The first and only film Larry Cohen directed that he did not write. He took the job in part to prove to people that he could direct someone else's material.
Larry Cohen had scriptwriter David J. Schow on the set every day to approve changes Cohen made to the script while filming, a courtesy Cohen wished had been extended to him on films he wrote but did not direct.
The fog in this movie was real fog, not an effect. Director Larry Cohen used it to his advantage to give this a more scary feeling.
In David J. Schow's original script, the semi driven by Michael Moriarty's character was just a regular pickup truck.