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28 challenges encountered by most airport operators include the these land uses and no reliable method exists for forecasting following. future trips. Lack of Data on Airline Passengers Effort Needed to Gather Required Data Most airport operators do not conduct regular surveys of Comprehensive surveys of originating and terminating air- the travel modes used by airline passengers, the occupancies line passengers can be costly and time consuming to plan, of vehicles transporting airline passengers, their lead and lag authorize, and conduct, with several months required to times, or their on-airport circulation patterns (e.g., the percent review and summarize the resulting data before they are using parking or curbside areas). It is estimated that fewer than available for release to others. 20 U.S. airport operators regularly conduct surveys of the travel modes and circulation patterns of airline passengers and have Resulting Accuracy access to current data. As noted, forecasts of the traffic volumes generated by air- line passengers are often prepared in substantially more detail Lack of Data on Hourly Passenger Volumes than forecasts of traffic generated by employees, air cargo, or Many airport operators do not have accurate data on hour- service/deliveries. However, although traffic generated by by-hour originating/terminating airline passenger numbers. At airline passengers may account for over 70% of the traffic many airports, for planning purposes, hourly airline passenger during the peak hour, it typically represents less than half of numbers are calculated using (1) reported aircraft arrival and the daily traffic generated by an airport. The costs and time departure schedules, (2) aircraft sizes (and corresponding seat required to gather the airline passenger data needed to fore- capacities) to determine the number of available seats per hour cast airline passenger vehicle trips should be compared with (or other time increment), (3) assumed load factors (by air- the benefits (i.e., anticipated level of accuracy). line)--the portion of seats occupied by passengers, and (4) the assumed portion of originating or terminating passengers (by Estimating Future Airport airline). A minor difference in the estimated load factor or the Roadway Traffic Volumes-- proportion of enplaned/deplaned passengers in the peak hour Alternative Approach can lead to significant differences in the numbers of peak-hour passengers. Furthermore, although planners recognize that air- An alternative approach to estimating future airport road- craft load factors vary throughout the day and by day of the way traffic volumes involves determining existing traffic week, typically, a single load factor is applied to all aircraft of volumes on each roadway segment (or major segments) and a given airline. Similarly, while the percentage of passengers applying a growth factor to the peak-hour volume to repre- who originate or terminate at an airport may vary signifi- sent future conditions. This alternative approach is com- cantly throughout the day, typically only a single originating/ monly called the "growth factor method." It is suitable for terminating factor is applied to all passengers of a given airline. quick analyses of airport curbside and terminal area roadway operations for planning purposes. Compared to the four- step forecasting approach, this approach can be applied rela- Lack of Data on Airport Employees tively quickly and inexpensively. The growth factor method As previously noted, most airport operators have little or requires (1) determining the existing peak hour(s) roadway no data regarding the numbers of employees reporting to traffic volumes on each roadway segment or major segments, work on a daily basis, and less data on the hour-by-hour (2) developing growth factors, and then (3) multiplying the arrival/departure patterns and travel modes used by these existing peak roadway traffic volumes by the selected growth employees. Few, if any, airport operators have forecasts of factor to develop an approximation of future conditions. future employment that are considered to be as reliable as the available forecasts of airline passengers. Growth Factor Method for Estimating Future Traffic Volumes Lack of Data on Air Cargo A growth factor is the ratio between traffic volumes in the and Service/Delivery Trips current peak hour and in the peak hour to be analyzed. A As noted earlier, additional research is required on air cargo growth factor can be based on the ratio of the forecast total and service/delivery vehicle trips. At most airports, little data annual airline passenger numbers (enplaned plus deplaned are available on the existing numbers of trips generated by passengers) for the future year to be analyzed to the equivalent

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29 existing airline passenger numbers. Seasonal growth factors Changes in choices of travel modes, parking facilities, or can be developed to adjust for peak-month traffic operations circulation paths that may result from new or improved using data commonly available at most airports. For example, public transportation services, changes in parking facilities seasonal factors can be developed using the ratio of parking or parking rates, or increases or decreases in the propensity revenues (or, preferably, public parking transactions) during of motorists to use curbside roadways. the peak month to the revenues during the current month or Changes in the proportion of airline passengers during the the ratio of month-to-month airline passenger numbers. future peak month, peak day, or peak hour, although these changes could be compensated for by adjusting the growth factor appropriately. For example, if the peak hour is Challenges with Use of expected to account for a smaller proportion of daily traf- the Growth Factor Method fic due to anticipated changes in airline schedules or a "flat- The major challenge with using the growth factor method tening" of the peak due to increased traffic volumes, the is that it is relativity simplistic. This method is based on the growth factor could be adjusted accordingly. assumption that existing patterns of activity and circulation Changes in the roadway network on or near the airport. will remain unchanged throughout the forecast period. This For example, the construction of a new major regional method also may not account for changes that may result from highway may affect how vehicles approach the airport and turning movement patterns at the airport entry/exit. Sim- New land uses on or near the airport that could affect the ilarly, a new or modified airport roadway could alter inter- paths that motorists follow when entering or exiting the nal traffic circulation and merging or weaving patterns on airport. the airport.