With his lean frame topped by a shock of thick graying hair, 42-year-old Alan Kayser is the epitome of a seasoned cyclist. He does not look like a criminal, but recently he was pulled over on his bike and fingerprinted by a sheriff’s officer.

“He fingerprinted everybody,” Mr. Kayser said. “That’s how they ID people.”

His crime? Running a stop sign on his bike.

As a growing number of people in the Bay Area take to bicycles  to be green, healthy or thrifty  there are signs of an emerging backlash against those who fail to follow the rules of the road.

Mr. Kayser was stopped in Portola Valley, an idyllic suburb of gently sloping, tree-lined roads. On weekend afternoons, cyclists are as ubiquitous there as smiles, following a route nicknamed The Loop. You will not find a single traffic light, but there is a three-way stop at the one significant intersection of Portola and Alpine Roads.