State police said northbound I-81 was closed for 20 to 30 minutes while the bridge was cleared.
At one point, traffic was backed up for about three miles into West Virginia, police said.
The drivers of the vehicles involved in the accident were: Michael Poet, 33, of West Side Avenue in Hagerstown; Leslie Rutherford, 30, of Dawrong Lane in Hagerstown; Curtis Vaughan, 40, of Michigan, and Jill Shipley, 18, of Falling Waters, W.Va.
A state police spokesman said that although some of the drivers were taken to the Washington County Hospital, none of them was seriously injured.
The Washington County sheriff's department handled 13 minor accidents between 4 p.m. and 10 p.m., a deputy said. "We were really moving, I'll tell you that," he said.
Pennsylvania State Police in McConnellsburg, Pa., reported a few accidents in Fulton County - none of them serious. Police in Franklin County said they were concerned about Interstate 81 icing up as temperatures dropped, but had handled no serious accidents Saturday.
State police in Washington County said a rash of minor wrecks started slacking off there between 7 p.m. and 8:30 p.m., when fewer people were taking to the roads.
The snow emergency plan went into effect at 4:30 p.m. in Frederick, where state police said they were busy handling minor wrecks.
Police in Berkeley, Jefferson and Morgan counties in West Virginia said they had not had a problem with accidents.
Saturday marked one of the few times this winter that Washington County Roads Department crews had to dip into the season's supplies of salt and chemicals.
Roads department Traffic Control Supervisor Craig Pensinger said workers were out early Saturday, and covered just about every route in the county. Roads were slick in spots, especially in the western and southern sections, he said.
As temperatures dropped toward evening, Pensinger said crews could only hope their efforts earlier in the day would keep road surfaces from freezing.
Pensinger said unlike recent past winters, the roads department has more than enough salt and chemicals on hand to treat roads should snow strike again. "We're in great shape," he said.
Today's weather is expected to be drier and warmer, according to the National Weather Service.
Forecasters called for mostly cloudy skies today, with highs in the upper 30s. Winds will be out of the northwest at 10 mph. It will be partly cloudy Sunday night, with lows in the low or mid 20s.
On Monday, forecasters said the temperature will climb back into the low 40s.