Lantley, that is true -- if I get rear-ended violently, there is a chance that the can could get knocked over. But the risk is essentially the same if the can is on the tongue -- a side impact or a rollover will spill the gas.
The same risk is present if the gas can is inside the cargo bed of the truck (where I have been carrying it). A bad crash will spill the gas.
By contrast, a true built-in gas tank is (in most vehicles) an armored metal container, designed to withstand impact. The little plastic tanks that most of us carry are much less safe, no matter where we put them.
