[SWHouston]

Besides your own piece of mind, having him leashed, there may be some legal issues to consider.

Many places require a Dog to be on a Leash full time, you need to verify that.
When he's off the Leash, YOU are responsible for his actions, regardless of where he is, and liable if he injures anyone, and that can include someone swerving a Vehicle to miss him, and having an accident.
However, when he IS on a leash, you are usually protected by the "common sense" application of the Law, in that you were attempting to control him in an acceptable manner, and the person/other animal came into that restricted area. This applies if he's injured as well.

Leashing is just a good idea in any case.