Vehicle Routing Problems

The VRPy package can solve the following VRP variants.

Capacitated Vehicle Routing Problem (CVRP)

In the capacitated vehicle routing problem (CVRP), a fleet of vehicles with uniform capacity must serve customers with known demand for a single commodity. The vehicles start and end their routes at a common depot and each customer must be served by exactly one vehicle. The objective is to assign a sequence of customers (a route) to each truck of the fleet, minimizing the total distance traveled, such that all customers are served and the total demand served by each truck does not exceed its capacity.

>>> from networkx import DiGraph
>>> from vrpy import VehicleRoutingProblem
>>> G = DiGraph()
>>> G.add_edge("Source", 1, cost=1)
>>> G.add_edge("Source", 2, cost=2)
>>> G.add_edge(1, "Sink", cost=0)
>>> G.add_edge(2, "Sink", cost=2)
>>> G.add_edge(1, 2, cost=1)
>>> G.nodes[1]["demand"] = 2
>>> G.nodes[2]["demand"] = 3
>>> prob = VehicleRoutingProblem(G, load_capacity=10)
>>> prob.solve()

Note that whether the problem is a distribution or a collection problem does not matter. Both are modeled identically.

CVRP with resource constraints

Other resources can also be considered:

  • maximum duration per trip;

  • maximum number of customers per trip.

Taking into account duration constraints requires setting time attributes on each edge, and setting the duration attribute to the maximum amount of time per vehicle.

Following the above example:

>>> G.edges["Source",1]["time"] = 5
>>> G.edges["Source",2]["time"] = 4
>>> G.edges[1,2]["time"] = 2
>>> G.edges[1,"Sink"]["time"] = 6
>>> G.edges[2,"Sink"]["time"] = 1
>>> prob.duration = 9
>>> prob.solve()

Similarly, imposing a maximum number of customers per trip is done by setting the num_stops attribute to the desired value.

>>> prob.num_stops = 1

CVRP with Time Windows (CVRPTW)

In this variant, deliveries must take place during a given time-window, which can be different for each customer.

Such constraints can be taken into account by setting lower and upper attributes on each node, and by activating the time_windows attribute to True. Additionally, service times can be taken into account on each node by setting the service_time attribute.

Following the above example:

>>> G.nodes[1]["lower"] = 0
>>> G.nodes[1]["upper"] = 10
>>> G.nodes[2]["lower"] = 5
>>> G.nodes[2]["upper"] = 9
>>> G.nodes[1]["service_time"] = 1
>>> G.nodes[2]["service_time"] = 2
>>> prob.time_windows = True
>>> prob.solve()

Note

Waiting time is allowed upon arrival at a node. This means that if a vehicle arrives at a node before the time window’s lower bound, the configuration remains feasible, it is considered that the driver waits before servicing the customer.

CVRP with Simultaneous Distribution and Collection (CVRPSDC)

In this variant, when a customer is visited, two operations are done simultaneously. Some good is delivered, and some waste material is picked-up. The total load must not exceed the vehicle’s capacity.

The amount that is picked-up is set with the collect attribute on each node, and the distribution_collection attribute is set to True.

Following the above example:

>>> G.nodes[1]["collect"] = 2
>>> G.nodes[2]["collect"] = 1
>>> prob.load_capacity = 2
>>> prob.distribution_collection = True
>>> prob.solve()

CVRP with Pickup and Deliveries (VRPPD)

In the pickup-and-delivery problem, each demand is made of a pickup node and a delivery node. Each pickup/delivery pair (or request) must be assigned to the same tour, and within this tour, the pickup node must be visited prior to the delivery node (as an item that is yet to be picked up cannot be delivered). The total load must not exceed the vehicle’s capacity.

For every pickup node, the request attribute points to the name of the delivery node. Also, the pickup_delivery attribute is set to True. The amount of goods to be shipped is counted positively for the pickup node, and negatively for the delivery node. For example, if \(3\) units must be shipped from node \(1\) to node \(2\), the demand attribute is set to \(3\) for node \(1\), and \(-3\) for node \(2\).

>>> G.nodes[1]["request"] = 2
>>> G.nodes[1]["demand"] = 3
>>> G.nodes[2]["demand"] = -3
>>> prob.pickup_delivery = True
>>> prob.load_capacity = 10
>>> prob.solve(cspy=False)

Note

This variant has to be solved with the cspy attribute set to False.

Periodic CVRP (PCVRP)

In the periodic CVRP, the planning period is extended over a time horizon, and customers can be serviced more than once. The demand is considered constant over time, and the frequencies (the number of visits) of each customer are known.

For each node, the frequency attribute (type int) is set, and the parameter periodic is set to the value of the considered time span (the planning period). All nodes that have no frequency are visited exactly once.

>>> G.nodes[1]["frequency"] = 2
>>> prob.periodic = 2
>>> prob.solve()

A planning of the routes can then be queried by calling prob.schedule, which returns a dictionary with keys day numbers and values the list of route numbers scheduled this day.

Note

The PCVRP usually has additional constraints: some customers can only be serviced on specific days of the considered time span. For example, over a \(3\) day planning period, a node with frequency \(2\) could only be visited on days \(1\) and \(2\) or \(2\) and \(3\) but not \(1\) and \(3\). Such combination constraints are not taken into account by VRPy (yet).

CVRP with heterogeneous fleet (HFCVRP)

In the CVRP with heterogeneous fleet (or mixed fleet), there are different types of vehicles, which can differ in capacities and costs (fixed costs and travel costs). Typically, a vehicle with a larger capacity will be more expensive. The problem consists in finding the best combination of vehicles to satisfy the demands while minimizing global costs.

First, the cost attribute on each of the graph is now a list of costs, with as many items as vehicle types (even if costs are equal). For example, if there are two types of vehicles, the following graph satisfies the input requirements:

>>> from networkx import DiGraph
>>> G = DiGraph()
>>> G.add_edge("Source", 1, cost=[1, 2])
>>> G.add_edge("Source", 2, cost=[2, 4])
>>> G.add_edge(1, "Sink", cost=[0, 0])
>>> G.add_edge(2, "Sink", cost=[2, 4])
>>> G.add_edge(1, 2, cost=[1, 2])

When defining the VehicleRoutingProblem, the mixed_fleet argument is set to True, and the load_capacity argument is now also of type list, where each item of the list is the maximum load per vehicle type. For example, if the two types of vehicles have capacities \(10\) and \(15\), respectively:

>>> from vrpy import VehicleRoutingProblem
>>> prob = VehicleRoutingProblem(G, mixed_fleet=True, load_capacity=[10, 15])

Note how the dimensions of load_capacity and cost are consistent: each list must have as many items as vehicle types, and the order of the items of the load_capacity list is consistent with the order of the cost list on every edge of the graph.

VRP options

In this subsection are described different options which arise frequently in vehicle routing problems.

Open VRP

The open VRP refers to the case where vehicles can start and/or end their trip anywhere, instead of having to leave from the depot, and to return there after service. This is straightforward to model : setting distances (or costs) to \(0\) on every edge outgoing from the Source and incoming to the Sink achieves this.

Fixed costs

Vehicles typically have a fixed cost which is charged no matter what the traveled distance is. This can be taken into account with the fixed_cost attribute. For example, if the cost of using each vehicle is \(100\):

>>> prob.fixed_cost = 100

Note

If the fleet is mixed, the same logic holds for fixed_cost: a list of costs is given, where each item of the list is the fixed cost per vehicle type. The order of the items of the list has to be consistent with the other lists (cost and load_capacity). See example Mixed fleet.

Limited fleet

It is possible to limit the size of the fleet. For example, if at most \(10\) vehicles are available:

>>> prob.num_vehicles = 10

Note

If the fleet is mixed, the same logic holds for num_vehicles: a list of integers is given, where each item of the list is the number of available vehicles, per vehicle type. The order of the items of the list has to be consistent with the other lists (cost, load_capacity, fixed_cost).

Dropping visits

Having a limited fleet may result in an infeasible problem. For example, if the total demand at all locations exceeds the total capacity of the fleet, the problem has no feasible solution. It may then be interesting to decide which visits to drop in order to meet capacity constraints while servicing as many customers as possible. To do so, we set the drop_penalty attribute to an integer value that the solver will add to the total travel cost each time a node is dropped. For example, if the value of the penalty is \(1000\):

>>> prob.drop_penalty = 1000

This problem is sometimes referred to as the capacitated profitable tour problem or the prize collecting tour problem.

Other VRPs

Coming soon:

  • CVRP with multiple depots