[iWhere they're used here, they are always on stretches of road that have a "speed checked by aircraft" sign.

I don't know if it is dependent on state law, but some states (like mine) aren't polite enough to post those signs and seem to have those white markers (like a /\ in WA) everywhere.

Aircraft traps are the easiest to spot, you can see them from miles away.

Generally true.

The plane will be flying low, parallel to the highway. You'd have to be not paying any attention to the sky at all to get nailed by an airplane trap.

But exceptions exist. My one aiplane ticket told me to watch not just the sky but the topography. Thankfully helicopters are much more expensive to operate than the little Cessnas they use for air speed traps or this problem would be worse.