3.3. Mathematical Model of the Profitability of the Car Sharing System
Let us consider the parameters that must be determined for the effective functioning of the car-sharing system. Since the car-sharing system functions cyclically, that is, every day in the system, depending on the type of system, cars may be relocated to those areas that are in demand by users at the appropriate time. Let T be the number of periods into which one iteration of the cycle of the car-sharing system is divided. We will assume that after the completion of the iteration, the system returns to the initial state. The system's initial state can be determined by relocating a certain number of cars to certain areas. As the demand for car-sharing services in these areas changes, the system's initial state changes in a new cycle. We will assume that one cycle iteration is divided evenly into T periods , , that is, we will determine the state of the car-sharing system at moments in time .
Let the car-sharing system be implemented in region R. Let us divide region R into subregions . The distribution can be performed by the hexagonal tessellation method. For each subregion , for the corresponding period , , we calculate the following parameters:
– – the number of occupied cars in the subregion , ;
– – the number of free cars in the subregion , ;
– – the number of cars that traveled from the subregion to the subregion , , , ;
– – the probability of a car trip from the subregion to the subregion , , , ;
– – maintenance costs (repair, washing, etc.) of cars in the subregion , ;
– – the cost of a car trip from the subregion to the subregion , , , ;
– expenses for a car trip from the subregion to the subregion , , , .
The cost of a car trip from the subregion
to the subregion
for the relevant period
,
can be calculated using the formula:
where
is the length of the route when traveling from the subregion
to the subregion
,
is the fare for travel to the neighboring subregion for the relevant period
,
. When using the hexagonal tessellation method, the adjacent subregion is defined by a hexagon whose side is directly adjacent to the hexagon that defines the current subregion.
Expenses for a car trip from the subregion
to the subregion
for the relevant period
,
can be calculated using the formula:
where
is the coefficient considers the fuel cost for travel to the neighboring subregion.
After calculating the system of parameters, it is possible to determine the objective function that will determine the maximization of the car-sharing system implemented in the specified region:
where
is the income received from all trips from each sub-region
to the subregion
for the complete cycle of the car-sharing system, i.e., for the period
;
is the costs from all trips from each sub-region to the subregion for the complete cycle of the car-sharing system, i.e., for the period ;
is the car maintenance costs in each subregion for the complete cycle of the car-sharing system, i.e., for the period .
The task is to find the optimal distribution of cars in all subregions at the initial moment. Such distribution can be determined based on statistical data on customers' trips in these subregions at specific points in time. For example, you can cover a region with hexagons, and for each hexagon corresponding to the corresponding subregion, determine the number of taxi orders in the morning period. Next, you can distribute the cars available in the car-sharing system proportionally across these subregions. That is, at the initial moment, the distribution of cars takes place according to the following formulas:
where is the total number of cars that are available,
is the number of free cars at the initial time point in the subregion , , is the number of free cars at the initial time point in the subregion , for the period .
At the following points in time, the distribution of cars will be determined according to the formulas that determine the restrictions on the use of free cars in the subregions. In particular, the number of cars that customers can use for trips from a subregion
to other subregions cannot exceed the number of free cars in this subregion:
where
is the condition that determines that at the initial moment, cars do not make trips;
is the number of free cars in the subregion for the period , ,;
is the number of cars that left the subregion to the subregion for the period ;
is the number of cars that left the subregion to the subregion for the period ;
is the number of cars that left the to the subregion for the period .
The relationship between the a priori probability of a car trip from the subregion
to the subregion
and the number of free cars in the subregion
is defined by the function
. Analytically, such a function is difficult to describe, so it is fashionable to use simulation modeling to find it. The trips of each free vehicle from each subregion over period
are simulated. We generate
times a pseudorandom number from the interval
. If the number falls into a gap
, then we simulate a car trip to the subregion
, if the number falls into a gap
, then we simulate a car trip to the subregion
, etc., if the number falls into a gap
, then we simulate a car trip to the subregion
, etc., if the number falls into a gap
, then we simulate a car trip to the subregion
. After carrying out the simulation modeling procedure, we get the integer linear programming problem (1)–(5). To find a solution to this problem, you can use the CPLEX application [
33].