Airplane seatbelts are only mandatory during take-off and landing as well as turbulence. It will be a very long time before any kind of driving is as safe as commercial air travel.
I wasn't talking about the laws, I was referring to the comfort baked into the design: I have slept while flying with my seat-belt on; it's not uncomfortable.
While not legally mandated, most airlines encourage people to keep their seat-belts on during cruise - this is good advice that I take to heart because turbulence is unpredictable; sudden loss of elevation are known to happen.
It's also the case that a loosely-buckled aircraft lap belt will probably keep you from making an acquaintance with the ceiling if the plane decides to suddenly make you very aware of the concept of inertia. On the other hand, should a car ram into something or something into it, the same thing is going to be pretty near useless.
