For years I've listened to nitpickers attack the ending. "Why didn't they just drive away?" If you recall, the attendant at the last gas station they stopped at warned, "You got a busted headlight on that thing". Not to mention half of the front end torn off, exposing the engine compartment.
Fonda's character pulls off the road to repair the headlight "while there's still enough light to see what I'm doing". The cultists last contacted them at the roadside construction project, and followed them to where they stopped -- at twilight.
When night fell, they either smashed the other headlight or disabled the engine by quietly pulling a cable or belt. In the dark, with no headlights, surrounded by hills and dales and rocks and streams and trees, that big behemoth isn't going anywhere, if it's even still running.
The couples originally see their $26,000 Vogue as a castle on wheels -- king of the road. "We don't need anything. We are self-contained, baby!" They quickly realize that it is in fact a lumbering, vulnerable deathtrap that makes them easy prey in isolated, unfamiliar territory. This refusal to accept that they are so out of their element -- in a race that they can't possibly win -- is the movie's entire point.