A ban on all nonemergency traffic on Long Island roads is in effect overnight, and virtually all of the region's public transportation is shut down amid a massive snowstorm predicted to reach its full fury through much of Tuesday, officials said.

Gov. Andrew Cuomo gave the order Monday for the ban to take effect at 11 p.m., with officials meeting before dawn to assess conditions for reopening.

State transportation officials said, just before the ban was to go into effect, they were bracing for the worst.

"It's coming down," said state DOT spokesman Beau Duffy, adding that there were no reports yet of serious accidents and that experts would evaluate the situation at 5 or 6 a.m. Tuesday. "We're starting to see the wind gusts pick up . . . Winds are picking up so conditions are starting to deteriorate."

"Anyone who decides to travel despite the ban is going to have very difficult conditions," he said.

In some parts of Long Island, the traffic ban had been activated even sooner.

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Said Brookhaven Supervisor Edward P. Romaine, who ordered people off the roads by 7 p.m.: "We're not looking to harass people. We're looking to protect public safety."Almost all surface transit -- trains, buses and roads -- were either canceled or drastically reduced by 11 p.m. And hundreds of flights were canceled at all area airports.

The LIRR was suspending service, officials said, with the last eastbound train leaving Hicksville about 11:30 p.m. and the last westbound train leaving from Valley Stream at 11:15 p.m.

Nassau's bus service, NICE, and Suffolk's, Suffolk County Transit, were quiet by 7 p.m. And bridges and tunnels operated by the Port Authority closed down at 11 p.m. to all but emergency traffic.

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And the snow will indeed keep mail carriers from their appointed rounds.

Christine Dugas, a spokeswoman for the U.S. Postal Service on Long Island, said there will be no mail delivery Tuesday on Long Island. It will resume on Wednesday "as soon as, and where it is safe to do so," Dugas said.

She said the path to the mailbox must be cleared and salted or sanded, especially on steps or icy walkways. For curbside boxes, customers must clear enough snow so the mail carrier can drive up to the box, deliver the mail and pull away safely.

Cuomo said the ban on city, state and local roadways will be enforced.

Those who violate the ban, he said at a Monday news conference in Manhattan, "will technically be committing a crime" and a summons will be issued. A fine of up to $300 could be imposed, too, he said.

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Cuomo stressed the need for roads to be clear of traffic so that plows handling Long Island arteries can do their job. That job will grow more difficult as the night goes on since snow is expected to accumulate at a rate of 2 to 4 inches per hour, with 24 to 36 inches expected and high winds.

The state Department of Transportation has 415 plows dedicated to Long Island, this includes 199 plows based on Long Island, 184 from other DOT forces around the state and 32 plows borrowed from Thruway Authority, a DOT spokesman said. DOT has 849 operators and supervisors dealing with plowing and road conditions on Long Island.

Drivers, for the most part, appeared to be abandoning the roads even before the travel ban went into effect, officials said.

By 10 p.m., as Nassau County Executive Edward Mangano traveled the roads, the streets were beginning to look a little like a ghost town, he said.

"The roads are well plowed," Mangano said. "The roads are passable. We thank our residents for heeding the warning because clearly the roads were . . . noticeably empty."

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Around the same time, Smithtown Highway Superintendent Glenn Jorgensen said managing the blizzard so far had been fairly easy -- although the peak of the storm was just starting.

"I find that the traffic is very minimal," he said. "We started plowing a little while ago; we usually plow when there's two to three inches of snow . . . it's still manageable."

Jorgensen said 100 town vehicles are out plowing the roads, along with about 70 private contractors. "The state has sent me 10 small plow trucks. I just dispatched them out to some areas."

Suffolk Transit, Suffolk's bus system, shut down Monday night and Suffolk Public Works Commissioner Gilbert Anderson said he did not expect service to be restored until Wednesday.

"I don't see that happening," he said.

The Nassau bus system, NICE, suspended all bus service as of 11 p.m. Monday.

Buses already on the roads will complete their routes, weather permitting.

"NICE anticipates that a.m. service Tuesday will not be provided," the agency said in a statement. "Service will be reinstated as quickly as snow removal efforts allow."

In the air, all flights at LaGuardia Airport and most at Kennedy Airport were canceled Monday, officials said.

All commercial flights scheduled to land at or depart from Long Island MacArthur Airport have been canceled until Wednesday morning, said airport Commissioner Robert Schaefer.

Amtrak announced shortly after 6 p.m. Monday that it had suspended all service between New York and Boston, and between New York and Albany.

Patrick Nowakowski, LIRR president, would not predict when rail service would resume. But he said that lines with higher ridership would be given priority.

"As soon as I can come back, we will," Nowakowski said. "If we can bring one branch back first, we will."

Cuomo said when subways and trains must be kept in yards where they can be protected to avoid some of the damage that was caused to the rails by superstorm Sandy, for instance.

All essential LIRR personnel were on call, Nowakowski, and some train crews were being put up in hotels near important locations to be able to quickly respond to problems. Nowakowski added that the LIRR's Jamaica situation room has been activated, with representatives from all the agency's different departments coordinating efforts.

Nowakowski predicted that the LIRR's $1.5 million, 80-ton snow spreading and ditching machine, nicknamed "Darth Vader," would be dispatched at daybreak Tuesday for the first time to clear tracks in eastern Suffolk, where high snow drifts are expected.

The agency called in extra personnel and took measures to ensure that track switches continue working through the storm, including by heating and lubricating them.

The LIRR also positioned track workers and snow-fighting equipment near important locations to be able to respond to problems.

With Alfonso A. Castillo and Lauren R. Harrison