A chain-reaction collision yesterday involving three trucks, including a tractor-trailer hauling lethal chemicals, closed part of Interstate 95 south of Fredericksburg, Va., for much of the day and kept nearby schools from opening.

Police allowed northbound lanes to reopen after they determined that no hazardous material had leaked as the result of the predawn crash. But the southbound lanes remained closed until the disabled trucks were cleared about 3 p.m., backing up traffic for at least six miles from the accident site between the Massaponax and Thornburg exits of the highway.

As a plume of black smoke rose from the wreck, hazardous-material teams were dispatched to the site after police spotted a placard on one of the trucks warning that it was hauling a poisonous cargo. Two state troopers who got there first were quarantined by the side of the highway for several hours, and one of the truck drivers remained trapped in his cab for four hours until emergency crews concluded that it was safe to approach the chemical truck, which was hauling 16 cylinders of methyl bromide. No one was seriously injured.

Three area schools separated from the interstate by a thinly wooded strip of land stayed closed for the day, affecting about 3,000 students at Massaponax High School, Thornburg Middle School and Riverview Elementary School.

Methyl bromide is a colorless gas typically used as a pesticide for controlling insects and rodents, primarily in agriculture but also in fumigating buildings. Exposure to the gas, which is gradually being banned by federal regulation because it contributes to depletion of the ozone layer, can harm the lungs, eyes and skin and cause fatal damage to the central nervous system.

"It could have been much worse," said state police spokeswoman Lucy Caldwell. "Due to the extreme nature and volatility of this material and the smoke coming from the wreck, you can't take any chances endangering other lives. We're very fortunate that it didn't breach [the chemical] containers."

While emergency crews evaluated the level of danger, thousands of motorists were detoured from I-95 onto Route 1. Thomas Drake, 42, an ink technician from Ladysmith, Va., said his typical 15-minute drive into Fredericksburg lasted more than an hour. "It was pretty hung up all the way through with tractor-trailers and cars everywhere," Drake said. "You don't realize how much traffic goes up and down 95 until you see it all on Route 1."

Roosevelt Johnson, 59, of Lake Anna, said he was ambushed by the massive backup because he had been listening to gospel music rather than traffic reports. "Everyone seemed to be aware of the accident but me," he said.

The collision occurred about 5:30 a.m. after the driver of a Toyota Camry lost control while headed south on the rain-soaked interstate and went into a spin, police said. The car, driven by Anna McCollum, 43, of Richmond, struck a truck hauling pet food to Florida, which in turn jackknifed, blocking all three southbound lanes.

The driver of the dual-trailer rig hauling the toxic chemicals from Baltimore to Charlotte, N.C., braked to avoid a crash. But it was struck from behind by a third truck and plowed into the disabled truck ahead, and one of the two trailers on the chemical-carrying truck overturned. The impact demolished the cab of the third truck, which was carrying boxes of fabric softener to Florida.

"Once we heard that there was a cloud, the worst scenario came into our heads," said Reggie Phillips, spokesman for the Fredericksburg Fire Department. "We couldn't be sure what we were dealing with, and we had to make sure that it wasn't going to be a threat."

The driver of the chemical truck, Jim Chandler, 61, of Charlotte, was uninjured, while McCollum and Marvin McDell Jr., 33, of Mobile, Ala., who was driving the pet food truck, were treated at Mary Washington Hospital in Fredericksburg and released. Despite being trapped inside his crushed cab, the driver of the last truck, Robert Dickle, 39, of Charlestown, N.H., was also treated and released. State police said they filed reckless-driving charges against Dickle.