Found this on the web:

"Now that you've got the right shoes, use them on the right surface. Avoid rock-hard surfaces like concrete sidewalks and aim instead for grass or dirt trails. The idea, of course, is to run where the ground will absorb more shock, instead of passing it along to your legs. Try to be consistent. A sudden change to a new running surface can itself be a cause of injury. Whatever you do, though, get off the concrete. It's approximately 10 times as hard as asphalt, and is easily the worst possible running surface. Runners World agrees, as evidenced by its ranking of surfaces on a scale of 1 (awful) to 10 (best):"

Grass 9.5
Wood chips 9
Dirt 8
Cinder track 7.5
Track 7
Treadmill 6.5
Asphalt 6
Sand 4
Snow 2.5
Concrete 1