Northern Virginia’s congressional delegation has written Defense Secretary Leon Panetta asking that he slow the transfer of thousands of defense workers to an Alexandria office complex until road improvements can be made to accommodate the traffic.

“We are concerned that the Department’s occupation plan is going forward before the appropriate transportation improvements and mitigation measures are in place,” said a letter signed by U.S. Sens. Jim Webb and Mark Warner, and U.S. Reps. Jim Moran and Gerry Connolly, all Democrats.

They expressed particular disappointment at a reduction from five to four in the number of shuttle routes operated by the Defense Department between Metro stations and the Mark Center complex.

Nearly 5,000 of the 6,400 defense workers being transferred there are currently scheduled to arrive before year’s end. The congressmen wrote that the transfers would cause “unacceptable congestion,” notably to the adjacent Interstate 395.