A pedestrian bridge spanning Kenilworth Avenue near the Minnesota Avenue Metro station collapsed into the roadway yesterday after its central support beam was struck by the raised bed of a dump truck.

No one was seriously injured in the incident, which occurred about 11:30 a.m., but lengthy cleanup operations closed the six-lane highway in both directions until 4:15 p.m., creating massive traffic jams throughout the afternoon and the evening rush hour.

"Cars are lined up solid as far as the eye can see on New York Avenue," a D.C. police traffic dispatcher said about 4:30 p.m. as he looked out a window from traffic division headquarters at the intersection of Sixth Street and New York Avenue NW. Bumper-to-bumper traffic was crawling at about 4 miles per hour along Bladensburg Avenue late in the afternoon.

George Schoene, head of traffic for the Department of Public Works, said that Kenilworth Avenue normally carries about 15,000 vehicles between 3 and 6:30 p.m. on Fridays, with the peak occurring between 3:30 and 4:30 p.m.

Miles-long backups were reported on the alternative outbound commuter routes of East Capitol Street, Benning Road, and New York, Rhode Island, Pennsylvania and South Dakota avenues, D.C. police reported.

One of the few pedestrian paths across Kenilworth Avenue, the bridge led directly into the Minnesota Avenue Metrorail station and was heavily used, officials said. They said it was miraculous that no one was on the bridge when it was struck.

"In 20 minutes the bridge would have been loaded down with kids from Woodson Junior High School just east of the northbound lanes going over to the 7-Eleven" store on the other side of the highway, said D.C. police Officer John Holton.

Wesley Luzier, a station attendant at the Minnesota Avenue stop, said, "I heard a tremendous noise. I thought two trains had crashed."

Deputy Police Chief Fred Thomas said that officials are not sure why the bed of the truck was elevated as the vehicle approached the overhead pedestrian walkway. He said that the truck was elevated to a height of about 19 feet when it struck the walkway, which had a clearance of 14 feet.

Police identified the driver as Leslie H. Steptoe, 54, of 1325 Fairmont Ave. NW. Officials at Howard University Hospital said last night that Steptoe was admitted in good condition with bruises and lacerations.

The truck, which is owned by Lee Washington Inc., an excavating and hauling company in Northeast Washington, was impounded by police for inspection.

Police said the truck bed crashed into a massive 60-foot I-beam that supported the eight-foot-wide, wire-enclosed footbridge, and the structure fell across the six-lane highway, which is about 130 feet wide.

In the northbound lanes, where the accident occurred, the I-beam was completely torn away from the bridge, and chunks of cement and steel were piled on the roadway two feet deep.

Workers with blowtorches and jackhammers broke the rubble into manageable sizes, and bulldozers plowed through the debris to clear the road.

The span of the bridge that crashed across the southbound lanes fell almost intact, and a huge crane lifted and hauled it to the shoulder of the road.

No estimate was available on how long it would take to replace the walkway. An official with the Department of Public Works who did not want to be identified said that the cost of a new bridge would probably be about $1 million.

D.C. police closed all access routes to the northbound lanes of the highway from Pennsylvania Avenue SE to the site of the accident and all southbound entrances from Eastern Avenue NE while workers frantically labored to clear debris from the roadway in time for the afternoon rush hour.

The accident also caused traffic backups in suburban Prince George's County, where Maryland State Police said that they closed the entrance ramps from the Capital Beltway onto the Baltimore-Washington Parkway, which leads into Kenilworth Avenue, and from Rte. 50 onto southbound Kenilworth Avenue.

U.S. Park Police said that southbound traffic on the Baltimore-Washington Parkway was diverted to New York Avenue.

"It's really lucky that it didn't happen during rush hour, when cars are lined up under the bridge and pedestrians are streaming across the top," said Metro police officer John Bresnahan, who was on duty inside the Minnesota Avenue stop when the crash occurred. "I thought from what I saw that someone blew it up."

Metro officials said that subway passengers who normally use the bridge to walk from the station across the highway to such communities as Mayfair would be provided with tranfer tokens for use on the M-16 bus as long as the bridge is out of service.