WASHINGTON - The Senate is trying to block congressional negotiators from mandating that states allow trucks with extra-long double trailers on interstates unless the vehicles can be shown to be safe.

The Senate voted Tuesday to instruct lawmakers negotiating the final language of a transportation bill.

Safety advocates say they're concerned negotiators will add a provision forcing all states to allow the longer trucks even though the provision wasn't included in transportation bills passed earlier by the House and Senate.

The Senate's instructions to negotiators would allow individual trailer lengths to increase from 28 feet to 33 feet if the Transportation Department finds the longer trailers wouldn't result in a statistically significant rise in fatalities.

Twelve states already allow longer trailers that bring the total length of a truck to 91 feet.