Federal Hill resident Dr. David Scharff is an avid cyclist who races with a group called Lateral Stress Velo. An internist, he regularly commutes by bicycle to his Canton office, a distance of about four miles. Scharff rides 5,000 to 7,000 miles each year, but he says experience is no guarantee of safety. Recently, he says, a friend was riding in Hampden when a car door opened in front of him. "He went flying over the handlebars," Scharff says. "Luckily, he was wearing a helmet and nothing happened."