IT seemed like a good idea at the time.

Save gas money, be environmentally correct, lose weight  just by biking to work. And so after two decades, Dan Cooley, 41, saddled up a silver 21-speed Raleigh in April to make the daily two-mile commute to his nursing job at a senior citizen center in Louisville, Ky. In four months, he lost 15 pounds. Way to go, Dan!

Friday morning, July 25, around 6:50 a.m., he was pedaling on a residential street, wearing his green hospital scrubs, when a Volkswagen roared out of a driveway in front of him. Swerving to avoid the car, Mr. Cooley cursed loudly and rode on.

The driver and his passenger cursed back. As Mr. Cooley pulled over to the sidewalk, the car turned onto a driveway, knocking him off his bike. In Mr. Cooley’s narrative, the passenger, swearing, jumped out and pummeled him. Then he got back into the car, which zoomed away. Mr. Cooley lay prostrate on the sidewalk, bloodied, with a concussion and a torn ligament.

“We’ve had a car culture for so long and suddenly the roads become saturated with bicyclists trying to save gas,” Mr. Cooley said 10 days after the attack, still feeling scrambled, in pain and traumatized. “No one knows how to share the road.” He doesn’t plan to bike to work again this season.