Arlington County Police Chief Edward A. Flynn presented some new and convincing evidence this week that bicycle helmets can save lives: his own noggin.

Holding his cracked bicycle helmet, Flynn told a crowd of reporters that he fell off his bicycle on Memorial Day and landed on his head. Had he not been wearing the helmet, his doctor told him, he could have been seriously injured or even killed.

"The word the doctor used was 'catastrophic,' " Flynn said. "I was lucky. I happened to put on my helmet that day."

Flynn, who called the news conference Monday to talk about his accident, said he didn't mind looking foolish in front of the Washington area if it keeps someone else from getting hurt. When the police chief fell off his bike, he hit the right side of his head, then his shoulder, fracturing a bone.

Wearing a sling on his right arm, Flynn stood near the spot on the Custis trail in Arlington where he had hit the pavement. Holding up his helmet--"you can see where it split"--he said the accident happened so fast that there was no time to react.

"The reaction time you have is minimal," Flynn said. "I didn't even have time to say 'uh-oh.' "

Flynn said he and his wife, Susan, were about six miles into a 10-mile ride on Memorial Day when he fell. Because it was his first time on his new mountain bike, he decided to wear his helmet.

They were riding down a rather steep hill at a pretty good clip, Flynn said, and he reflexively hit both of his hand brakes, instead of just one as he should have done.

"The bike stopped, and I didn't," Flynn said.

The impact of his helmet hitting the pavement made a loud noise, and his wife, who was ahead of him, quickly turned to come to his aid. "She heard this big noise and tried to do this big U-turn," Flynn said. She fell, too, and scraped herself, he said.

Flynn was rushed by ambulance to Arlington Hospital where, lying on a stretcher, he vowed that if everything was okay, he would share his story and implore parents to "be a good example to your kids."

There is no federal law requiring bicycle helmets, and states and localities began adopting laws in 1987, but they don't cover adults in most cases, according to Randy Swart, director of the Bicycle Helmet Safety Institute, who attended the news conference in support of Flynn.

"It's big of him to make a mistake and admit that he crashed," Swart said of the police chief. "Especially a man in authority."

In Arlington, only children 14 and younger are required to wear bicycle helmets, police said. Those who are caught without them typically will be required by the juvenile judge to take a bicycle safety class, police said.

"It's not meant to be punitive," said Kip Malcolm, an officer with the 2nd District bicycle patrol. Malcolm said only about half of the adults he encounters on the street are wearing helmets. "The disadvantage: You mess up your hair," he said. "The advantage: You save yourself from a serious head injury."

Flynn, who will be wearing a sling for four to six weeks, conceded that he has not always worn his helmet during bike rides. He will now, he says.

"The thing that struck me, besides the pavement, was how close I came to death," Flynn said.

CAPTION: Arlington County Police Chief Edward A. Flynn fell off his bicycle but injured only his shoulder because he was wearing a helmet.