I've had issues with my knees making me unable to run on hard ground. This has forced me to run on gravel outside the city or on treadmill (but treadmills are really boring). However, a few months ago I tried running on concrete for a few km and it wasn't an issue. Great. I can now run anywhere I want! So I've had a few runs in the city. Bleh. Much better to run outside a city and I gladly take the bike 5km just to have a good place in the forest to run.