Just stay home.

So said the region's transportation agencies as they prepared Friday for up to 20 inches of snow and scouring wind gusts up to 40 m.p.h.

"What we're really trying to hammer home is for people to stay off the roads," said Leslie S. Richards, Pennsylvania's transportation secretary. "The more people can stay off the roads, the more our equipment operators can do their job."

Through Friday, local, regional and national transportation agencies listed slowed, limited, or canceled services. SEPTA ended bus, trolley, and Regional Rail service until at least 4 a.m. Sunday, while planning to keep the subway rolling. PATCO trains will depart later. Amtrak was reducing trains on the Northeast Corridor. Highway agencies planned lower speed limits. Even bikers were out of luck. Indego, the bike-share program, was suspended as of 6 p.m. Friday, the city reported.

"It's going to be very difficult conditions," said Jeff Knueppel, SEPTA's general manager.

The risk, rail officials said, is not the snow accumulation. "The wind is what makes this storm unusual," Knueppel said. "It's the lack of visibility."

Wind can knock down trees, wreck electrical wires, and create whiteout conditions for operators, he said. If a train lost power or had its path blocked by tree limbs, it would be difficult for repair crews to reach the train to retrieve passengers or repair the line, he said.

A small army of SEPTA staff will be deployed, about 1,500 employees and 500 contractors, to clear platforms and make repairs as needed, Knueppel said. The agency will run some trolleys and Regional Rail trains Saturday, but that's just to keep rails clear of ice and snow, not to move passengers.

SEPTA's suspension of everything except the Market-Frankford and Broad Street Lines was to begin by 4 a.m. Saturday. The agency planned to have some service restored by 4 a.m. Sunday, but Knueppel said the time, and the level of service, could change depending on the severity of the storm and the damage that would need to be cleared. SEPTA warned that some bus service would likely not resume Sunday morning. SEPTA planned to release more information on its website, septa.org, Saturday afternoon or evening.

Though some of the Market-Frankford Line is above ground, Knueppel said the subway's "hardy lines" would weather the storm.

Amtrak's Northeast Corridor serves riders from Washington to Boston, outside the reach of the storm, spokesman Craig Schulz said. The need to transport those people was a reason Amtrak trains would keep running. Canceling some trains, though, gave the rail agency flexibility.

"The reduced schedule gives us a little bit of extra time to service equipment in the terminal," Schulz said.

Passengers whose trains are canceled were being contacted with information about alternative departure times, he said. Information about schedule changes will also be available on amtrak.com.

For those who brave the roads, the Pennsylvania Department of Transportation offered real-time information about snow plowing through its website 511PA.org, which tracks plows and offers video or pictures of road conditions.

PennDot was deploying 450 snow plows to state roads in the region, officials said, and 13 tow trucks would be roving the roads through Sunday. A 45 m.p.h. speed limit was planned on expressways, Richards said. The Pennsylvania Turnpike was reducing the speed limit to 45 m.p.h. at 12:01 a.m. Saturday from Exit 161 to the New Jersey state line, and banned some heavy truck traffic, officials said. The New Jersey Turnpike expected speed limits to drop to 35 m.p.h. while plows were on the road.

NJ Transit planned to run trains, but would cross-honor passes Saturday and Sunday, allowing riders to use bus passes on alternative modes if needed.

From SEPTA to PennDot, transportation agencies planned to have service back to normal, more or less, by the Monday morning commute.

"Unless there's a drastic change in the forecast," said John Krafczyk, the assistant district executive for maintenance, "Monday we'll be ready to go."