Boston police last year made 241 drunken driving arrests, or just two every three days, a sharp decline from recent years and far fewer than in cities of similar size, raising questions about how aggressively police are patrolling city roads.
Since 2009, in fact, arrests for impaired driving have fallen by one-third, to a total that pales in comparison with other jurisdictions. Denver, which has a slightly lower population, made more than 3,000 arrests last year. Police in Charlotte, N.C., a somewhat larger city in a smaller metro region, made nearly 1,600. Arrests in Philadelphia were nearly 20 times as high as Boston, according to arrest figures obtained by the Globe.
The vast disparity stunned law enforcement officials and opponents of drunken driving alike, and fed doubts about where enforcement ranks as a priority as city police fight violent crime, protect rough neighborhoods, and respond to emergencies.
"It can't possibly be a priority given those statistics," said Ron Bersani, who has lobbied for stricter drunken-driving laws since his granddaughter was killed by a drunk driver. "It's skewed beyond belief."
Boston police say the arrest levels reflect the city itself, a walkable place with widely available public transportation that reduces the number of impaired drivers. They also note that "nearly all the major roadways in and out of Boston," including Interstate 93, the Massachusetts Turnpike, and Storrow Drive, are patrolled by State Police.
In Boston, State Police have made more than 1,400 OUI arrests since the start of 2010. Most arrests follow direct observation of erratic driving or a response to a report from another driver.
But activists noted that arrests in Boston are far lower than in Washington, another walkable city in a large metro area with many college students and public transportation.
"It does raise questions," said Jan Withers, national president of Mothers Against Drunk Driving. "We really look to law enforcement to reduce drunk driving, and it could be they just don't have a focus on keeping the roads safe."
Through a spokeswoman, Boston police declined requests for further comment.
Thomas Nolan, a lecturer at Tufts University and former Boston police lieutenant, said drunken driving enforcement is seen as secondary, a distraction from more important duties.
"It never has been a priority," he said. "That's been a firmly ingrained part of the culture. It's not even on their radar."
During his years on the force, officers quickly got the message that Boston police had "better things to do" than make road stops, Nolan said.
With manpower at a premium, such arrests were seen as time-consuming luxuries, he said.
"If you have to get involved in an OUI situation, you're off the air for three hours," he said.
Police in other cities expressed disbelief at Boston's arrest figures, particularly given the size of the Boston metro area, the nation's 10th largest.
"You're kidding. That's a very small number," said Don Bickel, who directs the Traffic Safety Partnership in Marion County, which includes Indianapolis.
Bickel estimated there are about 3,000 OUI arrests a year in Marion County, which has a population of 918,000. Bickel oversees a specialized enforcement program that on its own made almost 800 arrests last year, more than three times the Boston total.
"That's six nights a month," he said. About six officers patrol the roads, and once a month conduct a well-publicized sobriety checkpoint.
"We have crime problems like everyone else, but we think it's a priority," Bickel said. "We could do a whole lot better."
Boston police do not conduct checkpoints. State Police conducted 73 that led to 542 arrests.
Overall, State Police arrests for drunken driving rose sharply last year, from 3,884 to just over 5,000, reversing three years of decline. Arrest numbers correspond to the size of the agency's patrol division, and that the first new class of recruits since 2006 arrived last year, a spokesman said.
In Washington, D.C., whose population is slightly lower than Boston's, police last year made 1,633 DUI arrests, nearly seven times Boston's total.
"It's a big priority," said James Crane, commander of the police department's special operations division.
The department conducts "saturation patrols" once or twice a week, and has a sizable presence during holidays and big games, Crane said.
Police there also focus patrols on high-crime areas, where stops often lead to discoveries of drugs and weapons.
"Any police agency will tell you, traffic stops can lead to more serious investigations," Crane said.
That kind of visible enforcement has been shown to deter drunk drivers, researchers say. Apart from arrests, prominent roadside stops and checkpoints send a strong message that driving under the influence has consequences.
"If police do something that's visible and frequent, and it's publicized, then people say 'I better be careful,' " said James Fell, a senior researcher at the Alcohol, Policy and Safety Research Center at the Pacific Institute for Research and Evaluation.
The Boston numbers, Fell said, suggest drunken driving enforcement must be a relatively low priority.
"If you do a traffic stop at night, the probability that police will encounter a drunk driver is pretty high," Fell said. "Those numbers look awfully small."
Statistics show that drunken driving arrests have fallen in most parts of the city, from Mattapan to East Boston. Downtown, arrests have fallen by nearly half since 2008.
Arrests in Roxbury and Mission Hill jumped slightly last year.
In February, after a Boston police officer drove the police commissioner's son home rather than arrest him for drunken driving, BPD officials said officers have the discretion to decide whether an arrest is necessary.
Forgoing arrests, which can take several hours, also enables officers to remain on patrol.
But with minimal fear of arrest, more people will get behind the wheel after drinking, advocates say.
"There's nothing more important than enforcement," said Anne McCartt, vice president of research at the Insurance Institute for Highway Safety, a nonprofit group dedicated to reducing driving crashes. "Study after study shows it."
Statewide, enforcement appears to be making gains. Alcohol-impaired driving fatalities have fallen from 155 in 2007 to 114 in 2011, government figures show. Stricter laws and greater public awareness may also be playing a deterrent role.
Robert Hedlund, a state senator from Weymouth, said he'd like to think there are fewer drunk drivers on the road, but isn't convinced.
Driving through the city late one recent night, he spotted several erratic drivers on the highway.
"If I were a state cop on duty, I would have had reasonable suspicion to pull [over] a number of them," he said. "Obviously they are on city streets before they get on the highways."
Peter Schworm can be reached at email@example.com. Follow him on Twitter @globepete.