mph/1000rpm is a constant for a particular type of vehicle depending on gearing and tyre circumference.

Have I advocated excessive speed on crowded roads? No.
Have I advocated excessive speed in poor conditions? No.

Reasonably clear motorway, enough room to dodge the ditherers - 70mph is unreasonably restricting.

Catty Carol, apart from the technicality of the speed limit (which is what I am arguing should be changed) while we both have the right to use the right of way, neither of us has any specific right to do it at any particular speed. You seem to feel you have the right to curtail my preference - but say that I have no right to curtail yours. You are demonstrating the absence of the mutual (vigilance and) consideration that I argued should apply all round.

Kendall, "professional traffic control experts" (whatever they are) don't set speed limits. Politicians do. Sometimes local authority "experts" get involved. They of course are employees of local authorities, so do what politicians tell them. One of the measures they use when considering what speed limits to advise to be set is the "85percentile speed". That means they clock all the cars going through (usually with the two pipes across the road) - then they discount the fastes 15% on the ground that they are untypical. Do they discount the slowest 15% who must be equally untypical? No they don't. Bias? Oh yes!

One factor that has not figures in the discussion here is the effect of speeed on road capacity. I don't understand the logic, but apparently it is empirically determinable that with a constant and concentrated stream of cars doing 40, more will pass a given point in an hour than a stream at 80. I supose it shows that even the much maligned driver does on average tend to leave a more than linearly increasing safety gap as speed increases. So in rush hour on the M25 it would flow more traffic at a speed limit of 40 than one of 70.

That does make a case for variable speed limits - but the congested road is not what I am talking about. It's the uncrowded motorway that does not need a limit of 70. It's the crowding that is the problem. When there is no crowding there is no such need for the restriction. Compare. In the 1960s many cars in the UK at 70 were nearing their limits. About a quarter of drivers still had licences from the 30s before the driving test became universal. My mother (now 96 and no longer driving) had an "all groups" licence that would have allowed her to drive a class 1 HGV or a tracked vehicle with skid steer. Now even my daughter's Daewoo Matiz with 797cc can see 100 with a following wind (I'm told). On paper my 1991 Volvo would clear 120 with some ease. Most 2007 Mondeos can manage getting on for 140. Now all drivers pretty well without exception are only allowed to drive vehicles on which their capabilities have been examined. There are written tests on theory. Surely the standard of driving must have improved (although I do see some examples that suggest otherwise). It is not realistic to continue to impose today a limit set in the light of 70s technology. It is not realistic to restrict all by reference to the standards of the least capable.