Bus bunching is often blamed on traffic or scheduling, but in my experience in NYC, a lack of enforcement and/or accountability plays a role too. I live near one end of a bus line and commute to the other end 5 days day a week. On a daily basis, there are large gaps where buses miss their scheduled times. Then, as they approach the end of the line, they arrive and depart in groups of three or four, which only worsens the problem.