Mass transit heuristics

Consider the bus lines of a major city. You're given a set of bus stops (which are set and immovable by your project) and you are helping to create the different bus routes that will service all of these stops. The plan will have a list of each bus route and how often buses come along each route. You can ignore rush hour peaks and changes in traveler load for the weekends-- assume the average load is the same at any time of day.

Your job is not to create this set of routes, but to write a heuristic that measures the efficiency of a given set of routes. You can use whatever metrics you like to determine the best route. First, what are some measurements that might help determine the "best" set of routes? And second, how would you code an evaluator to generate a single value that incorporates all of these measurements?