Re tread on road tyres:
http://sheldonbrown.com/tires.html
Tread for on-road use
Bicycle tires for on-road use have no need of any sort of tread features; in fact, the best road tires are perfectly smooth, with no tread at all!
Unfortunately, most people assume that a smooth tire will be slippery, so this type of tire is difficult to sell to unsophisticated cyclists. Most tire makers cater to this by putting a very fine pattern on their tires, mainly for cosmetic and marketing reasons. If you examine a section of asphalt or concrete, you’ll see that the texture of the road itself is much “knobbier” than the tread features of a good-quality road tire. Since the tire is flexible, even a slick tire deforms as it comes into contact with the pavement, acquiring the shape of the pavement texture, only while in contact with the road.
People ask, “But don’t slick tires get slippery on wet roads, or worse yet, wet metal features such as expansion joints, paint stripes, or railroad tracks?” The answer is, yes, they do. So do tires with tread. All tires are slippery in these conditions. Tread features make no improvement in this.
http://www.michelinbicycletire.com/michelinbicycle/index.cfm?event=smoothtread.view
Why a Smooth Tread on Road Tires?
The oval shape of a bicycle road tire contact patch permits effective water evacuation to help keep the tire from hydroplaning.
The footprint of a 23mm tire (approx. 7 sq. cm) is so small that the bike would need to be traveling at about 120MPH in order to hydroplane.
Nevertheless, some road tire models are designed with specific tread structures – primarily for cosmetics or to comfort the consumer. Sometimes tread features can help provide a harder rubber compound a better mechanical link with the road surface, for better grip.
