Californian's aren't the only ones that use California's freeways...
http://www.kcra.com/news/wildfire-spreads-across-socal-freeway-burns-cars/34227196
Here's a video showing more of the incident, as well as an interview with Russell Aleveto from Michigan
http://abc7.la/1Od4dYc
And the rest of the article
http://abc7.com/news/north-fire-shuts-down-15-fwy-burns-cars-prompts-evacuations-in-cajon-pass/859747/
Now, for this
The fire jumped the freeway ahead of where the cars were burned, effectively trapping all of the cars behind that point on the road. Initially everyone was told to stay in their cars, but as the fire came up to the road everyone abandoned their cars for safer ground. So, there wasn't anyone there to move the cars when they started burning, the driver's had already abandoned their cars for safety. After that they were kept away from the burning vehicles, not allowed to go back and move their cars away from those that were still burning.
With a fire that intense and moving that fast, the radiant heat would keep you from wanting to be within a hundred feet of the flame front. Maybe further. As the fire approached, you'd want to be as far from the edges of the road as possible to stay away from the fire and radiant heat. With wind gusts up to 45 mph and the type of dry brush that's there, the wildland fire would move through VERY quickly, probably not much slower than the windspeed. It would ignite the cars closest to the edge of the road that the fire approached from, or any that managed to trap burning embers. But the cars burn for a lot longer, so even after the wildland fire has come and gone the cars will continue burning for some time, and continue to catch nearby cars on fire as well. At that point, you'd want to be back out on the edges of the road, away from the other cars. But again, the driver's had left their vehicles and/or been evacuated.
The semi-truck fire burned so hot the lanes of the freeway were damaged.