It's always gratifying to hear about a successful off airport emergency landing. The pilot did a great job getting it down, and it appears the fire occurred due to a fuel tank breach when the aircraft hit the concrete freeway. As Jeff observed above, the Riverside freeway is always congested, and it's fortunate no one was injured.
This news report has a video of the landing and interview with the pilot. He put the plane down in between cars but the right wing clipped a vehicle or object, that spot erupted into flames, then the plane slid sideways into the wall. They are very fortunate. Could have ended much worse.