MADISON, Wis. -- Semitrailer trucks stuck on a Wisconsin freeway during a massive snowstorm left hundreds of motorists stranded for hours in a traffic backup Wednesday, and Gov. Jim Doyle called a state of emergency to have National Guard troops help them.

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State emergency management personnel said there were more than 800 vehicles in the backup as of Wednesday night, hours after the problem developed around 1 p.m. on Interstate 90 south of Madison. Authorities said traffic was backed up into Rock County.

Authorities strongly urged motorists to avoid Interstate 90 from Beloit to Madison so that crews would be able to clear the traffic stranded there. They said the goal is to get vehicles out of the area so that crews have more room to work.

In a conference call, officials said snowmobiles and all-terrain vehicles were used to check on the welfare of people in the stranded vehicles and also to bring them food or fuel as needed.

Authorities said their efforts have been slow-going because stuck vehicles and semitrailers are blocking lanes, making it difficult for snowplows and emergency crews to get through.

In a conference call at 1 a.m. Thursday, authorities said that while traffic was moving slowly in some sections of the interstate, there was no consistency and that traffic was still only moving in fits and starts early Thursday morning.

Lt. Col. Tim Donovan, spokesman for the Wisconsin National Guard, said 15 Humvees and two 5-ton trucks were being used to assist motorists with water and fuel, and two helicopters were being readied in case they would be needed for aerial searches or surveillance.

Donovan said that around 11 p.m. a Blackhawk helicopter was in the air to survey the scene. He said that from the helicopter they counted 552 tractor-trailers and 295 passenger vehicles at a standstill in both lanes.

Donovan said about 40 Guard soldiers were called in. The Wisconsin Army National Guard Armory in Janesville was also being readied for use as a possible shelter location if needed.

Authorities said that accommodations for motorists have been set up at Anchor Inn and County Inn & Suites off the Highway 59 exit.

One man who called WISC-TV from his vehicle said he has been stuck in traffic on Interstate 90 north of the Highway 51 exit since 1:30 p.m. Wednesday and had not moved for more than seven hours.

The Wisconsin State Patrol said that a tractor-trailer stuck near exit 154 in the northbound lanes, about 12 miles south of the Beltline, caused the backup. Authorities said there is an incline in the area that was just too steep for tractor-trailers to climb in Wednesday's snowstorm.

The Wisconsin Division of Emergency Management on Wednesday night activated its state Emergency Operations Center. Emergency response teams from several agencies gathered at Coachmen's Golf Resort, just off the Interstate south of Stoughton, to direct emergency response efforts.

"We're trying to monitor the situation throughout the state. We've got representatives from the Department of Natural Resources, Wisconsin State Patrol, state highways. We're watching the activity, seeing what's going on to see if there are any ways that we can support the local role," said Jerry Haberl, officer in charge at the Emergency Operations Center.

Dane County Emergency Management coordinated with at least 10 local fire departments and the Dane County Sheriff's Department to help provide medical assistance as well as food and water.

Authorities also stressed that there is no need for volunteers and that people should stay away from the area to give authorities room to work and coordinate efforts.

The State Patrol said that stranded drivers should stay with their vehicle. Drivers in the gridlock who are low on fuel are being told to turn their vehicles off until they need heat again, WISC-TV reported.

Although many motorists had been stranded for hours, authorities said that no emergency situations have arisen. They said that a couple of people were treated for medical conditions.

State Gets Almost 20 Inches Of Snow; 2 Die In Crashes

Snowfall reports by Wednesday evening ranged up to 21 inches near Beloit and 19.5 inches at Orfordville in Rock County, both in south central Wisconsin.

Jackson in Washington County, north of Milwaukee, saw 19 inches. Madison had 13.3 inches and Milwaukee 11.5 inches.

The National Weather Service said the storm was winding down across southern Wisconsin Wednesday night, but snow was expected continue for a couple hours in the southeast corner of the state.

All flights were stopped at Milwaukee's Mitchell International Airport Wednesday because of the blowing, drifting snow.

There have been two traffic fatalities reported.

A 51-year-old man was killed Wednesday morning in Milwaukee County when he lost control of his car on a slick portion of Interstate 94 and slid in front of a semitrailer.

A 59-year-old Oak Creek man died in Milwaukee in the afternoon when his pickup truck crossed the road, jumped a sidewalk and crashed into a pole.

The Milwaukee County Transit System suspended bus service at 5:30 p.m. after 23 buses carrying passengers became stuck in snow. Service isn't scheduled to resume until Thursday. The transit system said in its announcement that, "Out of concern for public safety we do not want to encourage people to travel in this weather."

Authorities throughout the state Wednesday were giving out the same message, discouraging people from traveling.

The severe snow also caused delays Wednesday for Madison Metro buses. Metro buses were running 10 to 20 minutes late, and all buses stopped running routes at 7 p.m. Wednesday due to weather conditions.

Hundreds of schools across southern Wisconsin closed on Wednesday morning as snow continued to pile up from a major winter storm.

The state's largest school district, Milwaukee Public Schools, also decided to close its schools and offices. That involves 87,000 children and 14,000 employees.

Even the University of Wisconsin-Milwaukee canceled classes until 3 p.m. The University of Wisconsin-Madison canceled afternoon and evening classes beginning at 3:30 p.m.

About 143 schools in the Madison area closed or delayed opening on Wednesday.

Milwaukee's all-time record for a 24-hour snowfall is 20.3 inches, set back in Feb. 4 and 5 of 1924.