Traffic on the Parkway East and Parkway West faced a bit of a hangover from the weekend storm on Monday night and yesterday, as scores of snow-capped trucks triggered overheight warning monitors at the Squirrel Hill and Fort Pitt tunnels.

The Pennsylvania Department of Transportation yesterday issued an advisory asking tractor-trailer drivers to abide by a state law requiring all motorists to remove snow and ice from their vehicles before driving.

Meanwhile, forecasters warned of another round of winter storms that could paralyze Christmas Eve and Christmas Day travel both east and west of here.

A blizzard was expected to blast the Great Plains and severe ice storms threaten the central mid-Atlantic region, which is still digging out from record December snowfalls from the weekend, according to AccuWeather, based in State College.

Sensors in place on the approaches to the tunnels are designed to detect overheight trucks and flash a warning to them.

When the sensors are triggered, red signals are activated at the entrance to the tunnels to prevent trucks that are too tall from getting stuck or causing damage. The trucks are ordered to turn around, PennDOT spokesman Jim Struzzi said. That can take several minutes, causing severe backups at peak times.

"It really caused some problems" during yesterday morning's rush hour, he said.

Snow-covered trucks triggered the warning system 25 times Monday night and five times yesterday morning at the inbound Squirrel Hill Tunnels and three times Monday night and twice yesterday morning at the Fort Pitt Tunnels, he said.

"In addition, some trucks are still entering the tunnels (despite the red signals), causing the snow and ice to be pushed from the trucks onto other vehicles and the roadway, creating extreme safety hazards," Mr. Struzzi said.

A state law enacted in 2006 imposes fines of $200 to $1,000 if snow or ice dislodged from a vehicle causes a death or serious injury. It was passed after the death of a woman in Carbon County on Christmas Day 2005 when an ice chunk flew off a tractor-trailer and crashed through the windshield of her sport-utility vehicle.

AccuWeather meteorologist Tom Kines said Pittsburgh likely would be spared the worst of the ice storm forecast to strike beginning tomorrow night.

"I think you'll have mostly rain," he said, with the possibility of some freezing rain between 3 and 10 a.m. on Christmas Day that could slicken roads.

Areas to the east, including Altoona, State College and DuBois and the Interstate 95 corridor that was buried by last weekend's blizzard, could have severe icing that brings down tree limbs and knocks out power, he said.

"The question is how big an area gets the significant icing," Mr. Kines said. "It is going to be a very slippery travel day Friday across the state."

AAA has projected that 87.7 million Americans would travel 50 miles or more from home over the holidays, a 3.8 percent increase from last year.

The Pennsylvania Turnpike is expecting 4.2 million vehicles, a slight increase from last year, spokesman Carl DeFebo said.

Turnpike travelers can get traffic and weather updates at www.paturnpike.com or by calling 1-866-976-TRIP (8747). Updated traffic and weather for other Pennsylvania highways is available at www.511pa.com or by dialing 511.

Port Authority bus, rail and incline service will be on holiday schedules on Christmas Day. The regular weekday schedule operates tomorrow.

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