During their drive east in Walter's car through the Sierra Mountains, Irene's lover Torrence attacks Walter and leaves him for dead.
Then ... Here we see them driving past a substantial retaining wall with minimal protection from a precipitous drop.
... and Now, it was a lot more convenient for the film crew to choose the nearby Angeles National Forest area north of Pasadena for this shot. It's on Mount Wilson Red Box Road, 3.0 miles from the junction with the Angeles Crest Highway and 2.0 miles from the observatory (map). In the recent photo below we can see the retaining structure has held up amazingly well these last 64 years but it still has a very low wall along the side of the road.
You might wonder how CitySleuth tracked down this location? While watching TV he was astonished to see a Mercedes Benz commercial with ... hallelujah ... a shot of the same place! Whadayaknow! A few phone calls and emails later, the location was revealed. Here is that image from the commercial, beautifully captured by professionals in the magic light of dawn in July, 2011.
Then ... They continue along another stretch of mountain road.
... and Now, CitySleuth is pretty sure this was filmed on the Angeles Crest Highway which snakes up from the lowlands to the Mount Wilson location described above through terrain just like this. He can't be 100% certain but this curve is very similar.
Torrence had earlier let air out of one of the tires and when it goes flat he pulls over. While Walter replaces the wheel Torrence, snarling "From Irene and me, sucker", thwacks him with a lug wrench and dumps him over the roadside edge, leaving him for dead.
He then frantically drives off when a passing Bekins van stops and in his haste meets an approaching gas tanker head-on. The ensuing explosion reduces him to toast, a punitively fitting end to a thoroughly bad apple.
Then ... Walter, however, survives the ordeal and staggers back to the road. He climbs in the back of the van just as it drives off and, some time later, hops off at Vera's rest stop in Nevada (below) where he reads in a newspaper of his reported death. The authorities, not knowing of Torrence, have assumed it was Walter who died in the car crash.