If you just tell someone you can use the bridge then you can probably revoke it. But if you keep letting them use it and they build a house on the other side of the bridge or something then you've probably granted an implied easement[1] and can't, in fact, suddenly stop letting them use the bridge. But in this case there's actually a legal document saying that anybody can use the bridge so that would be analogous to an express easement.

[1]http://en.wikipedia.org/wiki/Easement