In real life if someone is blocking you in a hallway or aisle you simply say, "Pardon Me." But it's tricky when you're driving. If someone is parked in the fast lane, and going the same speed as someone in the right lane, there's no easy to way to signal, "GET OUT OF MY WAY!"

Tailgate You get their attention quickly and effectively communicate your urgency It's illegal and incredibly unsafe. If someone tailgates me I ignore them for being rude
Honk It's the preferred method for communicating with other automobiles They may not hear you and some people think honking is ruder than tailgating
Flashing High Beams You can do it from a safe distance and flashing is usually a polite or helpful signal They may not notice, or get confused