Poor reliability of bus services is a common problem in cities in developing countries. There are several aspects of reliability. Most fundamental is whether a service operates at all, and if it does, whether it operates on schedule, if it has one. Also significant is the reliability of the vehicles themselves: frequent mechanical failure will detract considerably from service reliability. And in some cities, buses frequently fail to complete their journeys, particularly if there are few passengers remaining on board.
Reliability is difficult to measure, and standards and expectations vary considerably from one place to another. Where services are operated to schedule, reliability may be measured in terms of the percentage of journeys operating on time or less than a specified number of minutes late, or by the percentage of scheduled kilometers actually operated.
Poor reliability may be due to inadequate enforcement of rules and regulations, so that operators may disregard them with impunity. 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, may also result in an erratic and unpredictable service.
Poor vehicle maintenance is the main reason for unreliable vehicles. This is particularly common where there is a predominance of small operators, but there are also some large operators with poor standards of maintenance. The number of kilometers per breakdown is a useful measure of reliability.
Some causes of unreliability are largely outside the operators’ control. Traffic congestion is a particular problem in many cities, often making adherence to schedules extremely difficult.