IMO, the more rubber in contact with the road, the better. Why would drag racers run slicks if they don't provide better traction?
A slippery surface is a slippery surface. Tread or no tread. But, having LESS tire surface in contact with the surface reduces the traction on a slippery surface.
The ONLY reason that auto & motorcycle tires need tread is to channel standing water away from the tire face/road. Auto tires, in particular, need this, because a) they're flat, with a large tire surface in contact with the road, and b) they travel at a high rate of speed. This means that there's a point, at some speed, where the tire can't push all the water out of the way.
A bike tire has so little surface in contact with the road, that the contact pressure is substantially higher than an auto or motorcycle tire. And, it presents a rounded surface. this, in conjunction with the fact that there's more time (since a bike is slower than a car) to push the water our of the way.
Wikipedia mentions the formula that Brown referred to. (the actual formula is speed (in knots) equals 8.6 times the square root of the tire pressure in psi) And, they mention
Bicycles, motorcycles, and similar vehicles with a round-shaped surface toward the pavement are far less likely to hydroplane in normal road use. The contact area with the road is a canoe-shaped patch that effectively squeezes water out of the way.
And, at How Things Work, mention is also made of this.
How Things Work said:
Is hydroplaning a form of sliding friction?
Not exactly. Sliding friction refers to the situation in which two surfaces slide across one another while touching. In hydroplaning, the two surfaces are sliding across one another, but they aren't touching. Instead, they're separated by a thin layer of trapped water. While hydroplaning still converts mechanical energy into thermal energy, just as sliding friction does, the lubricating effect of the water dramatically reduces the energy conversion. That's why you can hydroplane for such a long distance on the highway; there is almost no slowing force at all.
Dan Barker, one of my readers, informed me of a NASA study showing that there is a minimum speed at which a tire will begin to hydroplane and that that speed depends on the square root of the tire pressure. Higher tire pressure tends to expel the water layer and prevent hydroplaning, while lower tire pressure allows the water layer to remain in place when the vehicle is traveling fast enough. As Dan notes, a large truck tire is typically inflated to 100 PSI and resists hydroplaning at speed of up to about 100 mph. But a passanger car tire has a much lower pressure of about 32 PSI and can hydroplane at speeds somewhat under 60 mph. That's why you have to be careful driving on waterlogged pavement at highway speeds and why highway builders carefully slope their surfaces to shed rain water quickly.
Wikipedia also mention that narrow tires, and tires at higher pressures are less succeptable to hydroplaning, and that underinflated auto tires contribute to hydroplaning.
There are lots of references to hydroplaning and speed. The only factors mentioned to determine hydroplaning speed is tire pressure. If you are approaching the speed at which hydroplaning will occur at your tire pressure, at that point, a tire's tread WILL help you out a bit, by channeling some of the water away. But, it does so by removing rubber that is in contact with the road, and thus reducing
traction at all other times.