For fixed-route service, service planning includes a number of important tasks, including:

· Network design
· Route design
· Frequency determination
· Scheduling (or timetabling)
· Vehicle assignment
· Crew assignment

These six tasks are often conducted in a sequential manner, with various models and methods being used to solve each task.

In this module, we will investigate the challenge of vehicle assignment. In this case, one can assume that a timetable (schedule) already exists for the service, and the challenge is to assign individual vehicles to particular parts of the schedule. For many transit agencies, this is conducted as often as the schedule is re-constructed, perhaps several times per year. However, compared with crew assignment, which is often subject to considerable constraints due to work rules, vehicle scheduling is relatively straightforward.

Formally, the vehicle assignment problem involves assigning each vehicle to a piece of work: a single run or set of runs for a single route. Pieces of work are then assembled into a full schedule across the day, and assigned to a single vehicle. The desired outcome is that all runs, for all routes in a day, are covered by a vehicle. There can be any number of objectives in this process, including: (1) minimizing the total number of vehicles placed in service during the day; (2) minimizing the total vehicle-hours of service; or, as a related measure, (3) minimizing the total operating cost of the service.

One might observe that the ability to have vehicles move from one route to another during the day may afford considerable flexibility in assigning vehicles. The assignment of individual vehicles to different routes during the day is called "interlining." The flexibility brought about by interlining often results in considerable savings in cost, mileage, and time for the transit agency.

One of the additional considerations in vehicle assignment is that the vehicle may need to move around during the day, but without being in revenue service. This could occur for a number of reasons: the vehicle may need to re-fuel at a garage, it may need to move to a terminal or garage to allow a crew change, or it may simply need to move to another location as it switches between different pieces of work. The latter movement of vehicles between pieces of work is commonly called "dead-heading." Together, the non-revenue movement of vehicles can add considerable time, mileage and expense to the transit service.