Chapter 9 A Hybrid Genetic Algorithm, Simulated Annealing and Tabu Search Heuristic for Vehicle Routing Problems with Time Windows

The vehicle routing problem with time windows (VRPTW) is an extension of the vehicle routing problem (VRP) with earliest, latest, and service times for customers. The VRPTW routes a set of vehicles to service customers having earliest, latest service times. The objective of the problem is to minimize the number of vehicles and the distance travelled to service the customers. The constraints of the problem are to service all the customers after the earliest release time and before the latest service time of each customer without exceeding the route time of the vehicle and overloading the vehicle. The route time of the vehicle is the sum total of the waiting time, the service time and distance travelled by the vehicle. A vehicle that reaches a customer before the earliest release time incurs waiting time. If a vehicle services a customer after the latest delivery time the vehicle is considered to be tardy. The service time is the time taken by a vehicle to service a customer. A vehicle is said to be overloaded if the sum total of the customer demands exceed the total capacity of the vehicle. The quality of the solution is measured in terms of the minimization of the number of vehicles followed by the minimization of the total distance travelled respectively in that order. That is, a solution for the VRPTW with a lower total number of vehicles and greater total distance travelled is preferred over a solution that requires greater number of vehicles and smaller total distance travelled.