Introduction to the Model 4. 1.1 The Passenger Demand 4.1.2 Model Assumptions 4.1.3 Travel Time 4.1.4 Passenger Travel Time Cost 4.1.5 Passenger Waiting Time Cost 4.1.6 Dispatching Cost 4.1.7 Total Cost 4.2 The Calculation of the Optimum Headways 4.2.1 Proof of the "Minimum" 4.2.2 Equal Headways 4.2.3 Conditional Requirement of Passengers per Bus and Bus Capacity 4.2.4 Optimal Fleet for Combined Regular/Express Bus Operation 4.3 Numerical Example 4.4 Combinations of Scheduled and Unscheduled, Regular and Express Bus Operations 4.4.1 Case 1: Scheduled Regular and Scheduled Express Bus Operation 4.4.2 Case 2: Scheduled Regular and Unscheduled Express Bus Operation 4.4.3 Case 3: Unscheduled Regular and Scheduled Express Bus Operation 4.4.4 Case 4: Unscheduled Regular and Unscheduled Express Bus Operation 4.