Solving NP-hard optimization problems to optimality is a topic, that has challenged researchers
almost since the beginning of computer history (long before the concept of NP-hardness was
discovered). Significant progress has been made in the recent decades, but for many problem
types only fairly small instances can be solved. Vehicle routing problems belong to a class of
problems that has proved to be difficult to solve. Only moderately sized problems can be solved
to optimality consistently.