James Frederick was in Manhattan cycling west in the Prince Street bike lane on a recent morning when a green Ford parked in the lane forced him to swerve into the narrow roadway where cars and vans were rushing past.

“It’s kind of scary because the cars next to you just keep going,” said Mr. Frederick, 49, a messenger who lives in the Bedford-Stuyvesant neighborhood of Brooklyn. “The city just put this lane in a few months ago, but it’s not respected by drivers.”

On streets clogged by pollution-emitting cars, buses and trucks, New York City’s quest to establish reasonably safe cycling paths by adding to its roughly 300 miles of bicycle lanes has been welcomed by cyclists. But the lanes are often battlegrounds between cyclists and drivers who seem undeterred by the clearly demarcated paths.

Although city regulations forbid cars from blocking bike lanes  a violation that carries a $115 fine  those rules are routinely ignored by drivers who use the lanes as parking spots, loading zones and places to pick up passengers. Such maneuvers have enraged cyclists who say they are unlawful, rude and dangerous.