The failure of police to record the race of all drivers is bound to complicate efforts to monitor racial disparities in traffic stops, according to independent researchers and the law's sponsor. And the missing information could influence whether a department draws the attention of the state attorney general, the American Civil Liberties Union, or private attorneys seeking remedies.

Although most officers did record the race or ethnicity of the driver - in 97 percent of tickets statewide - the failure rate is greater than 10 percent in three towns: Webster, Acton, and Nahant. Boston police didn't record a race on 8.7 percent of tickets.

A Boston police spokeswoman said that often it is impossible to know or even to guess the race of every driver - for example, when an accident investigator mails a ticket to a driver long after the accident occurred. And some of the tickets with no race were handed to a driver, she said, but it was actually the owner who was cited - such as for improper equipment - and the officer can't know the race of the owner.

''In a lot of these cases, the numbers don't tell the whole story,'' said spokeswoman Mariellen Burns.

But an officer in the community with the state's highest failure rate, Webster, echoed sentiments many officers expressed privately: He said he rarely records the race of a driver because he doesn't believe officers should be making guesses about such sensitive information as race. If the state thought it was important to collect information on race, Officer Brian Barnes said, the state should have put that information on all drivers' licenses before such a study.

''I believe that the law definitely leaves the police officer in a predicament,'' said Barnes, who described himself as one of the most aggressive traffic enforcers among the roughly 30 officers in Webster. ''Why should it be incumbent upon me to determine the race? Why do I have to guess? I don't guess your address, I don't guess your name, I don't guess your date of birth.''

Every month, the Executive Office of Public Safety receives computer printouts from the Registry of Motor Vehicles, showing how many tickets from each police department reported no race or gender, according to the office's spokesman, David Goggin. The data collection was ordered by the Legislature in 2000. A Boston Globe analysis of tickets shows a wide racial disparity in the ticketing and searching of minorities.

Public safety officials sent a summary of the data to every police department about nine months ago, with a form letter reminding departments to record the race of drivers on all tickets. But the state has not contacted any department to inquire about high rates of failure to record race.

''This is the first I've become aware of the number of tickets that aren't being checked off,'' said Taunton's police chief, Raymond L. O'Berg, whose department is one of 33 that left at least 5 percent of tickets blank. ''We have a large Portuguese population, and the guys, instead of asking, may be leaving it blank. There's Cape Verdean Portuguese - are they black, or are they white?''

Goggin, the public safety office spokesman, said the agency had not taken any action on racial disparities because it is awaiting a state-sponsored study of the same traffic tickets by Northeastern University. But Goggin also acknowledged that no study is necessary to determine which departments were leaving the race box blank more often than others.

The 2000 law requires police officers to mark one of six races or ethnicities on traffic tickets: American Indian, Asian, Hispanic, Middle Eastern, black, or white. There is no option for `other' or `unknown,' or any bi- or multiracial options. When officers are unsure of a driver's race or ethnicity, they must guess it, from appearance, surname, or language.

Since the law was passed, police in the state have known that the tickets would be studied for evidence of possible racial bias. That might be part of the problem, said O'Berg, the Taunton chief.

''I know the union was very concerned that if you put the wrong race down, it might come back to haunt you,'' O'Berg said. ''So rather than take a stab at it, you leave it blank.''

Most often, police did follow the law, leaving the race blank on only 3 percent of tickets, or 22,000 out of 756,000 tickets that were otherwise complete, according to a Boston Globe analysis of the state's database of tickets. Statewide, the failure rate has hovered around 3 percent each month since the law took effect in April 2001.

But some departments far exceeded that state average. Webster, on the Connecticut border, led with 25 percent of tickets showing no race, or 189 out of 748. Boston police failed to record the race on nearly 9 percent of tickets, or 6,225 out of 71,432.

Other large communities where at least 5 percent of tickets are blank include Lawrence, Fitchburg, Cambridge, Quincy, Methuen, Worcester, Newton, Chelsea, Taunton, and Weston.

The inaction by the state to enforce the law outraged the sponsor of the racial-profiling law, state Senator Dianne Wilkerson, a Roxbury Democrat. Without full information for a study, ''the whole effort is being compromised,'' she said.

''We wanted the officer's own assessment of who the drivers were,'' Wilkerson said. ''So there is no excuse for it being left blank. This isn't a case of, `I wasn't allowed to ask,' or `I couldn't tell.' Who did you think you were talking to? That's what is significant.''

A spokesman for police chiefs said the failure rates merely show a lack of training in enforcing the two-year-old law to combat racial profiling.

''We had recommended that they do some training of officers,'' said John M. Collins, general counsel for the Massachusetts Chiefs of Police Association. ''But there's no money available for training, no video. Politically, the sponsors of the bill wanted a phone number, 1-800-I-HATE-COPS, and a study to prove that police are racist. We wanted more training to change the culture.''

A surprising pattern appears in the records from four urban police departments - Boston, Cambridge, Chelsea, and Worcester. Officers there left the race of the driver blank on traffic tickets far more often when they searched vehicles than when they merely wrote tickets. A stop with a search is a longer, more confrontational encounter than a normal traffic stop.

The pattern is most striking in Chelsea, the second most Hispanic community in the state, where police left the race blank in 48 percent of the searches. In only 5 percent of standard traffic tickets with no search, by contrast, Chelsea police left the race blank.

Chelsea police spokesman Captain Brian Kyes said yesterday that when the Globe informed him of the department's statistics, he sent an e-mail to the entire department reminding officers to comply with the law.

''It's extremely important for the citation that the officer fills in to use their best judgment and fill that race box,'' Kyes said. He also suggested that the numbers of searches are small enough - there were 42 searches, 20 of them with no race recorded - so one or two officers could account for most of the problem, due to a lack of training.

The authority to enforce the law falls to a former police chief in Chelsea, Edward A. Flynn, the new secretary of public safety in Governor Mitt Romney's Cabinet. His office receives the monthly reports showing racial disparities in ticketing and searching. The law allows him to decide when to refer the monthly statistics to the attorney general for investigation, and whether to require police departments with apparent disparities to collect information on every traffic stop, whether or not a ticket is written.

Flynn took office only on Friday. His spokesman said that under the outgoing secretary, James P. Jajuga, the office forwarded a copy of the statistical reports once last winter or spring to every police department in the state, with a form letter reminding them of the law. And Jajuga's predecessor, Jane Perlov, had sent a letter before the law took effect, encouraging police chiefs to follow the law.

But the office has not asked the attorney general to take action against any police department, and it has taken no action to require more data collection or to reprimand departments for not following the law.

Prior to 2001, filling out the box for race on a traffic ticket was optional, said Boston's former police superintendent in charge of training, Ann Marie Doherty, so officers may not be accustomed to checking it. She said police commanders had sent reminders to officers soon after the law took effect. But she said she wouldn't assume that officers who leave the race blank are trying to hide anything.

''I'd be making a huge assumption to say that someone is doing something intentionally, without really looking at the data,'' Doherty said. ''I think it's a huge leap.''