The driver, Coffman's daughter, tried to brake as she slowed for congested traffic and skidded into the rear of a 1993 Freightliner flatbed, police said.
The Mustang continued across I-70, striking a 1991 Toyota car and then a road sign, police said.
Debris from the road sign hit a 1986 Volkswagen, police said. The Toyota also was struck by an unidentified tractor-trailer that did not stop.
Also injured in the wreck were Coffman's daughter, Tonya Marie Cline, 23, of 162 Cumberland St., Clear Spring, and Coffman's 7-month-old granddaughter.
Tonya Cline was in critical condition at Washington County Hospital on Friday night, according to hospital spokesman John Costopoulos.
The baby, whose name was unavailable, was in critical condition on Friday night at Johns Hopkins Hospital's Pediatric Intensive Care Unit in Baltimore, a hospital spokeswoman said.
Coffman and Cline were not wearing seat belts, police said. The baby was in a child restraint seat.
No other injuries were reported from the accident, which closed westbound lanes of I-70 for about three hours, police said.
Other accidents were reported on Interstate 70 as well as dozens of fender benders across the Tri-State area.
Stretches of roads west of Hagerstown were slippery from slush and freezing rain, causing a tremendous number of minor accidents, said Maryland State Police Sgt. Donald Stottlemyer.
The further west, the worst the roads were, Stottlemyer said.
State roads were in fairly good shape Friday night except for isolated patches of ice, said Gary Shank with the Maryland State Highway Administration.
Overpasses and bridges still were slippery, but road crews were expected to be out throughout the night to check on road conditions, Shank said.
If temperatures rise as expected overnight, the roads should be in better condition today, he said.
Snow changed to freezing rain and then rain on Friday. The rain was expected to end Friday night, said National Weather Service meteorologist Dave Thede.
Temperatures were expected to rise to the upper 40s today before a cold front moves in, knocking temperatures down to the 30s in the late afternoon, Thede said.
Some Tri-State area schools closed early on Friday because of poor road conditions caused by the weather.
The snow emergency plan for Washington County was lifted at 9 Friday night after more than seven hours, according to the State Highway Administration.
A winter weather advisory for the Tri-State area was lifted at 8 p.m. on Friday after more than 16 hours, according to the National Weather Service.
Slippery roads contributed to an 1:30 p.m. accident on Pa. 233 about three miles south of U.S. 30 in which a southbound 1991 Hyundai slid into a northbound Pennsylvania Department of Transportation, according to Pennsylvania State Police. There were no injuries.
Franklin County emergency services reported about a dozen minor traffic accidents, almost all of which occurred on Interstate 81.
Staff Writers Terry Talbert and Richard F. Belisle contributed to this story.