Gates don't really work; they rely on someone opening and closing them, and the assumption is that if they are open, the trail is open. Since, as has been stated, conditions change by the hour (in summer as well as winter), it is pretty much impossible to stay on top of them.
Hard and fast rules don't really work, either, because conditions are so variable. Sometimes a 4" tire is will rut, sometimes a 1" tire is okay.
The only thing that will work is education and peer-pressure. Whether that is from signs, fellow riders, an ad campaign, or some combination, "Don't ride if you are leaving a rut," needs to be spread far and wide.