Since I have been recovering from surgery following a sidewalk injury sustained while WALKING (did not fall), there's lots of reasons to get off the sidewalk.

But ironically, there's no, er, concrete evidence that concrete versus tarmac is more likely to cause injury. However, running on a poorly maintained sidewalk versus running on a well maintained road is far more likely to cause injury. (Well, ignoring those people behind the wheels at least.)

But the most common reason cited for training on a road is that you should train on the surface you are going to run on. Last time I checked, the Boston, Chicago and New York marathons are all ROAD races.

-mr. bill