The River Road Tunnel was a tunnel along River Road.
Biff Tannen took the River Road Tunnel from his house to the Enchantment Under the Sea dance at Hill Valley High School. While going through the tunnel, he almost heard Marty in back of his car talking to Doc as the tunnel shielded radio signals from reaching the car radio and Marty's walkie talkie.
Driving back home from the dance through the tunnel, Biff realized Marty was following him, and fought with him for the Grays Sports Almanac he was given by his older self, attempting to smear Marty McFly off the side of the car onto the wall. After several attempts, Marty rebounded off the wall of the tunnel and grabbed the almanac. Biff performed a quick u-turn at the far end of the tunnel and charged at him. Marty was nearly run over at the school end when Doc Brown came to the rescue with a string of pennants hanging out the door of the DeLorean time machine.
As Biff looked up in shock, he did not see the manure truck that he had passed in the tunnel before turning around, and crashed into it.
Behind the Scenes
The tunnel scenes were filmed at Observatory Tunnel, in Griffith Park, Hollywood. Forced perspective tricks, including a wide-angle lens and unevenly-placed interior lights, made the tunnel look much longer on-screen than it is in reality.