Officer Lynn Davis has a map of Rosslyn in his head. Somewhere under the crown of the baby-blue helmet is a mental microchip, packed with the timing of traffic signals and the patterns of rush-hour motorists and the first names of shopkeepers.

After 24 years with the Arlington Police Department, 22 of those on the motor squad and 18 patrolling in Rosslyn, Davis turned in his 1985 Harley-Davidson and retired last Friday.

"When you needed to know something about Rosslyn," said Cpl. Phil Beuchert, Davis' supervisor for the last year, " other officers would say, 'Oh, you need to ask the mayor,' and we all knew that was Lynn.

"I can tap him as a resource," Beuchert said last week. "When the memory bank has left us, we're going to have a little bit of a vacuum . . . . We'll kind of have to fill that gap."

To some, motor patrol is a thankless necessity of police work -- a dry routine of handing unwelcome tickets to irate motorists and untangling the same traffic knots day after day.

But it was the glamor of the motor squad officers, with their sparkling machines and independent style, that appealed to Davis shortly after he joined the department on Oct. 16, 1962.

"It was just something about the motors themselves . . . . In those days, it was just common knowledge that the motor officers were the elite of the police, spit and polish-type individuals, able to go out and do a job with little supervision," Davis said.

His colleagues say Davis has carried the stripe of old-fashioned police work through a quarter-century of change in the county and in the department. Over the years, as the motor squad shrank from 30 officers to fewer than 10, Rosslyn's skyline jutted upward and its streets grew more crowded.

Davis "still has some of the old officer-type things," says Phil Talbott, who worked with Davis on the motor squad for 13 years.

"He knew all the business people," Talbott said. "The new officers don't have time for that . . . . He saw the county go from a little quaint place he was policing to a major urban city."

Some things didn't disappear with the years, such as the respectful distance between the man and his motorcycle, Davis said.

At the beginning, "I was scared of something that big, knowing that if you lost control and your hand slipped and you let the throttle up . . . . "

In time, he learned how to ride and not to fall, but he never forgot that fear.

"I look at it this way: The day you lose your fear of it, that's the day it's gonna get you down and stomp on you."

One morning of Davis' last week at work, the Harley-Davidson stood at the corner of Wilson Boulevard and North Oak Street, wheezing exhaust into snowy air. Davis sat straight up, baby-blue helmet on his head, baby-blue stripes on his pants disappearing into boots planted firmly on the asphalt.

Moments later, in the Commonwealth Carry-Out in Rosslyn, customers greeted Davis by name and owner Tom Grammen put a slice of pie in reserve. "It was 20 degrees this morning; he never once complains," Grammen said. "It's unbelievable that he drove that thing so many years. He's my protector; I know I can get him in here in a hurry."

Davis said there have been times when the work seemed less than heroic. "Sometimes, it's hard to do a job where you know you're only doing temporary relief," he said. "You ticket an illegally parked car one afternoon; the next day someone else tries to sneak into the space. The county grows, and the traffic just keeps coming.

"If you stay around long enough, the same things will pop up again. The problem disappears for a while, then it starts sliding back into its old pattern," he said.

"You go out and do what you can. You know deep down that you're not going to totally solve a problem . . . . If you stop someone, and that prevents an accident, your job is done."

In 22 years of doing his job astride a Harley-Davidson, Davis became the police department's in-house expert on matters ranging from funeral escorts to traffic light wiring, his colleagues said.

"He goes out, he goes to Rosslyn, and he knows every inch of it," said Rich Baker, a radar officer who has worked with Davis for three years. "He knows what time to be where in order to regulate traffic. You can learn just from hanging around him."

Problems that would send other officers scurrying for traffic manuals could be resolved with a glance from Davis, Beuchert said.

When someone reports a malfunctioning signal, for instance, "If I go look at it and it's red and then it's green, it looks okay to me. If you send Lynn, he can say, 'Well, it's in its rush-hour cycle at 10:09 and it was supposed to change nine minutes ago' . . . . He keeps all of this in his head."

Younger officers talked of Davis as a taciturn teacher who pointed out the right way of policing by quiet example.

"He's not a talker. For him to explain how Rosslyn works . . . he can't do that," said Beuchert. "But all my new officers, that's who we put 'em with to learn how to do a funeral escort. It didn't take a lot of words -- you just watch Lynn."

In retirement, Davis said, he plans to "take it easy and relax, do what I want to do" during the winter and spring, then start looking around for a new challenge. "There are too many things you can do if you're willing to work."

For now, motorcycle-riding is not part of the plan. "I have no desire to buy one . . . . Lord, no; they're too expensive," Davis said. "I'm not going to give up my operator's license, though."