Understanding how Podaris calculates the vehicles required for a given trip, service, or agency.
Peak Vehicle Requirement (PVR) is the number of vehicles required to operate a given trip, service, or agency. In the most simple instance, PVR is simply the run length of a trip divided by its headway. For example, if a round trip takes 60 minutes to complete (including layovers at either end), and operates at a frequency of fifteen minutes during its peak hours of operation, then 60/15 = 4 vehicles are required for the service.
This calculation can become greatly complicated, however, by many different factors, including:
- Non-periodic vehicle departures (using exact times rather than headway- or fleet-based trip schedules)
- Service patterns with complex topologies, or which change over the course of the day
- Deadheading (vehicle movements that are not part of a service pattern)
- Movements to and from depots ("pull-in/pull-out"s)
- Sharing vehicles between services ("Interlining")
To calculate PVR, Podaris runs an internal simulation which creates a blocking table for every vehicle movement, including layover and deadheading requirements. Podaris assumes that vehicles will be shared between trips and services whenever possible, automatically interlining services if this creates efficiencies. Note that vehicles are only shared between services that are defined as using the same type of vehicle, and they are never shared between agencies however.
Because of the way that PVR is calculated -- incorporating the efficient re-use of vehicles from non-overlapping trips -- the PVR for a given service may be less than than the sum of its trips. Similarly, the PVR for an agency, which can be seen in the Active Services window, may be less than the sum of its services, since vehicles may be recycled between (for example) weekday and weekend services.
Limitations of the PVR calculation
It's important to understand what is not included in the PVR calculation. There are additional factors which should be considered when sizing vehicle fleets, including:
- Routine vehicle cleaning & maintenance schedules.
- Reserve vehicles for accommodating breakdowns or service disruptions.
- Vehicle charging or fueling requirements.
- Depot pull-in / pull-out times.
- Driver schedules and labour rules.
Podaris' PVR estimates should thus be taken as an absolute minimum. For strategic planning purposes, you should consider the operating conditions of your agency and add an appropriate margin. For detailed planning purposes, detailed scheduling software which takes all of the above into account should be used. If you would like to know how Podaris can interoperate with the scheduling software of your choice, please contact firstname.lastname@example.org.