"TT - any scientific basis for the assumed difference in coefficient of friction between asphalt and grass? Any factor applied for the lentgh of the grass? Or the temperatures of the surfaces? Runways and roads seem to be designed to have a high coefficient of friction rather than a low one."
Well Crab, the short answer to that is no.
Mind you, I have been attempting to get a bit of project off the ground that may shed a bit of light on the matter and have approached Wimbledon and Lords about using their well-mown surfaces to measrue some solid run-ons. Still waiting for a reply there. But I have had an intruiging offer from Aintree saying I could take part in an event involving a variety of terrain, namely grass; asphalt; hedge row(!) and even water. To be honest, I got the impression that someone was sending me on a bit of a wild steeple chase.
In the meantine, I will have to concede that I haven't got a jot of evidence. Somewhere in there is the thinking (righlty or wrongly) that above grass the laminar flow will be more disturbed, and that even if tarmac is the higher friction surface, this "stickiness" will occur very close to the surface allowing a more laminar flow to develop almost immedeatly above.
Point taken about temperature differences. Late morining with a baking hot sun on the asphalt there might be some pretty thin air above the runway. Maybe that adjacent grass strip in the shade deserves a second look after all.
TT