David Mann: Fill it with Ethel.
Gas Station Attendant: As long as Ethel doesn't mind.
David Mann: You can't beat me on the grade. You can't beat me on the grade.
David Mann: That truck driver's crazy, he's been trying to kill me, I mean it.
Bus Driver: Well, mister, if I was to vote on who's crazy around here, it'd be you.
David Mann: I'd like to report a truck driver who's been endangering my life.
David Mann: Come on you miserable fat-head, get that fat-ass truck outta my way.
David Mann: Where's the summit? Please... please... come ooooon.
David Mann: Well it's about time, Charlie.
David Mann: Come on, car! come on, LET'S GO.
Chosen answer: That is never answered for the film, and not knowing adds to the mystery and intrigue. No mentally stable person would target someone just because they overtook their vehicle on the road. It appears that Mann happened to cross paths with a psychopath. Steven Spielberg has commented that the multiple out-of-state license plates attached to the truck's front bumper may be "trophies" that indicate that the trucker is a serial killer who has run down other drivers. This could be a deadly game to the truck driver.
raywestI just wonder why the driver door on the big rig was open while it took a dive over the mountain.
Perhaps a hint that the truck driver escaped. You notice that the truck doesn't explode on impact, although the studio insisted it must; Spielberg fought the studio over the inclusion of a cliched fiery finale, as he wanted the crash to convey an ambiguous ending, suggesting that the driver might not have died. Spielberg even explained that the red liquid seen in the truck cab was not blood, but was some sort of automotive fluid. This all lent to the mystery of what actually happened to the driver, whose body we never see.
Charles Austin MillerI heard the stuntman driving the truck had to jump out of the truck right before the truck went over the edge of the mountain. The door was open on the truck.