Their FAQ answers a lot of these questions:
http://solarroadways.com/faq.shtml
What are you going to do about traction? What's going to happen to the surface of the Solar Roadways when it rains?
Everyone naturally pictures sliding out of control on a smooth piece of wet glass! Actually, one of our many technical specs is that it be textured to the point that it provides at least the traction that current asphalt roads offer - even in the rain. We hesitate to even call it glass, as it is far from a traditional window pane, but glass is what it is, so glass is what we must call it.
We sent samples of textured glass to a university civil engineering lab for traction testing. We started off being able to stop a car going 40 mph on a wet surface in the required distance. We designed a more and more aggressive surface pattern until we got a call form the lab one day: we'd torn the boot off of the British Pendulum Testing apparatus! We backed off a little and ended up with a texture that can stop a vehicle going 80 mph in the required distance.
