Traffic droned on the New Jersey Turnpike just as on any other Tuesday morning. Commuters passed through the rest area here just as sleepy-eyed. But something was amiss, and if Robert Cieplensky hadn't spotted it someone might be dead now - in New York, perhaps, or Washington or Toronto - from the shrapnel of a homemade bomb.

Cieplensky, a New Jersey state trooper, saw a disheveled young man in the rest area parking lot, walking in circles, looking under a car, heading one way, then another. When the man got into a brown Mazda and pulled out - narrowly missing parked cars, the trooper said - he stopped him.

The Mazda turned out to contain not just Yu Kikumura, later identified as a member of the terrorist Japanese Red Army, but also electrical fusing devices, two clocks, precision tools, U.S. airline timetables, maps of New York City and other places, a bottle of Ten High whiskey and 36 $100 bills.

A search also found three foot-long steel canisters packed with gunpowder and lead shot.

Kikumura faces sentencing Monday in Newark following his November conviction for transporting the bombs "with the knowledge and intent that they would be used to kill, injure and intimidate." He faces 27 to 33 months behind bars. But U.S. Attorney Samuel Alito argues that this case requires a judge to go beyond the guidelines, that Kikumura intended to kill and maim U.S. citizens in retaliation for the 1986 bombing of Libya.

Though no one knows the bombs' intended target - sources speculated it was a meeting of finance ministers in Washington or an economic summit in Toronto or that pinholes in a New York map marked the spot - Kikumura's case and others like it raise a troubling question:

Are Americans, who have long felt safe at home, becoming more vulnerable to terrorists?

"The fact that we have been free from terrorist acts to the extent we have is in some ways amazing," said U.S. Attorney George Terwilliger III, who last year prosecuted three Lebanese natives stopped just inside the Vermont-Canada border with makeshift bombs. Federal investigators said they belonged to an organization that assassinated Lebanon's President-elect Bashir Gemayel.

"Are we vulnerable? Absolutely," said Frank Bolz Jr., whose consulting firm trains other nations' police in handling terrorist incidents.

"This is a democratic country, with wide-open borders," added Brian Jenkins, a terrorism expert at the Rand Corp. think tank in Santa Monica, Calif. "It's not as though we're protected by a high wall."

Nonetheless, Jenkins said, the United States generally lacks the driving forces of home-grown terrorism: ideology (usually "Americans kill in personal quarrels, not for causes") and ethnic grievances (the independence movement in Puerto Rico is an exception).

As for foreign terrorists, why attack on American soil when there are convenient U.S. targets abroad, from embassies to fast-food restaurants? "It's easier to cross a street than cross an ocean to commit a terrorist attack," as Neil Livingstone, author of several books on terrorism, put it.

Still, despite safeguards, bombers have slipped through.

Even though their numbers have not been great - the FBI counts seven domestic terror incidents in the last year - law enforcement officials know the potential for destruction and intimidation of even a single well-placed explosive. (The Vermont bombs blew 2-foot-square steel plates the length of a football field when agents set them off.)

Americans' best protection, authorities say, will come from continued toughening of anti-terror laws and swift justice for those caught. At least as important is intelligence work.

In Bolz' words, "We have to get to know more about the enemy."

"We can't wait for the bomb to go off," said the FBI's counterterrorism chief, Neil Gallagher, who added agents would use "whatever resources the judicial system would allow . . . (to) neutralize a terrorist act before it occurs."

Civil liberties groups are watching for overzealousness, something the FBI tacitly acknowledged last year in disciplining agents for an undercover probe of the Committee in Solidarity with the People of El Salvador.

The FBI's counterterrorism effort costs tens of millions of dollars annually, said Gallagher, who declined to be more specific because the budget is classified.

"I always say it's been 95 percent blood, sweat and toil by those agencies involved, but we've had a bit of good luck, too," said Victoria Toensing, a former top terrorism policy official at the Justice Department.

She said the tools of anti-terrorism have improved.

There's better cooperation among investigators and prosecutors - the FBI, which once balked, she said, now "has the coffee waiting" for Justice Department lawyers called to an investigation. And laws have been toughened - extradition agreements have been revised, and it's now a federal crime to attack an American abroad, she noted.

These changes have not halted terror altogether. In 1986 alone, the worst recent year for domestic attacks, the FBI recorded 24 incidents and nine preventions.

In 1987, the FBI's undercover sale of an inert anti-tank rocket to members of the Chicago-based El Rukns gang led to conspiracy convictions and long prison terms. The government said they had offered to blow up U.S. targets for Libya.

"A domestic group was willing to commit a terrorist act in the United States," the FBI said, "in order to receive funding from a known state sponsor of international terrorism."

In Hartford, Conn., members of Los Macheteros, a Puerto Rican separatist group, are on trial now, charged with conspiracy in a violent $7 million armed robbery there. And three alleged revolutionaries accused of plotting a series of robberies and bombings, including one that injured 22, are being tried in Massachusetts.

A 1983 explosion 30 feet from the Senate chamber in the U.S. Capitol was blamed on a group protesting U.S. foreign policy.

Before and since, groups have detonated explosives at banks, at government offices, under police cars - from anti-Castro organizations in Miami to affiliates of the Aryan Nations in Idaho. The tradition of American terrorism goes back to the Weather Underground, to the 19th century coalminer-terrorist "Molly Maguires" in Pennsylvania, and beyond.

In recent years, the acts and threats have changed the American landscape and lowered Americans' resistance to searches. Security barriers shield the White House and other public buildings, and airport scanners, having turned up 41,000 guns since 1973, are a fact of life.

Not all of the security gates are closed, however, according to Robert Kupperman of the Center for Strategic and International Studies in Washington: "If you hit some of the key nodes" _ gas transmission lines, water systems or electric power grids _ "you're going to have big trouble."

In a 1987 mock attack, an Army Special Forces team acting as terrorists symbolically "knocked out" power substations, an emergency operations center and valves of the Strategic Petroleum Reserve in Louisiana and Texas. The U.S. Department of Energy its tightening security for the oil stored against an embargo.

It was no mock attack two years ago when a bomb destroyed an employee's car at the Lawrence Livermore National Laboratory in California, where nuclear weapons are designed. Claiming responsibility was the Nuclear Liberation Front, a group never heard of before.

There is no "profile" to use to spot a potential bomber, the FBI's Gallagher said, citing the case of Raymond Levasseur, on trial now in Massachusetts with two others on seditious conspiracy charges.

"Their group began up in Maine as some husbands and wives who formed up a leftist terrorist group. What identifies them in advance as terrorists?

"Once they began the series of bombings, bank robberies," he said, "there is a series of trails they left. They kept records. They kept safe houses. All that has to be methodically pieced together."

Sometimes the piecing together is beforehand. The decade-old Foreign Intelligence Surveillance Act court is described as an important anti-terrorism tool.

The court, which convenes in a secret room in the Justice Department headquarters with judges rotated every two weeks, hears 500 applications a year for electronic surveillance in foreign intelligence, spying and terrorism cases.

It was a FISA court-approved wiretap that alerted FBI agents to a bomb aboard a Los Angeles-to-Boston flight in 1982. Three men convicted of conspiracy and transporation of explosives appealed, saying their rights had been violated, but the convictions were upheld.