All good stuff, but I'm increasingly perplexed as to HOW one might give consideration to the 85%ile speed? I mean, in the early days of the M1, when there was no speed limit, it would seem like a perfectly sensible thing to do in order to set a limit (assuming, of course, that it was necessary in the first place)! These days, however, there's no situation where one can observe the "natural" free speed of motorists on any given section of road. Partly because we have a generation of motorists that have grown up with a variety of speed limits, and partly because even those motorists who tend to exceed them (i.e. the vast majority!

) only tend to do so, for the most part, by about the same amount as the enforcement tolerance. This isn't necessarily the natural speed at which they would choose to drive, it's more like the most they think they can get away with!