Many public bus services are run to a specific timetable giving specific times of departure and arrival at waypoints along the route. These are often difficult to maintain in the event of traffic congestion, breakdowns, on/off bus incidents, road blockages or bad weather. Predictable effects such as morning and evening rush hour traffic are often accounted for in timetables using past experience of the effects, although this then prevents the opportunity for drafting a ‘clock face’ timetable where the time of a bus is predictable at any time through the day. Predictable short term increases in passenger numbers may be dealt with by providing “duplicate” buses, where two or more buses operate the same slot in the timetable. Unpredictable problems resulting in delays and gaps in the timetabled service may be dealt with by ‘turning’ a bus early before it reaches it terminus, so that it can fill a gap in the opposite direction, meaning any passengers on the turned bus need to disembark and continue on a following bus.