Transit 101: The Spare Ratio
The Spare Ratio refers to the percentage of total vehicles that are not needed in peak service out of all the vehicle that are used in peak service. For example, suppose a transit agency has 200 buses, of which 160 are needed in peak service. The remaining 40 buses are used in case there is a problem with the first 160 buses, thus, the spare ratio is 40 / 160 or 25%.
Of course, transit agencies have a clear incentive to operate as low a spare ratio as possible, as each additional vehicle operated means one more bus that has to be parked , maintained, insured, etc. On the other hand, having a spare ratio that is too low may result in some bus runs not being operated because there are no vehicles available - not a good situation. Having a lower spare ratio also means that it would be hard to expand service without adding more vehicles, as all available vehicles may be needed to operate the existing service.
The Federal Transit Administration states that normally, the spare ratio for agencies receiving federal aid and who have more than fifty vehicles should not "normally" exceed 20%. While is there is some leeway in this number, transit agencies can be forced to dispose of excess vehicles when the ratio moves higher than 20%, a situation normally experienced only after a significant service reduction (which unfortunately has happened frequently in the past few years due to the economy).
For a variety of reasons similar to why there is such a wide variation in spare ratios for rail systems, described below, it is often difficult for American bus systems to achieve the magic 20% number. Thirty-six North American bus agencies, ranging in size from 39 to 3,644 vehicles, responded to a query about their spare ratio in TCRP Synthesis 11. Ratios ranged from 10% in Toronto to 32% in San Diego and Alexandria, VA, with most located around 20%. There was a slight trend towards a lower spare ratio in larger agencies, which might be because they can afford to replace their buses more frequently and have more advanced maintenance procedures.
In contrast with buses, there is no required spare ratio for rail lines. In fact, in the United State the spare ratio for rails varies widely. In TCRP Synthesis 15, a variety of factors were described that each influence the rail vehicle spare ratio for different operators (many of these factors would also apply to bus operators):
Factors that tend to raise the spare ratio include the following:
- Aging fleets with high maintenance downtime
- Gap trains used in service but not included in peak vehicle requirement
- Difficulties in disposing of surplus cars
- Cost of retiring cars that have not reached the end of their useful life
- Concerns that fleet reductions will compromise quality service objectives
- Concerns that downsizing will impair ability to meet unexpected future demand
- Hopes that current ridership declines will turn out to be temporary
- Heavy overhaul and rebuild programs that take vehicles out of service
- Difficulties in providing passenger amenities that take vehicles out of service for extended periods of time
- Extensive use of vehicles in married pairs and multiple-unit consists
Factors that tend to reduce the spare ratio include the following:
- Quality maintenance programs that lead to high vehicle availability
- Ability to temporarily remove excess cars from total active fleet for short periods
- Capability to move cars between lines in same system
- Maintenance labor agreements conducive to quality improvements
- Quality maintenance employees
- A "lean" fleet management philosophy
- Ability to lend/borrow or buy/sell vehicles from other agencies
- Limited or costly storage space, particularly in city centers
- Ability to procure "standard" cars that are interchangeable within a system and with other systems
- Recognition that inactive cars often lead to maintenance problems
- Ability to combine with other agencies in joint railcar procurements
- Effective use of "options to buy" procurement strategies
For heavy rail systems in North America, spare ratios ranged from a low of 6% in New York to 78.9% in Miami, with an average of 24%. For light rail systems in North America, spare ratios ranged from a low of 12% in Toronto (the legacy streetcar system) to 31% in Boston, with an average of 23%. For commuter rail systems, spare ratios ranged from a low of 3.5% in Metra (Chicago) to 21% for SEPTA in Philadelphia, with an average of 13.48%. Spare ratios for commuter rail are typically smaller than for buses and other rail lines because commuter rail cars are usually not powered.
There are some people who argue that instead of stating a fixed spare ratio, say 20%, transit agencies should determine an optimum personal spare ratio taking into account several variables. For example, Klein (Method of Calculating Spare Ratios. Transportation Research Record: Journal of the Transportation Research Board, 1838-10, 2003, 73-80) took an industrial engineering approach and calculated a spare ratio for commuter railroads based on annual use, reliability, inventory support, train length, and how long it took maintenance to repair vehicles. How long the peak number of vehicles were required was as important as the number of peak vehicles required. The Federal Transit Administration does take some of these variables into account by allowing for "contingency" buses in addition to the normal spares that may be needed with certain kinds of new technology such as hybrid and electric buses .
There has been a trend in the past thirty years of spare ratios declining, perhaps due to more aggressive replacement of older buses and better maintenance. However, at this time there have been no comprehensive studies of how new vehicle technology (electric, hybrid, CNG buses) have affected spare ratios. Anecdotal stories suggest poor reliability on electric and CNG buses - if true, higher spare ratios would be required for transit agencies utilizing these kinds of buses in the future.