The governor and other officials urged motorists to stay off the roads Saturday unless it is absolutely necessary to go out. They urged drivers who do venture out to take it slow, take special care at highway interchanges and give snow plows a wide berth.

 “Never pass a snow plow or a snow plow train,” said state Highway Administrator Neil J. Pedersen, using the term for a convoy of plows acting as a team to clear multiple lanes.

 There were few serious incidents to impede the traffic flow yesterday. An exception was a collision in which a vehicle carrying two adults and three children hit the back of a salt truck on the shoulder of I-95 at Route 462 in Harford County. Officials said one adult and one child were critically injured.

Earlier Friday, O’Malley declared a state of emergency that cleared the way for the Maryland National Guard to offer assistance – and the use of their Humvees – to local first responders.

 The declaration could also clear the way for Maryland to receive federal emergency assistance if it meets the threshold of 28 inches of snowfall. Such assistance could help the state and local governments cope with the budget-busting effects of the most snowy winter in recent memory – one that was expected to put the state well above the $60 million it had allocated to removal in the current budget year.

 “We hope our federal partners measure in a snowdrift,” O’Malley said.

Most commuters avoided the worst of the storm yesterday as they departed work en masse at midday Friday – encouraged by liberal leave policies adopted by the state and federal governments and many private employers. O’Malley said the biggest surge in homebound traffic occurred between noon and 2 p.m., and mo

st roads appeared to be lightly traveled by the time the snow intensified and visibility waned in late afternoon.

Transit riders joined in the early exodus as MARC trains and Maryland Transit Administration commuter buses left early Friday afternoon in an attempt to get commuters home before the worst of the storm.

State Transportation Secretary Beverly Swaim-Staley said both MARC and the commuter buses performed well.

The MTA was planned to run local buses as long as possible but Swaim-Staley said it might discontinue operations earlier than it did during the last major snowstorm in December, when some buses got stuck and had to be towed out. She said some buses will run on primary routes only, requiring some passengers to walk farther to catch a bus.

MTA officials were planning to run unoccupied “snow trains” through the night to keep tracks cleared on the subway and light rail lines. But the Washington Metro was expected to suspend operations once the snow exceeded 6-8 inches.

BWI’s biggest airline, Southwest, cancelled more than 200 flights Friday and Saturday in preparation for the storm, forcing ticket holders to improvise. Southwest, like most other airlines, won’t start flying again until Sunday.