R.O.P.S.

1928.51(b)(3)

Protection from spillage. Batteries, fuel tanks, oil reservoirs, and coolant systems shall be constructed and located or sealed to assure that spillage will not occur which may come in contact with the operator in the event of an upset.

Cited from:   OSHA 

To ensure safety during the transport of class 3 combustible liquids, the LEE D.O.T. line of diesel trailers is equipped with R.O.P.S. This cage-like structure that surrounds the fuel pump at the top of the trailer reduces the chance of spilling liquids in the case of an overturned vehicle. 

Along with various warning placards placed on the towed vehicle, the D.O.T. trailers follow compliance with the Department of Transportation to drive this model on highway roads.