A footbridge over the Baltimore Beltway collapsed today onto rush-hour traffic, crushing the driver of a sport-utility vehicle and disrupting the evening commute for thousands of motorists.

Five people were injured, three seriously, when the concrete bridge tumbled onto the four-lane stretch of the Beltway southwest of Baltimore after a backhoe mounted on a passing tractor-trailer clipped the bottom of the structure shortly before 5 p.m.

Witnesses said they heard the crunch of metal striking concrete followed by a loud boom. Houses tucked amid trees behind the highway noise walls shuddered as the span collapsed in a V, spewing debris across the inner loop of Interstate 695 just northwest of the interchanges with Interstate 95 and Route 1.

The bridge plummeted onto the front of a blue Dodge Durango traveling in one of the left-hand lanes, crushing the hood to the pavement and killing the driver, Robert Norman Taylor, 54, of Baltimore, instantly. Amid the sharp squealing of brakes, two cars traveling in the slow lanes smashed into the structure, witnesses said.

"I saw a large cloud of dust," said Dave Atkins, 44, of Arbutus, who was driving along the Beltway. "People were jumping out of their cars. I did the same. I couldn't believe this bridge was lying on the Beltway."

Before rescue crews arrived at the scene, several people tried to free the victims from their crippled vehicles, using crowbars and pickaxes to pry the doors open, Atkins said.

Three women were evacuated by ambulance and a pair of helicopters to the Maryland Shock Trauma Center.

Elizabeth Freeman, 68, who lives in a retirement village and was driving a Nissan, was reported in critical but stable condition undergoing treatment for a bruised lung in connection with cardiac and blood pressure problems plus an ankle injury.

Regina Lee Brehon, 51, a passenger in Taylor's Durango, suffered knee injuries and ruptured tendons and was reported in serious but stable condition.

Henri Patrice McQueen, 27, of Linthicum, Md., who was driving a Toyota, suffered leg injuries and was reported in fair but stable condition.

Two other people suffered minor injuries when they were struck by flying debris, but they declined medical treatment, authorities said.

Taylor's body was removed from his vehicle after 7:30 p.m., and within an hour, two cranes had brought down the still-standing portion of the pedestrian bridge over the outer loop of the Beltway. Crews hoped to have the roadway cleared before the Wednesday morning rush hour.

The collapse severed one of Baltimore's prime commuting routes, the 33-mile Beltway built in 1962. Traffic in both directions was rerouted as police carried out an emergency detour plan. But state police said the effect was not nearly as severe as the disruption last week of rush hour on the Capital Beltway, after a truck hauling explosive powder crashed in the Springfield interchange, paralyzing travel in both Virginia and Maryland.

"Traffic wasn't all that bad considering how serious the accident was," state police spokesman Pete Piringer said. Piringer said the backhoe had been improperly loaded on a flatbed truck that had picked up the piece of construction equipment at Wagner's Point, in Baltimore.

The driver of the truck, owned by TTK Transport of Ontario, Canada, was Paul McIntosh, 23, of Ontario, state police said. They said McIntosh told authorities that he did not realize the backhoe had clipped the pedestrian bridge or that it had collapsed. Piringer said tonight that the driver "knows that he loaded [the truck] improperly now. He did not know that, however, until he hit the Westland bridge."

Police said that their investigation was continuing, and that McIntosh had a permit to operate the truck in Maryland.

No one was on the pedestrian bridge when it collapsed. It has been closed since November 1996 because of community complaints about vandalism and crime on the span, and it was to have been demolished as part of planned Beltway widening, according to state officials. But State Highway Administrator Parker F. Williams said the structure, built in 1957, was sound and had last passed inspection early in May.

"We have had vehicles that have hit bridges before, but nothing this catastrophic," Williams said. He said the clearance for the bridge was 16 feet, considerably higher than the standard of 13 feet 8 inches.

Piringer noted that the truck already had passed under several other bridges on its way from Wagner's Point.

State engineers were dispatched to the scene to examine the remaining span of the bridge over the outer loop, which was rudely converted to a walkway into thin air when the portion over the inner loop was ripped away. They also were inspecting the Westland Boulevard bridge, about 500 feet away.

The National Transportation Safety Board also sent a representative to the scene. The pace of the investigations could determine how quickly the rubble can be cleared from the highway and the lanes reopened.

"We are talking tons of concrete. The removal process is going to be very serious and will take us well into the morning hours," said Capt. Michael Robinson, of the Baltimore County fire department.

In 1989, a vehicular overpass being built at Route 198 over the Baltimore-Washington Parkway near Laurel collapsed during rush hour, injuring 14 motorists and construction workers. Federal officials said apparently faulty scaffolding used to support the uncompleted span was to blame in that incident.

Staff writers Linda Perlstein, Robert E. Pierre and Fern Shen and research editor Margot Williams contributed to this report.

CAPTION: Rescue workers face the wreckage of a footbridge and three vehicles on the Baltimore Beltway.

CAPTION: The collapse of the pedestrian bridge forced the closure of the Baltimore Beltway in both directions. Authorities hoped to clear the wreckage in time for the morning rush hour.