Bus Route Optimization Calculator

Because the number of buses needed depends on the RELATIONSHIP between round trip time and headway, not on round trip time alone. A longer round trip does increase Active Buses Needed for a fixed headway (more buses have to be simultaneously on the road to maintain the same departure frequency over a longer route), but the headway itself is set independently by ridership demand and bus capacity. Round trip time and headway together determine fleet size; neither one alone tells the whole story.

Because Required Headway comes from dividing peak ridership by how many buses per hour are needed to carry it, and buses per hour is ridership divided by capacity per bus. A higher-capacity bus (an articulated or double-decker bus, for example) can carry the same peak ridership with fewer trips per hour, which means each bus can depart less frequently -- a longer headway -- while still moving the same number of riders through the route in an hour.

Because buses need to leave revenue service periodically for scheduled maintenance and inspections, and unscheduled breakdowns happen regardless of maintenance quality -- a fleet sized to exactly the number of buses needed at any one moment has zero capacity to absorb either. A roughly 10-20% spare ratio is standard transit industry practice for exactly this reason; this calculator uses 15% as a representative figure within that range, not a number every agency uses identically.

It increases Round Trip Time, which does tend to increase Active Buses Needed for a fixed headway, since more buses must be simultaneously on the road to cover a longer trip duration at the same departure frequency. Average speed already reflects real operating conditions -- traffic, stops, and dwell time, not top road speed -- so a route through heavier traffic or with more frequent stops both lower average speed and, through this same mechanism, tend to raise the number of buses the route requires.