It may be expressed as a percentage of the owned or licensed fleet which is available over a period; the two definitions are usually different since all vehicles owned are not necessarily licensed; sometimes vehicles are classified as “owned” if they remain on the company’s books, when in fact they have been delicensed for a considerable time and have been cannibalized to the extent that there is virtually nothing left. It is therefore usually more useful to base this ratio on the number of buses licensed rather than owned.

Availability may also be expressed in terms of average days available per year, but this is less usual.

Because vehicles spend varying times undergoing maintenance or repair, the number available will tend to vary constantly throughout the day. The most relevant time for calculating vehicle availability is at the time of peak vehicle requirement, and it is appropriate to record the number of vehicles available at this time each day, and to calculate the average over a period, such as a week or a month.

There will rarely be 100% availability except possibly for short periods, since every vehicle requires time out of service for routine maintenance, and there will always be an element of unscheduled maintenance and accident repairs.

Some operators regard vehicles as available when they are undergoing scheduled routine maintenance, but this is incorrect, since they are not available for revenue-earning service at these times. If availability is calculated on a daily basis, there may be days on which 100% is achieved: for example, on weekends or public holidays when no routine maintenance is carried out and there are no vehicles out of service for unscheduled repairs or accident damage.

Some operators may achieve 100% availability during peaks if routine maintenance can be co-ordinated with operational requirements so that all maintenance is undertaken during the inter-peak periods, although this is unlikely to be achieved on a regular basis.

With effective preventive maintenance it should be possible to obtain average availability figures of 90% of licensed fleet; 85% is a reasonably good figure in most circumstances, although 75% or below is more typical in developing countries. However, in many countries, particularly in Africa, extremely low availability figures are recorded, where large numbers of vehicles are unserviceable for various reasons, but principally due to shortage of spare parts: availability figures of 10% or less are not unknown.