David Mann: I'd like to report a truck driver who's been endangering my life.
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: Come on you miserable fat-head, get that fat-ass truck outta my way.
David Mann: Well it's about time, Charlie.
David Mann: Come on, car! come on, LET'S GO.
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: Where's the summit? Please... please... come ooooon.
Answer: At that point in the film, the protagonist David Mann is ready to confront the truck driver. When he sees the old Peterbilt truck outside, David mistakenly assumes the truck driver has already entered the diner, so he confronts a likely suspect that he sees at the counter (but he has misidentified the man). The misidentified man takes offense and punches David out. By the time he recovers his senses, David sees the old Peterbilt truck leaving the parking lot. Which means the actual homicidal truck driver never entered the diner in the first place and was waiting outside the whole time. If David had first gone outside to the Peterbilt, there was a good chance the waiting homicidal truck driver would have killed him right there, and the story would have abruptly ended. So, David's misidentification of the truck driver allowed the film to move ahead into its next act.
Charles Austin Miller
Yes, I get why the filmmakers did that, but I still think it is a plot hole. If the Dennis Weaver character was afraid of getting killed by the truck driver, I doubt he would have confronted him in the cafe.