On paper it looks neat and clean, but the plan to evacuate residents from parts of five counties seems also to have the potential for creating history's biggest traffic jam.

It is no laughing matter, of course, but the scenario has the seeds of ultimate irony for our times - chased from their homes by the errant atom, the residents stall bumper-to-bumper behind their internal combustion engines.

"Don't quote me," said an American Red Cross official, "but you're right - if that siren blows, you're going to see the damnedest rush imaginable. Nobody is going to follow that plan of a scaled evacuation."

State and federal officials are weighing the possibility of evacuating an estimated 630,000 people to safer ground if the danger of a nuclear disaster increases at Three Mile Island.

It is foregone, as President Carter suggested after a visit to the crippled nuclear power plant today, that at some point in the coming days people will likely be ordered out of this area.

The idea would be to get them out of the range of the lethal radiation that could be emitted into the atmosphere if there is a reactor core meltdown at the Metropolitan Edison Co. plant.

In buses, government and military vehicles and private cars, they would be expected to leave the area within a 20-mile radius of the plant, which is about 15 miles south of here.

According to the plan, the residents of the downwind quadrant closest to Three Mile Island would be mobilized to evacuate first.

The two key imponderables how ever, are the wind itself - withs its unpredictable shifts - and whether evacuation is forced by a sudden crisis at Three Mile Island.

Said another Red Cross official at Middletown, a community three miles from the plant: "I feel it's going to end up in chaos and cause more problems than the accident. We're going to have to trust in other people and right now I don't think we can do that."

The Red Cross has geared up to receive people who need food and shelter at designated schoolls, churches and community centers outside the 20-mile perimeter.

Each county government is responsible for arranging transportation for residents who have no other way out.

The Red Crossm beefed up with disaster aid personnel from across the country, handles shelter and feeding for those with no place else to go.

Here in Duaaphin County, for example, residents would be sent to centers that are from 40 to 80 miles north and northeast. The county has lined up 1,000 buses and today asked President Carter for more federal vehicles.

About 400 National Guardsmen have been put on alert to help with traffic control, which county civil defense chief Kevin Molloy said "will be the key to whatever we do."

He said that about 150 county prisoners, held for serious offenses, would be sent to other jails outside the area. But 80 others, convicted of lesser offenses such as drunkeness or prostitution, might be freed by judges now reviewing their cases. "You might call it an accelerated work-release program," Molloy quipped.

Many people left this area before the weekend after Gov. Richard Thornburgh urged pregnant women and preschool childfen who lived within a five-mile radius of the plant to go. Those departures left restaurants and shops shorthanded, a situation expected to continue until the emergency ends.

The 40-mile-wide swath of central Pennyslvania that would be covered by an evacuation order includes some good-sized towns that have traffic problems even on their best days-Harrisburg (pop.68,000), York (47,000), Middletown (11,000) and Hershey (9,000).

The area happens to be blessed with a system of modern, criss-crossing interstate and four-lane highways that lead out in all directions, which would ease movement somewhat.

"Yes," said an official at the state Emergency Management Agency, "a lot will depend on weather and conditions down at Three Mile Island when and if we go to the evacuation."

"We do have a good highway system and we will make heavy use of these interstates. I just don't think you're going to see a traffic jam.".

He'll have other things to worry about, of course, but at least Gov. Thornburgh wouldn't have to fight the traffic. He and other top government officials would spend the duration in a radiation-proof bunker 50 feet below the Department of Transportation building here.