I have a pair of them back there, I put them there but did see them mounted like that many times and I believe a few on original military vehicles. Anyway like said I don't carry fuel in them, motor oil mix, which is what I run in my truck(plus the laugh factor if anyone tries to siphon them for their truck)
Anyway no worries about getting hit and blowing up!! first I am 12 feet away from them and second have you ever hit a gas can......it just breaks and spills all the gas-no big boom!! In order for it to blow you would need one or more things to happen.......enough compression(contained) in order to detonate(like in your motor) or you would need a large enough spark(after it has spilled all over and mixed with oxygen) I have tired many times to make gas tanks blow up.....it is actually very hard to do.
The worse I can see happening is a really bad accident, somehow involving flames, and my cans get ruptured and spill all the gas out and it travels to the flame and then WHOOSH larger flame that goes out when the fuel is burned up.
Again, all this isn't going to matter as I said I don't carry "gas" or "diesel" in them. When summer comes about I may even get one to swap out back there and fill it with water, especially if I get the second tank mounted that I want.