A major U.S. transport union on Tuesday opposed a request by Alphabet Inc’s (GOOGL.O) self-driving unit Waymo and autonomous driving technology company Aurora Innovation Inc (AUR.O) for an exemption from rules on warning devices for large semi-trucks, citing safety issues.

The Federal Motor Carrier Safety Administration (FMCSA) said last month it received a joint application from Waymo and Aurora seeking a five-year exemption from rules that require drivers to place reflective triangles or a flare around a stopped truck to alert other drivers and help prevent a crash.

Aurora and Waymo instead want to use warning beacons mounted on the truck cab to avoid needing human drivers.

The Transport Workers Union of America said the petition is “inappropriate, represents an overreach and a misuse of the waiver and exemption process, and would significantly diminish the safety of our roads. It should be rejected in the strongest possible terms.”