You can now buy Nathan/Airchime horns for over the road use. My best guess is that these have been choked down to not exceed the maximum loudness loudness for highway use.

Link: Nathan Over the Road Air Horns

Ironically, the P series horn was originally supposed to be a truck horn. Since the K series was stillin development, the P was rushed into production for railroad use to complete with the Leslie Supertyphon series, which did not have to be tuned like the Nathan/Airchime M series and used less air.

Since truckers can now buy legal horns, there is no excuse to steal them!

Word of advice - Do not buy the new 5 chime P series. These are sand cast bells, the later patterns are out of tune and the chord sounds pretty bad. The P3 aren't too bad (think most late SP horns). The common K horns are die cast, and are a higher pitched chord, but much closer to being in tune.