I don't know if it is truly all that big a risk, don't recall hearing of a large number of such accidents back when I was in high school and we did it all the time. Must have been some problems somewhere though because it is illegal now here in Texas.
I haven't let mine ride in the back except on the farm, not particularly worried about "riding in the back" as much as just wanting to keep an eye on 'em. On the other hand, I have let them ride in the back seat of my '72 Jimmy with the top off, on the highway - and that probably isn't the safest vehicle around in a crash either.
Society often has double standards on how child safety is addressed. Examples:
- Kids can't ride in the back of a pickup truck on the streets legally
... but they can if it is called a parade
- Kids can't be paddled at school, because their little butts are so tender and precious Mom and Dad can't handle the agony
... but the same parents can sit in the bleachers and cheer as their kids are beaten black and blue by strangers playing football, soccer, etc.
- Kids can be kicked out of school, sometimes permanently, for even the most minor fight or squabble because violence absolutely cannot be tolerated by the adults
... but many of the same adults encourage Golden Gloves, martial arts, etc
- Kids can't rollerblade or ride skateboards outside in most cities without a helmet
... but they don't have to wear a helmet when paying to skate at roller rink or ice skating
- It is illegal to let your minor child get behind the wheel of a car while you teach them to drive, the way all of us used to get to drive with Dad
... but it is legal to let kids run mini-dragsters, race motorcycles, etc.
One of the "safety" things that really fascinates me now is the school mentality of "locking down" if there is an emergency or threat situation. Instead of getting the children out of the building away from potential fire / collapse / bombs, they lock the doors and won't let parents in to get their child. In years past, the idea was always to get the kids away from the building and out where they could run and not be massed up. I don't know about you guys, but in the event of another Columbine I'd rather see the students running wildly all over heck and half of Georgia to get away than sitting trapped in their rooms waiting for someone to walk up and shoot them, while the cops dither outside.
Safety is a tough ideal to achieve, especially since life is a terminal condition... .