SAN FRANCISCO -- A fatal accident on the westbound section of the Bay Bridge Tuesday afternoon forced the closure of several lanes, triggering fears of a rush-hour commuter nightmare.

Shawn Chase of the California Highway Patrol said the fatal accident occurred when a truck drove directly into the back of a stalled big rig. The accident occurred at about 1:15 p.m. on the westbound portion of Interstate Highway 80, the section of the bridge before the tunnel at Treasure Island.

The big rig, which has a utility crane attached, was stalled in the No. 5 lane of the highway with its emergency lights on, Chase said. The box truck, from a business called "Diet to Go," drove directly into the back of the big rig.

The man in the box truck was killed, but the man in the big rig was unharmed, Chase said.

Debris from the accident scattered across the lanes of the highway, and the Nos. 3, 4 and 5 lanes have been closed. A Sig-alert for the highway was issued at 1:41 p.m. and is still in effect, according to Officer Kevin Kroncke.

Lanes were not expected to open for a few hours, and traffic is backed up into the East Bay, Chase said.

The CHP was asking drivers to take Interstate Highway 580 or state Highway 92 as alternative routes, Chase said.