Now, it's BMX bikes again. It's much more interesting to see someone on a bike doing a somersault six feet off the ground than watching someone pedal down a street. A four-story fall to the sidewalk would truly ruin your day. Underground steam pipes are hot and can sear flesh. The unwary can drown in storm drains.
Arrest is certainly a very real danger.
But with care, adventurers can reduce their chances of getting hurt. Don't climb higher than you care to fall. Plan your route carefully. Test your holds before you commit. Don't touch hot pipes.