Just askin' whats the right / legal...

[BP]

Park behind them and dont move

That's a good way to get tazed and to have your car dismantled by a bomb squad.

To the OP: Secret service generally plans several routes, anyone living on a potential route has probably already been briefed so they'd expect something like this. The local police are there as a first line of defense, people see them and tend to not do stupid things. If something gets past them though secret service has several additional ways to protect the president and most of them involve gunfire.