SUDDENLY, YOU LOOK in the rear-view mirror and there's a patrol car behind you.

All the usual questions race through your mind. Was I going too fast? Did I make an illegal lane change? Does the car look suspicious? You check your speed, ease your foot off the accelerator for good measure, and continue to glance nervously back and forth from the road ahead to the rear-view mirror.

Eventually, the patrol car turns off the road or speeds up and passes you. Maybe the fact that it seemed to lock onto your rear bumper for a mile or so was just a coincidence. Or maybe you were being checked out. Officers on patrol in all 50 states are able to routinely check the numbers on auto license plates and discover within seconds whether the plate has been stolen. It doesn't matter if you plate is from Virginia and youreriving in Idaho. The patrol officer makes radio contact with a computer operator who enters the number into a system linked to a ntional computer network; and the reply comes back in a flash.

The license check is just a small example of the investigative wizardry that law enforcement officers throughout the nation can perform because of the NCIC -- the National Crime Information Center. Few communications systems in the world are more complex than the NCIC. It is law enforcement's central computer system, established in 1967 and overseen by the FBI. State and local jurisdictions throughout the country -- on line with their own computer systems -- are also hooked into the NCIC, which offers instant access to information on stolen property, missing persons, criminal histories, and wanted persons.

Last year alone, more than 240 million inquiries were channeled through the center. Within its data base are 12 files, most with self-explanatory labes -- "License Plate," for instance. A sampling of others includes "Missing Person," "Vehicle," "Wanted Person," "Article" (a file for miscellaneous stolen goods), "Gun," "Canadian Warrant" (for fugitives from Canada). Another file is for stolen boats. Stolen securities are also listed.

A case cited recently in an NCIC newsletter tells of a New Jersey police officer who ran an auto check through the "License Plate" file, discovered the car was stolen, and arrested the driver. The suspect was booked, and put up his own bail money. But another officer ran the serial numbers of the money through the "Securities" file and learned that the bills had been taken during a Phoenix bank robbery in 1983. The suspect was rearrested and shipped to Arizona to stand trial.

Similar success stories are legion among law-enforcement officers, who view the NCIC as an invaluable tool. Their enthusiasm, however, is balanced by critics who say the NCIC files offer the possibility of tens of thousands of false arrests and that it is a too-powerful "1984"-like police tool.

"We don't have to choose between good, hard-hitting law enforcement and due process," says Ken Laudon, a New York University computer expert who conducted a congressional study of those electronic criminal records. "The real choice is whether we're going to run these systems responsibly."

For as the computer interconnections between law-enforcement agencies increased into the 1980s, we have already seen a number of flesh-and-blood concerns raised by citizens whose contact with the system revealed its flaws by wrongly landing them in jail.

Worries over the NCIC center on two of the 12 files: the Interstate Identification Index and the U.S. Secret Service Protective.

The Interstate Identification Index, known as "Triple I," makes the NCIC a switching point for the exchange of criminal history information among state and local jurisdictions. Although technically there is no monolithic federal file containing a history of every criminal offender in the country, the FBI maintains an index of every person with an arrest record which is used to refer inquiries to the states which hold the record.

The FBI says it is a "decentralized" system because the actual files are, in fact, held by state and local authorities. Only federal offender records are maintained in the criminal-history file of the NCIC.

But Jerry Berman, legislative counsel of the American Civil Liberties Union and director of the ACLU Project on Technology and Civil Liberties, claims, "It is time to recognize that Triple I is a national data bank controlled by the FBI. More important, it is a national data bank operated in ways which do not assure that records disseminated are accurate or complete . . . . The system is capable of merging intelligence and investigative files so it can be used for surveillance and intelligence purposes."

The potential effect of a national system of access to arrest records is made more dramatic by statistics which indicate that one in five Americans will be arrested at some time in their lives. Of nine million persons arrested in 1980, according to Ken Laudon's study, two-thirds were released or acquitted or referred elsewhere. But, says Laudon, "A mere arrest increasingly tends to produce a permanent criminal-history record, whatever the merits of the arrest in the first place."

Laudon's statistics indicate that nearly one third of the work force in the United States has a criminal-history record, primarily for misdemeanors. Although it is FBI policy to protect the privacy of criminal-history records, they are, according to Laudon's study, "increasingly being used in the employment process . . . . In the FBI's largest criminal history file, 53 percent of the requests for information are for non-criminal- justice purposes -- generally employment by state, local, and federal agencies, and a growing number of private employers."

The U.S. Secret Service Protective File, which became operational in April, 1983, has been used to provide leads and continuing updates on people who pose a threat to the president and others under the protection of the Secret Service. Again, the FBI emphasizes that information in the files "must not be used for licensing and employment purposes," and again, its value has been demonstrated in the identification of individuals who have threatened the president and who were encountered during routine checks.

"Because of the extremely important interest that the Secret Service serves," Berman says, "it was very difficult to oppose the dangerous-persons list. But it is an extremely dangerous precedent for using NCIC and Triple I to communicate subjective, raw intelligence and investigative information over the system, in addition to 'tracking' persons who may have no arrest record or outstanding warrant for their arrest."

Berman's underlying concern is that it may be used as a "wedge" to add other such files to the NCIC.

The debate over the existence, expansion, and use of the NCIC files has existed since its inception. Adding fuel to it now are the real experiences of people whose lives have intersected with the criminal-justice system as a result of computer files. It is a development Ken Laudon predicts will continue.

"We're seeing just the tip of a huge iceberg. In the 1980s . . . people will find that the systems are becoming more connected and more powerful."

Laudon's study showed that a substantial percentage of the warrants listed on the law- enforcement computer networks should have been removed or updated. "Eleven percent of the warrants listed on the system were no longer actionable. They had been vacated. Six percent of the warrants were inaccurate . . . that is, the people were wanted, but not for the crimes listed. And perhaps most importantly, we found about 20 percent of the warrants were more than five years of age, which means they are not likely to be prosecuted."

That immediately places "in excess of 14,000 Americans" at risk of being falsely detained and perhaps arrested because of invalid warrants, according to the study. Eight thousand Americans are at risk of being detained, but subsequently neither indicted nor prosecuted because of the non-serious nature of the offense. And 19,000 Americans whose outstanding warrants are valid but more than five years of age are at risk of being detained or arrested -- but not prosecuted -- due to the age of the warrant.

In addition, innocent citizens with no prior contact with law enforcement are reporting, in some situations, that they have been arrested becuase of a "hit" -- an electronic "wanted" file on a criminal bearing the same name or using the name as an alias.

"We aren't talking here about a small group of hardened criminals," Laudon cautions. "We're talking about average, ordinary citizens who could be stopped on vacation and be subject easily to periods of detention simply due to errors in these poorly managed computer files."

In New Orleans, the ACLU has filed a class-action lawsuit on behalf of "hundreds of persons" who may have been involved in situations of computer-generated false arrest. Bill Quigley, an attorney who has handled such comnplaints in the New Orleans area, says several instances of which he is aware led to imprisonment.

One of the most dramatic cases involves 26-year-old Eugene B. Russell of New Orleans, whose ID was routinely checked by police in the French Quarter on Sept. 4, 1981. He was arrested after local computer records listed a warrant for a man with the same name who was delinquent in child-support payments. The Eugene Russell who was taken into custody had attended the Belle Chasse school for the mentally retarted in Louisiana. He could neither read nor write nor express himself well to authorities.

And he was the wrong man.

"Then, when they brought him to be booked," according to Quigley, "the person who typed his name into the computer typed 'Eugene Brussells' instead of 'Eugene B. Russell' . . . just a mistake on the keyboard.

"From the on, Eugene Russell no longer existed in the prison system. He was transformed into 'Eugene Brussells.' He was put into the general prison population on Friday night, and the next Monday morning when the court said, 'Bring over Eugene Russell,' the people at the computer center of the prison scanned their list and said, 'We have no Eugene Russell.' He stayed there for over 40 days, never having seen a lawyer. And because he had a difficult time communicating and articulating his problem, no one would believe him. It wasn't until his family got a letter that was written by another prisoner and they repeatedly went to the sheriff's office that he was finally brought to court and released."

Russell won a $20,000 settlement from city and Orleans parish authorities.

Sheila Virginia Jackson of Alexandria, Va., is an Eastern Airlines flight attendant. She is 35 years old -- bright, articulate, businesslike. On Oct. 28, 1983, as a crew member of an Eastern flight that had stopped in Mexico, she was passing through customs after her flight had terminated at New Orleans.

"I gave them my passport," she recalls, "and the customs agent ran it up in the computer, left, and came back with a supervisor. The crew was told to go on, that I wouldn't be leaving with them. And I was told they had a warrant for my arrest."

The passport check had turned up a computer warrant for a Sheila Virginia Jackson who had violated parole following a conviction of charges of cocaine possession in Houston. The warrant had been listed on the equivalent of an electronic bulletin board.

"The first thought I had was that one of the crew members had played a joke on me," Jackson said. "Halloween was just a couple of days away, so I turned and said, 'Okay, Halloween's not here yet,' and looked at the customs supervisor. He was not laughing."

Around 4:00 p.m., customs officials took her to a room on the other side of the inspection station. There, she was searched and told she would be transferred back to Texas to serve a prison term. The fact that Jackson could verify that she had been employed by Eastern for 11 years -- including the period of time in which the conviction occurred and the warrant was issued -- did nothing to sway the odds in her favor.

Officers from Kenner, La. -- the police department with jurisdiction over the airport -- took custody of Jackson at 6:15 p.m. Around 8:30, she was transferred deeper into the maze, to the Jefferson Parish (county) jail. Still dressed in her flight attendant's uniform, she was fingerprinted, photographed, and placed in another holding cell. A cardboard sign posted past the jailhouse entry, reads: "This is not Burger King. You cannot have it your way." From there, Jackson telephoned a friend, John Pritchard, who is an FBI agent in New York City.

"I figured if somebody could get me out of it, he could," she said. But even Pritchard could not interrupt the momentum that had sent her to jail.

"We're not gonna turn her loose just because somebody vouches for her," said Jefferson Parish Sheriff Harry Lee. "The only thing we're holding her for is to get her back to Texas to face charges." In other words, it will take a judge to override a computer warrant from another law-enforcement agency.

For the same reason, a New York union representative of Eastern's flight attendants, Glenda Moss -- who had been notified of the arrest by other crew members -- was told during phone calls to both Kenner and Jefferson Parish that there was nothing she could do for Sheila Jackson.

"It was a Friday night," she remembers. "Because I was in a holding cell, I had to sleep on the floor. Criminals were coming in and out all night long. There was one lady, she was so drunk . . . she just looked at me in my uniform and said, 'What are you, the army or something?' And she was just laughing and crying and acting crazy. But she got out before I did. I was treated just like another criminal."

Sheriff Lee doesn't dispute that. "Once we arrested her, she was treated like a criminal. We've become very callous in regards to people saying, 'It's not me.'"

Before dawn on Saturday, Jackson was transferred again, this time to the Jefferson Parish Correctional Center in Gretna. There, she remained in another holding cell until late in the afternoon. That's when someone finally stopped to listen. Correctional-center officer Vanessa Shropshire agreed to help verify her story. "I told her I was getting very tired and I wanted to go home. She was due to get off work in about an hour, and she stayed the rest of the evening to get me out."

Shropshire and other officers, after speaking again with Jackson's friend, FBI agent Pritchard, contacted Jefferson Parish Judge Tom McGee. McGee, in turn, authorized Sheila Jackson's release on a $1,000 bond with a restriction to the state of Louisiana. Eastern Airlines posted the bond and a New Orleans representative of the airline met Jackson at the correctional center and took her to a motel, where she awaited a court order that would allow her to leave the state. On Nov. 2, five days after she was arrested, Jefferson Parish Judge Joseph F. Grefer signed the order that sent Sheila Jackson back home.

The object lesson of Sheila Jackson's experience is clear. "As much as we live in the computer age, operating in billionths of a second," says Ken Laudon, "it still takes hours and days and maybe even weeks to verify a record in the local jurisdictions."

Her arrest also highlights Laudon's concern for the use of such records outside the criminal justice system, as in employment. Even though Sheila Jackson was innocent, a proper inquiry into the Triple-I file from anywhere in the country will reveal the fact that she has now been arrested.

Can't she have her name wiped out of the files? Kenner Police Chief Lentini suggests that it's possible, but expensive -- requiring an attorney and a court order.

In a precedent-setting agreement this month, a court system for the first time has taken an active role in restraining police use of computers. The agreement, filed Sept. 4 in Los Angeles Superior Court, settles a class-action suit filed by the Center for Law in the Public Interest on behalf of five plaintiffs who spent from one to 26 days in jail on wrongful arrests caused by the computerized warrant system used in Los Angeles county.

Among other things, the settlement requires that police apply more stringent criteria in matching a possible suspect with a computer warrant. If a person claims to have been falsely arrested after a computer "hit," the arresting agency is now obligated to make a manual search of its records before jailing the suspect rather than simply deferring to the information displayed on the computer screen. And warrants that have caused a wrongful arrest must be purged from the system and may not be reentered until they are made more reliable.

Attorney Lucas Guttentag, who helped bring this class-action suit, emphasized that it was not the intent of the plaintiffs to dismantle the computer system. The result, he says, is a "step towards insuring that the use of computer technology in law enforcement will not, by design or accident, violate fundamental constitutional or civil rights."

Yet Jefferson Parish Sheriff Lee was still able to come up with the topper when he was responding to inquiries about the use of computers in his parish. Lee produced a thick, folded computer printout from his office desk.

"I had a friend I was with last night," he said. "We were sitting on his patio and he explained to me that when he was in college his driver's license was stolen, and that later they'd arrested someone who'd been using it."

Lee peeled back the top layer of the computer printout and began to unfold it, holding it an arm's length above the top of the desk.

"Just out of curiosity this morning," he continued, "I called and got my friend's date of birth, and then we ran his name on the computer. We came up with this."

Lee allowed the printout to drop to its full length, which stretched from the tips of his fingers to just above the floor. It was a rap sheet of felony offenses, at least a yard long.

"This is my friend," said Lee. "And what aggravates the situation even more, this is a lawyer. If a lawyer doesn't know about this, what does the ordinary citizen know?"

Through the Triple-I file, that record, like a bad credit rating, could follow the lawyer "anywhere in the country," allowed Sheriff Lee.

It was in the computer.