I was watching a special (PBS?) about big mega-crashes on highways, the 60-vehicle-pileup variety. A lot of research happens when those occur. Probably the worst problem is fire, and the worst causes of fire are the saddle tanks of 18-wheelers. They talked about that a bit, even showed some crash testing of a tractor that showed how those tanks are pretty easily torn off and/or ruptured, bouncing around pretty much anywhere. But they also explained iddibhai's point...there really isn't anyplace else to put them, short of some very drastic redesigns of tractors. They're just too bulky.
I think they said the greatest cause of such crashes was fog, followed by other weather circumstances where people drive too fast for the conditions. Interestingly, in fog, even if you're trying to drive slow you probably are going faster than you realize.
rusty