One hears the tire noise, not the engine. The cars are coming downhill towards that end of the tunnel, so 1) not much engine noise and 2) braking increases the tire noise...fortunately. The cars are coming around a lot of curves, so headlights are no clue (plus being under the darkcloth). And if one did happen to miss hearing the car, the visibility is high.
I'd love to this in the Zion tunnel, but my luck the only vehicles coming would be the rangers (no stopping allowed in the tunnel, I don't think pedrestrans are allowed either..but allowed in the Wawona). Zion's is one incredible tunnel!