I've always wondered the reasons behind the regional difference in popularity of these. Trucks in the middle of the country seem to have them, trucks on the coasts seem to not. And you'd think longer haul trucks would be the most likely to have them, and so you'd end up seeing some on the coasts regardless.

Is it something with state by state safety regulations, like prohibitions on protruding structures that aren't part of the main body? Or differently-defined overall length restrictions? Does a given long haul truck have them deployed in the middle of the country, but then they're removed before getting to the coastal states?

> I've always wondered the reasons behind the regional difference in popularity of these.

There's an interesting mis-alignment of incentives. It comes down to who owns the truck vs the trailer.

* Truck owner pays for fuel.

* Trailer owner pays for the flaps.

So, unless you own both the truck and trailer, there's little incentive for trailer owners to install them.

That by itself isn't enough to explain the stark discrepancy though. Surely in the regions where tail flaps are extremely rare, there are still some operators that own both trucks and trailers.