David's car was given a nice splattering of gasoline on the hood when the gas attendant got sloppy with the hose. This is what caught on fire, not the car itself.
Looking closely at the truck's front, you see license plates, which could be innocent...and notches carved into the headlights. This is your first foreshadowing that the driver is a Serial Killer, something made explicit by Word of God.
Just before the truck goes over the cliff and crashes in the end, the door to driver's seat is open and the driver is missing... which, combined with the darkness of the fluid in the subsequent Gory Discretion Shot, could mean the driver survived and escaped to continue his actions.note As detailed on the Triva subpage, the truck door being open was basically a mistake that was left in the movie because the director thought it added to the film, which undermines this Fridge Horror — the original intent was that the driver died in the movie as explicitly as possible given the release date and rating.
