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Curb Requirements Model 25 enforcement is not present. Most airports today enforce a policy of not allowing vehicles to stop at the curbfront unless the driver can see their arriving passenger waiting at the curb. Long dwell times are less of a problem at the departure curbfront, where most drivers drop off their passengers and depart immediately. Figure 27 shows the Curb Requirements model and illustrates the basic flow that yields the outputs of required curb frontage from the peak 15 minutes of demand and the comparison to existing curbfront length with the percentage utilized. Process for Estimating Curb Length The Curb Requirements model uses the following approach in estimating frontage demand. Passenger survey data is used to find the modal splits and vehicle occupancies for passenger traffic to and from the airport. The factors are used with the design hour passenger volume to gen- erate demand for autos, taxis, limousines, and some other commercial vehicles. Bus and some shuttle schedules should be consulted to determine their peak frequencies. The peak 15 minutes of the design hour can be determined through a design day analysis or by observations. If data at the 15-minute detail level is not available, design hour or peak hour data can also be used with a peak 15-minute percentage. If traffic is considered to be evenly distributed during the design hour, the peak 15 minutes would equal 25% of hourly activity. Other inputs include dwell times, vehicle lengths, and multiple stop factors. These inputs will generate the associated frontage demand for each vehicle type and the total curb frontage required. A primary element of curbfront LOS is the ability to find a space for loading or unloading. The probability of finding an empty curb space or having to double park is typically used to describe LOS. The curbside capacity is considered to be the double parking capacity of the curb, assum- ing a four-lane roadway with double parking allowed. LOS is then based on the percentage of the double parking capacity as follows: A--Parking demand equal to or less than 50% of double parking capacity. B--Parking demand is between 50% and 55% of double parking capacity. C--Parking demand is between 55% and 65% of double parking capacity. D--Parking demand is between 65% and 85% of double parking capacity. E--Parking demand is between 85% and 100% of double parking capacity. F--Parking demand exceeds 100% of double parking capacity. Figure 27. Example of curb requirements.