Irregular and unpredictable service frequencies make a bus service less convenient and may sometimes be a serious problem.
In practice, it is impossible to ensure that services operate with identical intervals between buses. Where services are operated to schedule, a divergence of no more than three minutes from the scheduled time for at least 90% of journeys is normally acceptable on urban services.
If services are not operated to schedule, the level of irregularity may be regarded as unacceptable if the interval between buses exceeds the average interval for the route by more than 100% on more than 50% of occasions. On a very intensive service, if fewer than 10% of passengers must wait for more than 10 minutes, the question of regularity is largely insignificant.
Irregular bus frequencies may be caused by inefficient operating practices such as the system of “full-load dispatching,” where buses wait at terminals until they are fully loaded before they depart instead of operating to schedule.
Inefficient route planning may also be a contributory factor, especially where several routes operate along a common corridor and their schedules are not coordinated.
Inappropriate operating structures or company size may be a contributory cause: it is far more difficult to provide regular services when they are provided by a large number of small operators working independently than by a single operating unit responsible for the entire service on a particular route.
Even if services are scheduled to provide a regular frequency, determined according to passenger demand at different times of the day, adherence to schedules is often extremely difficult because of traffic congestion.