Cars collided, tractor trailers jackknifed and traffic backed up for miles last night in Prince George's County and many other sections of Maryland as plummeting temperatures turned wet roads into sheets of ice.

"So many cars were wrecked . . . the salt trucks couldn't get through," said state police Cpl. Harold Sullivan of the College Park station. At one time, he said each of seven troopers working out of the station faced a backlog of five accidents.

Wrecks forced the closing of Rte. 50 in the Bowie area for an hour starting shortly after 8 p.m., Sullivan reported. State police in Montgomery County described Rtes. 29 and the Capital Beltway as "a wreck all night."

State police imposed snow emergency plans in Prince George's and in seven other Maryland counties, including all those in the Baltimore area where state police spokesman Chuck Jackson reported "instant skating rink conditions."

An unidentified New Jersey man was killed about 7 p.m. on I-70 in Howard County in a five-vehicle collision in which weather appeared to be a factor, said Maryland state police trooper Victor DeCesare.

Jackson said hundreds of calls reporting disabled or abandoned vehicles poured in on a special statewide police telephone hot line. For evening rush hour commuters, "arriving two to three hours late was not uncommon," the spokesman said.

Congestion appeared to be easing in the Washington area by late last night and a Maryland State Highway Administration employee said trucks were patrolling highways ready to spread abrasives on icy patches.

The National Weather Service blamed the icing on a cold front that moved through the area late in the afternoon from the west.

Forecaster John Forsing said the front followed the storm that brought heavy rain to the area Thursday night and yesterday morning. "When the cold air arrived," Forsing said, "roads had not had time to dry out."

At National Airport, the temperature reached 39 degrees at 1 p.m. but fell below the freezing mark between 4 and 5 p.m. Winds gusted to more than 40 miles an hour during the night.

The cold front also transformed yesterday afternoon's drizzle into snow and sleet. Some grassy patches turned white, but there was little accumulation otherwise.