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 Bus Service Planning: Theory and Concepts

 
Preliminary Schedule Design

Designing a schedule can be quite complicated, so preliminary schedule design will be portrayed here in the form of an example.

Consider a transit route that connects a residential neighborhood to a central business district. The distance between the neighborhood and the downtown area is 5 miles. The transit vehicles average 12 miles per hour between the two terminal points. The goal is to provide transit service every 15 minutes along the route.

The first step is to determine the time required to travel from one end of the route to the other. The one-way trip time is given in the equation below:

One-Way Trip Time = Route Length / Average Operating Speed
One-Way Trip Time = 5 miles / 12 mph
One-Way Trip Time = 25 minutes

The total round-trip time is twice the one-way trip time, or 50 minutes.

The next step is to determine the number of vehicles required in order to operate at the desired level of service. Now suppose that the desired headway is 15 minutes. That is, the frequency of service is one vehicle every 15 minutes. How many vehicles would be required to provide this service?

Number of Vehicles Required = Total Round Trip Time / Headway
Number of Vehicles Required = 50 minutes / (15 min/vehicle)
Number of Vehicles Required = 3.33 or 4

The revised round-trip time can now be calculated.

Revised Round Trip Time = (Number of Vehicles)(Headway)
Revised Round Trip Time = (4 vehicles)(15 minutes/vehicle)
Revised Round Trip Time = 60 minutes

This leaves 10 minutes for recovery and layover time, since the actual round-trip running time is 50 minutes.

The capacity of the route can also be determined.

Capacity = (Vehicles)(Capacity/Vehicle)
Capacity = (4 vehicles/hour)(75 passengers/vehicle)
Capacity = 300 passengers/hour

Now suppose that the forecasted demand for this transit route is 400 passengers per hour at the peak loading point. We need to re-estimate the required vehicles because the capacity calculated above is insufficient to carry this projected demand.

# Vehicles = (400 passengers/hour) / (75 passengers/vehicle)
# Vehicles = 5.33 vehicles/hour
Headway = 60 minutes / 5.33 vehicles
Headway = 11.25 = 10 minutes/vehicle

Note that we use an even "clock headway" of 10 minutes, rather than the cumbersome and potentially confusing value of 11.25 minutes that we initially calculated.

# Vehicles = (50 minutes + 10 minutes) / (10 minutes/vehicle)
# Vehicles = 6

At this point, we have completed the preliminary calculations in schedule design. The final computations involve the development of the schedule and the vehicle 'blocks.' These computations are presented in the 'final schedule design and blocking' discussion.