Damn slippy wet london!

I think that the issue is the coefficient of friction, not the tread per se.

Just to throw a spanner in the works, part of the problem in the quoted article by sheldon and all the hifalutin terms used by physicists etc. does little to examine the properties of the rubber compound itself in the wet vs the dry - they just discuss "tyres" or general physics.

If a compound has a very fine "ribbed" surface, at a microscopic or even at molecular level, what happens is that it can draw water down those fine channels very fast, and spreads it very evenly very quickly. Some parasitic plants have a surface like this, so that in the wet the insects slide in more easily - google it if you're bored, they're fascinatingly devious little bastards. Anyway, potentially, the rubber compund itself could cause its own hydroplaning with a very limited amount of water - it's called the dispersal factor of the material, or something like that.

That would seem to hold good as a theory with the description of how the kendas handle, because the tarmac of the road has a pretty grippy spiky surface which is probably pushing through this thin layer and into the rubber, but come to something smooth like metal drains or well-worn stones where this effect is no longer happening, so the very thin water layer is now able to come between the rubber and the stone and BAM! - it's ice-skating time.

So I'd guess that whatever tread you have on the kendas may be largely irrelevant if the rubber itself is holding a thin film of water and is just waiting for something nice and smooth to come along, so it can lift off and slide your arse across the road...