Commuter traffic was backed up for about eight miles on southbound I-270 near Gaithersburg for nearly three hours yesterday morning after a tractor-trailer collided with a car and both overturned, Maryland state police said.

The truck swerved and jackknifed about 7 a.m. after trying to change lanes. It then hit a Honda Civic in the left lane and flipped over, blocking all three lanes, the right shoulder and part of the left shoulder, according to Trooper William Vogt.

The truck's load of mulch spilled out all over the highway and it took until 9:45 a.m. to clean the area. "It was a mess," Vogt said.

Because the accident happened near the entrance ramp to Rte. 124 traffic was directed onto Rte 124 and then to the next I-270 entrance ramp at Rte. 117, police said.

"Luckily it happened right there by those entrance ramps so we could . . . let people off the highway," Vogt said. Had it happened north of the interchange commuters would have been trapped, he said.

The truck driver, Carson Mayst of Spring, Pa., was unhurt and the car driver, Suzanne Davis of Fairfield, Pa., was taken to Shady Grove Adventist Hospital where she was treated for minor injuries and released.

The accident also happened near a State Highway Administration facility, so dump trucks and front-end loaders were readily available to clean up the mulch, according to police.