Peer pressure works. It seems like every city goes through this same issue. They develop a few off-leash areas, and then say they don't work because people are still walking their dogs outside the areas. When dog parks are successful at policing themselves and co-existing with a community, however, they're often heartily embraced. Dog owners in New York City were among the first to push for official off-leash areas, and they've been tremendously successful.
People observe and have fun.
The dogs, of course, have a ball.