I've never driven in Britain. And on all the TV footage I've seen I can't remember noticing what we call an "advisory speed sign" of the sort we have here in Australia and NZ. But they're so normal here that I'm not sure I would notice one even if I saw it. And in a quick internet look I can't see whether you have them.
Its a black and yellow sign we have on all our main roads that tells which which direction the next corner in road goes, and how fast you can get around it.
But the speed on it is the speed that you can get around it at 0.25G cornering power. And since a good car on good tyres in good conditions can pull close to 1G cornering, you can actually negotiate the corner at pretty close to twice the speed the sign says. Yeah, its a square law, you've got to pull 4x the cornering force to do 2x the cornering speed. Remember that when they say racing karts and F1s on fat sticky racing tyres at low speeds (too low for the wings to be working, ie "mechanical grip" only) can pull 2.4Gs, thats about 50% faster than you could do in your road car, and high speeds when the wings are working they can pull 4Gs, that twice as fast.
A car magazine here looked at how well a range of new vehicles actually corner in the wet and the dry. Sports car. Ordinary family sedan. Four wheel drive. Van. When I translated back to how fast they could all get around the sort of typical country highway corner, one with a 65 km advisor speed sign, it turned out that the best would get around it 126 km/h and the worst at about 90 km/h. Of course older cars and cars with mechanical problems would be even worse. Buty it gives an idea of the range of typical cornering ability.
