Why do cars pull over on the freeway?

Q. Has anyone done a study to determine what is the most frequent cause of “stalled” cars on freeways? Modern cars do not stall, so what’s really going on? I think a large number of these cars are running out of gas because of the negligence of their operators. Do they suffer any consequences if that’s determined to be the case? The amount of lost time, productivity and increased pollution because of such negligence must be astounding.
– Kay Romer, Orange