Scheduled mass transit (i.e. buses or trains) trades off service frequency and load factor. Buses and trains must run on a predefined schedule, even during off-peak times when demand is low and vehicles are nearly empty. So to increase load factor, transportation planners try to predict times of low demand, and run reduced schedules or smaller vehicles at these times. This increases passengers' wait times. In many cities, trains and buses do not run at all at night or on weekends.