I wondered, too. It was deliberately visually ambiguous (sic) for Spielberg to have not made it clear whether what was dripping was oil or blood; and thus wise of David to have kept vigil after the truck went over the cliff, although the prospect of a living and healthy driver returning from the wreckage (or nearby it) strikes me as highly unlikely due to the injuries he must surely have sustained after he exited the truck.
That was a steep embankment, and from the way it looked in the movie, and from what I remember of it, the driver wasn't going to emerge, a la Steve Reeves in a Hercules picture after vanquishing an army of dragons or something, looking strong and healthy, ready for yet another fight. This can't happen in real life under those circumstances. If it was an "Arnold"-like Terminator sort of character, okay, maybe, but that would have pushed
Duel
into the real of science fiction.
The trucker would have to be suffering from injuries due to the terrain; and nasty ones, including possible head injury and trauma. Even if the physical pain wasn't that acute from such a fall he'd be dazed and confused; and whether he knew it or not, suffering from trauma. He wouldn't be the same dude who tormented David earlier in the film. Even if alive he'd have to be slow; and no threat to anyone, due to his being a physical wreck. He would be in shock. There's no way around this.

I was wondering if that was blood or oil dripping. There is no real indication if the truck driver died. I wonder if that was why Mann was keeping vigil over the crash site at the end. Perhaps he needed to make sure that the truck driver was not going to re-emerge from the wreckage.

I'm just watching it, and even though I assumed it was blood the first time I saw it, now I see that you can't really tell. Actually it's more like oil. It's more brown than red. It looks like burnt sugar. But liquid.
If it was supposed to be blood, that was one bad blood faking attempt by the film crew!

I didn't realize that was blood. I thought that was oil leaking from somewhere.
R.I.P. Rick Ducommun and Tony Longo

It looks as if the truck driver jumped out of his truck right before it went over the cliff. This is evidenced by the open driver side door as the truck goes over the edge of the cliff. This theory is shortly dismissed as we see blood on the truck steering wheel a few moments later letting us know that he was indeed in the truck when it went over.
"Don't tell me your little problems son, all I'm interested in is results."

Thanks!!!
I love cinema

He clutches, shifts to a lower gear, engages a Jake brake but by then it's too late. I'm sure he stomped the clutch to keep the engine from dying while shifting/dropping speed.