Line blocking with vehicle interchange differs from that without vehicle interchange, in that the vehicle combination to be used is not strictly defined for each vehicle journey section. In fact, the procedure has the possibility of selecting the best vehicle combination specified in the attribute vehicle combination set. Different criteria are possible, which can be weighted against each other in a subordinate objective function:
- Selection according to costs: Different costs are involved with the selection of a vehicle combination, which flow into the objective function.
- Selection according to capacity: For the selection, a comparison between the trip volume (Assignment results or count data) can be carried out on the one hand and the (seat) capacity of the vehicle combination on the other hand. Not the covered demand provided by the capacity, is included in the objective function.
- Selection according to availability: The number of available vehicles can be predefined on the unit level. The selection is made, so that this restriction is adhered to. The number of vehicles used in addition to the ones available are included into the objective function.
Line blocking with vehicle interchange thus goes beyond the application area of line blocking without vehicle interchange (Line blocking description without vehicle interchange) and also covers the following application areas.
- Planning the vehicle use depending on the demand, at the same time considering block-related restrictions.
- Reduction of the calculated vehicle requirement by making the vehicle use more flexible, with the (possibility of) replacing a vehicle combination with another, for example because of technical restrictions.
- Consideration of different vehicle combination-specific minimum layover times
The procedure is based on the line blocking without vehicle interchange and integrates this as a procedural step into its entire process. Compared to this one it is not about an analytical procedure, but an iterative search procedure which in general finds very good solutions, but never an optimal one regarding the objective function.
As another distinctive feature, several complete and equal solutions of the given line blocking task (parameter number of solutions per iteration), exist for each time of the procedure. These are changed iteratively and evaluated. If there is no improvement of the objective function value (convergence) or if the defined maximum number of iterations has been reached, the procedure is stopped and the best solution is provided.
The procedure includes the following steps.
1. Initial selection of the vehicle combination from the specified vehicle combination set for each vehicle journey section and each solution.
2. Line blocking without vehicle interchange for this selection for each solution
3. Evaluation of the solution and convergence check.
4. Determining and merging selections, which have lead to good solution properties, and new start from step 2.), until convergence applies or the maximum number of iterations has been achieved.
Because the line blocking is carried out as in the procedure without vehicle interchange (Line blocking description without vehicle interchange), for defined selection of the vehicle combinations, the following additional components are necessary to understand the procedure:
- Selection principles of vehicle combinations
- Solution evaluation via objective function
- Parameters and convergence
- Consideration of different vehicle combination-specific minimum layover times
