Trails in parks are the best places for running. First, they are normally made from crushed gravel or limestone, so they are easy on the body. Second, they are likely isolated from traffic, which gives you better air to breathe. Finally, the natural beauty gives you something else to enjoy while you run. I prefer running on dirt or paths with crushed gravel or limestone. These surfaces are simply easier on the body than concrete or asphalt. Asphalt is better than concrete since concrete is much harder and less forgiving on the body.