is not true. The cameras do force the lower speed, but the driver is much more likely to flip into a red-mist situation. He can see that enforcement of the limit is not justified in the current conditions, but he can do nothing about it except fume to himself. the cameras effectively force the driver into a road-rage mood, and there is nobody else about. Concentration in this situation is not good and the driver is likely to be very dangerous and will start missing simple things like red lights (which are also contributing especially if nobody comes the other way).
I will admit that I do use speed in order to maintain concentration. It can be used safely where conditions allow (both on empty Motorway at 100mph and on deserted urban streets at 40mph), and I am certain that the vast majority of experienced and responsible drivers will recognise these situations. I am not suggesting speeding is valid all the time, only that when all other risks are removed it is a valid option for maintaining safe driving.
I am sure that critics will say "But you have been told many times: Don't Drive Tired". But get in the real world - if you have got up at 3:30 am in order to catch the only flight of the day from the only airport which is 100 miles away, you do not have the option of saying "oh I feel a little sleepy, perhaps I should pull over and have 20 minutes kip". If you did, then you wouldn't wake up again unless you set an alarm, and then would still be sleepy, and would have missed the flight. In this situation driving at 40mph on the deserted urban streets and 100mph down the deserted (except for the occasional truck) motorway is a perfectly reasonable and safe approach. The draconian enforcement of the speed limits has the opposite effect - it increases stress, and the motorist gets pushed into a red-mist zone on the far right of the curve, they will drive unnecessarily aggressively and take more risks.
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