There was one survivor of an explosion
He was not mentally handicapped, but had been fooled into thinking he was simply transporting a bomb to leave at a location. Naturally, he was a bit pissed off, and turned in a few bad guys.
That's only one real data point. I find it hard to believe that after so many car bombs, that new drivers wouldn't get the general idea figured out on their own, which implies to me that some of them are okay with the suicide bit in the first place.