Trivia: Steven Spielberg "interviewed" over 100 trucks before finding the forboding, evil looking one for the movie.
Trivia: At the end of the film when the truck goes off the cliff, we hear an odd groaning sound. This is a dinosaur growl from an old movie Spielberg likes. Interestingly, this same sound is also used at the end of Jaws as the shark sinks to the bottom of the ocean. Spielberg did this to show that Jaws and Duel would always be closely connected.
Trivia: This film garnered so much critical praise that it was theatrically released in quite a few countries, though it was originally a made-for-TV-movie for NBC in 1972.
Trivia: The scene where the truck goes over the cliff had to be perfect the first time - the production team could not afford to purchase other trucks due to the extremely low budget.
Trivia: This was the full-length feature directorial debut of Steven Spielberg.
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.