Kirsten Gwynn has no memory of the moment she was hit by a bicyclist while jogging last month along Boathouse Row. When she tries to picture it, all she sees is a bike tire. The bystanders calling 911, the ambulance rushing her to intensive care, the doctor telling her she had a skull fracture? All a blank.

Gwynn was in Hahnemann University Hospital for three days and bedridden for weeks after she returned to her Center City apartment.

Gwynn, 25, a nursing student at the University of Pennsylvania, who expects to graduate next month, struggles with dizzy spells, hearing loss, and, if she moves too quickly, a sensation that her brain is "jumbled."

But she feels lucky things weren't worse. At least two Philadelphia residents died last month after cyclists hit them: Tom Archie, 78, who was struck by a bicyclist going the wrong way down a street in South Philadelphia, and Andre Steed, a 40-year-old paralegal, whom police found bleeding on the ground at 16th and Locust Streets after an apparent collision with a bike.

"I still can't believe this happened to me," Gwynn said. "This was something I never even thought of as a possibility. But no matter how bad it gets for me and how frustrated I am, I'm back to normal life for the most part."

More people are biking in Philadelphia than ever, according to the Bicycle Coalition of Greater Philadelphia. More than 11,000 people pedal to work on a typical day, and an estimated 36,000 make that trip by bike at least once a month, the coalition's bike counts of 2008 showed.

But the growing biking population has also brought more rogue cyclists who don't follow traffic laws and can cause accidents.

And unlike reckless drivers, bike scofflaws typically get off free. They often flee accidents, and though police can issue tickets for violations, they almost never do. Philadelphia police wrote just 14 tickets to bicyclists last year, compared with more than 200,000 to drivers.

"Are we as diligent about it as we should be? Probably not," Philadelphia Sgt. Ray Evers said. "But we have to prioritize. We can't even stop every car violation we see."

Police are hoping to change that, at least for a few hours, when they launch an effort to inform cyclists of the laws of the road. In the next few weeks, officers from the Ninth Police District, which includes Rittenhouse Square and much of Center City, will flood the area to write tickets. Members of the Bicycle Coalition are expected to be there to hand out information about responsible biking.

"I've always received complaints," said Philadelphia Capt. Dennis Wilson of the Ninth District. "People get so fed up that I've had patrolmen get yelled at for biking on the sidewalk."

Bicyclists are bound by the same laws as drivers. They are supposed to stop at lights and stop signs and to signal before turning. On roads that don't have a bike lane, cyclists are supposed to keep to the right when possible. Drivers are supposed to treat bicycles as cars and to stay out of bike lanes, except when making a turn. And in Philadelphia, only children 12 and younger may legally ride on sidewalks.

In reality, cyclists often zip along sidewalks, weave between cars, and speed the wrong way on busy streets. Some bike messengers and others ride bikes with fixed gears and no brakes.

Some cyclists consciously flout the rules, but many are unaware of them, said Breen Goodwin, educational director for the Bicycle Coalition.

"A lot of people don't understand that a bike is a legal motor vehicle, whether you're a motorist who's annoyed that you're stuck behind a bike or whether you're a cyclist who's riding on the sidewalk because you feel safer there," Goodwin said.

The city has worked in recent years to accommodate the growing number of bicyclists. The addition of bike lanes on Pine and Spruce Streets has more than doubled the bicycles there, according to the coalition, and many bikers say the corridors have been made safer for cyclists and motorists alike.

Goodwin said she believed stronger enforcement of traffic laws would have an impact on irresponsible bikers.

"If a cyclist runs a red light, they should be stopped," she said. "A huge part of this is just educating people and teaching everyone that if you give respect, you'll get respect."

Police don't keep data on how many people are hit by bicyclists, largely because combing through the city's thousands of injury reports would be extremely difficult, they said. But anecdotal evidence from people who have reported a wide variety of injuries suggests the accidents are relatively common.

Gwynn doesn't know what caused the cyclist to strike her from behind Oct. 14. By the time passersby called 911, the cyclist was long gone.