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17
ger pickup area) roadways be conducted during the 3 hours Average travel speeds--particularly along a roadway seg-
including and after the 60-minute period with the most arriv- ment having a length of 1,000 feet or more--can be used to
ing flights. The 60-minute departures and arrivals flight peaks support evaluations of airport roadway operations. Measur-
do not necessarily coincide. ing instantaneous speeds (also known as spot speeds) is not
useful in airport roadway analyses because the speeds of
individual vehicles tend to vary significantly on the roadway
Surveys of Traffic Characteristics
network.
and Operational Patterns
· Other data. In addition to the data listed above, depend-
In addition to surveys of traffic volumes, analyses of airport ing on the nature of the traffic operations problem being
roadway operations frequently require other surveys to deter- addressed, data on vehicle mix (i.e., the proportion of pri-
mine the following: vate vehicles, taxicabs, limousines, vans, buses, etc., using
the roadways), recirculation volumes (i.e., the proportion
· Vehicle mix. In an airport environment, vehicle mix (or of vehicles passing the curbside or other location multiple
vehicle classification) refers to the portion of the traffic vol- times, typically determined by recording and matching
ume accounted for by individual modes, as defined by both the license plate numbers of passing vehicles), and curbside
the type of service each mode provides (e.g., taxicab, cour- occupancies (observations or video recordings of curbside
tesy vehicle, charter bus) and the type of vehicle used (e.g., use patterns) are sometimes gathered as part of airport road-
sedan, passenger van, minibus, full-size bus). These data are way operations analyses. Surveys of airline passengers and
required to analyze curbside roadway operations. visitors are commonly used to gather such data as vehicle
· Dwell time. This is the amount of time a vehicle spends mode-choice patterns, passenger arrival patterns, passenger
parked at a curbside lane (or other passenger loading or regional approach/departure routes, place of origin/
unloading area). Typically, the dwell time is the length of destination, and use of airport parking facilities.
time between when the driver parks (i.e., the vehicle comes
to a complete stop) and when the driver first attempts to
Estimating Future Airport Roadway
rejoin the traffic stream (it does not include any time dur-
Traffic Volumes--Traditional
ing which the driver may be ready to depart, but is pre-
Four-Step Approach
vented from doing so by other vehicles). For some analyses,
it is also helpful to measure "active" dwell times (i.e., the Developing a comprehensive estimate of future traffic vol-
length of time a vehicle remains at a curbside while actively umes on airport roadways using the traditional four-step
loading/unloading passengers and their baggage) as opposed approach involves the following:
to the "total" dwell time, which reflects the time difference
between when a vehicle first stops at a curbside until it leaves · Trip generation. Estimating the traffic volume generated
the curbside. Dwell time data are required to analyze curb- by each on-airport land use during the future airportwide
side roadway operations. peak hour(s) as well as the peak hour(s) of activity for each
· Queue length. Queue length is the distance, time, or number land use.
of vehicles in a line of vehicles waiting to proceed along a road- · Trip distribution. Determining the points where trips gen-
way in which (1) the flow rate of the front of the queue deter- erated by each airport land use enter the airport roadway
mines the average speed within the queue and (2) the rate of network.
vehicles arriving in the queue is greater than the rate of vehi- · Mode-choice analysis. Analyzing the travel mode choice
cles leaving the queue. Queues form when a group of vehicles patterns of passengers and employees.
is delayed because of downstream congestion or bottlenecks. · Trip assignment. Assigning the estimated traffic volumes
The length of a queue can be measured by observing, at fixed to the on-airport and regional roadway networks.
intervals, the length of slow moving or stopped vehicles, and
the time of a queue can be measured by observing how long In regional planning, the third step--mode-choice analysis--
it takes a vehicle to travel from the back to the front of a queue. is conducted using sophisticated travel demand forecasting
The number of vehicles in a queue and the duration, or per- models. These models are used to estimate future mode-choice
sistence, of the queue also can be determined through obser- patterns or changes in existing patterns caused by the intro-
vations. These data are used to support evaluations of airport duction of new travel modes (e.g., rail service) or changes in
roadway operations. travel time or travel cost. Such models are rarely required in an
· Travel speeds. Average travel speeds can be measured by airport setting. It would be appropriate to include mode-choice
recording the time it takes random vehicles to travel a analysis during the analyses of airport roadways if a significant
known distance, such as between two fixed objects or points. change in the existing travel modes were anticipated (e.g., new
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scheduled public bus or rail service or expansion of existing terminating) arrive at, or depart from, the airport. Separate
service) and if this service were expected to attract significant analyses of these three peak periods (originating, terminating,
numbers of airline passengers or employees who currently and total) are required because peak periods of demand on
travel by private vehicles. some roadway segments coincide with the originating passen-
The three steps applicable to airport roadway operations, as ger peak periods (e.g., the departures curbside area), and some
well as challenges to using this approach, are described below. coincide with the terminating passenger peak periods (e.g., the
arrivals curbside area). The total peak period traffic volume
may not coincide with the peak period of either the originating
Estimating Traffic Volumes
or terminating passengers, but may instead reflect the busiest
(Trip Generation)
overall period at the airport (e.g., the hour with the largest traf-
The key generators of airport roadway traffic are airline pas- fic volumes on the airport entry and exit roadways).
sengers and accompanying visitors, employees working at the At airports with significant numbers of connecting pas-
airport, air cargo and airmail services, airlines, in-terminal sengers, the peak hours of airline passenger activity may not
concessionaires, and other building tenants plus airport ten- correlate with the peak hour of roadway traffic volumes. For
ants with service or delivery needs. At most airports, the data airports with multiple terminals or multiple large concourses,
required to estimate the volume of traffic generated by airline it may be necessary to gather these hourly data for each termi-
passengers are more readily available than comparable data for nal or each concourse.
employees, air cargo, or service and delivery vehicles. Existing originating and terminating airline passenger num-
Reliable statistics on existing monthly and annual volumes bers are available through the Origin-Destination Survey of
of airline passengers and air cargo tonnage and forecasts of Airline Passenger Traffic, Domestic, an online database pub-
airline passengers and air cargo tonnage are available for all lished by the FAA, which is based on a 10% sample of all air-
commercial-service airports. However, as described in greater line tickets collected by U.S. airlines. Since foreign flag airlines
detail in subsequent paragraphs, most airport operators have are not required to participate in this ticket sample, the pub-
limited-to-no data available on the number of employees lished originating-terminating airline passenger data may
working at their airports, or the types of air cargo shipments underreport passenger numbers at major international gate-
(e.g., overnight deliveries, small parcels, international, or other way airports.
types of freight). As a result, forecasts of traffic generated by air- Future peak-hour airline passenger numbers are a function
line passengers are often developed in substantially more detail of the future flight schedules of each airline, the anticipated
than forecasts of traffic generated by employees, air cargo, or size of aircraft operated (i.e., number of seats), and anticipated
services and deliveries. However, traffic generated by airline passenger load factors. Forecasts of airline passengers can be
passengers may represent less than half of the total (daily) obtained from recent airport master plans, the FAA Terminal
vehicular traffic generated at an airport. Area Forecast (TAF) (see http://aspm.faa.gov/main/taf.asp),
and other sources. Master plans may present forecasts of
annual or daily airline passenger numbers, as determined
Traffic Generated by Airline Passengers
using an average day of the peak month or standard busy day
Estimating the volume of traffic generated by airline pas- rate. Such forecasts may be based on the assumption (partic-
sengers requires the following inputs. ularly at small and medium commercial-service airports) that
the existing relationship between peak hour and daily airline
Number of originating and terminating airline passen- passenger numbers will remain constant through the forecast
gers. Roadway traffic operations are analyzed considering period unless a significant change in airline operations is
the peak-hour volume (i.e., the traffic volume occurring dur- expected.
ing the busiest 60 consecutive minutes). Analyses of airport
roadway traffic begin with the hourly numbers of originating Passenger characteristics. When possible, it is helpful to
and terminating airline passengers (or preferably the num- disaggregate the numbers of originating and terminating air-
bers occurring in 15-minute increments). Originating and line passengers by trip purpose and place of residency rather
terminating airline passenger numbers (rather than enplaned than just considering the total passenger numbers because air-
and deplaned passenger numbers) are used to generate traf- line passenger travel patterns (e.g., vehicle occupancies, cir-
fic volumes because these volumes exclude those passengers culation, and mode-choice patterns) are a function of their
transferring between flights. trip purpose (business vs. nonbusiness), place of residence
Analyses of hour-by-hour airline passenger numbers indi- (local residents vs. nonresidents), and type of flight (short-
cate when the largest numbers of originating passengers, ter- haul domestic, long-haul, transborder, overseas, or other).
minating passengers, and total passengers (originating plus Typically, these data are obtained from surveys of airline
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passengers or from data at peer airports. For example, resident times referred to as an earliness distribution). Similarly, a rep-
travelers are more likely to use private vehicles and park for resentative distribution of lag times is sometimes referred to as
the duration of their trips, while nonresidents are more likely a lateness distribution.
to travel to the airport in rental cars or hotel/motel courtesy
vehicles and not use parking facilities. Travel mode choices. To convert person trips into vehicle
trips, it is necessary to first determine the travel modes used by
Lead and lag times. Airline passenger numbers are airline passengers (or the percentage of passengers using each
reported by the airlines according to the time aircraft are sched- available travel mode). Regional transportation planning often
uled to depart (push away from the gate), and arrive (touch considers just two travel modes--private vehicles and public
down). Since these times do not coincide with the times transit--whereas airport roadway planning requires consider-
motorists enter and exit airport roadways, to analyze airport ation of taxicabs, limousines, courtesy vehicles, rental cars,
roadway traffic operations it is necessary to adjust these times scheduled buses, and other travel modes.
to reflect how much time passengers arrive at the airport in As noted, travel modes are a function of trip purpose and
advance of their scheduled flight departure times (lead time) place of residency. Airports serving a large proportion of
and depart from the airport after their scheduled flight arrival leisure passengers have distinctly different travel-mode-choice
times (lag time). International passengers typically have longer patterns than those serving business markets. However, at
lead and lag times than domestic passengers (because of most U.S. airports, 70% to 80% of all airline passengers arrive
the 2-hour advance check-in required by most airlines and and depart in private vehicles or rental cars. Typically, fewer
time required for immigration and customs processing), and than 5% to 10% of all passengers use public transportation
leisure travelers typically have longer lead and lag times than (e.g., scheduled buses or trains, or door-to-door shared ride
business travelers (because they are more likely to have checked vans). The remaining passengers typically use taxicabs, courtesy
baggage). Typically, these data are obtained from surveys of vehicles serving hotels/motels, parking facilities, rental cars, or
airline passengers or from data at peer airports. Lead time data transportation services that require prior reservations (e.g.,
may be aggregated to form a representative distribution (some- limousines, charter or tour buses/vans). Table 3-1 presents the
Table 3-1. Typical vehicle mode choice and occupancies at selected airports--
originating airline passengers.
Typical vehicle
occupancy
Salt Lake (number of
Los Angeles (a) San Diego (b) Tampa (c) City (d) people)
Private Vehicles
Curbside 42.4% 25.5% 36.3% 27.0% 1.2
Short-term parking 4.4 17.0 8.5 1.3
Long-term parking 2.5 19.5 7.0 1.3
8.3
Off-airport parking (e) 10.0 4.5 1.3
Subtotal (private vehicles) 54.8% 55.0% 55.9% 47.0%
Rental cars 11.4 19.1 36.9 35.0 1.4
Subtotal 66.2% 74.1% 92.8% 82.0%
Commercial Vehicles
Taxicabs 9.3% 7.3% 2.3% 1.5% 1.5
Limousines 2.0 1.3 -- 2.0 1.5
Door-to-door shuttles 10.0 9.5 2.0 4.0
3.3
Hotel/motel courtesy vehicles 5.1 5.8 10.5 2.6
Public transit 4.1 1.0 0.3 0.5 5.0
Charter/other bus 3.0 1.0 1.4 1.5 15.0
Subtotal (commercial vehicles) 33.5% 25.9% 7.3% 18.0%
Total 100.0% 100.0% 100.0% 100.0%
(a) Applied Management and Planning Group, 2006 Air Passenger Survey: Final Report. Los Angeles
International Airport, December 2007
(b) Jacobs Consultancy, Interim Report 1: San Diego County Regional Airport Authority. Destination Lindbergh,
December 2008.
(c) http://www.tampaairport.com/ground_transportation/transit_survey_presentation.pdf.
(d) HNTB, Landside Report, Salt Lake City International Airport, December 2002.
(e) Passengers typically arrive at the curbside in courtesy vehicles.
Source: LeighFisher, July 2009, based on the documents noted above.
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mode-choice patterns for typical large-hub airports. These parked for the trip duration), drive back to the terminal to
data were obtained from recent studies prepared for Los pick up passengers, and then exit the airport (e.g., a long-
Angeles, Salt Lake City, San Diego, and Tampa International term parking patron).
Airports. Using the format shown in Table 3-1, some airline
passengers are counted twice (e.g., a private vehicle driver who Similarly, rental car customers may go to the curbside area
parks in an economy lot and rides a courtesy vehicle or a rental before they drop off rental cars or after they pick up rental cars.
car customer who also uses a courtesy vehicle). Commercial vehicle drivers may drop off customers, wait in a
holding area, and then recirculate back to the terminal to pick
Vehicle occupancies. Vehicle occupancies (the number of up additional customers. Table 3-2 presents the travel paths
passengers per vehicle) are used to translate or convert "person and proportion of airline passengers using these paths for a
trips" by travel mode into vehicle trips. When analyzing airport typical large-hub airport. Medium- and small-hub airports
roadways, vehicle occupancies represent the number of airline have similar patterns, but at these airports there may be greater
passengers in each vehicle (i.e., excluding visitors accompany- use of private vehicles and less use of taxicabs, limousines,
ing airline passengers or the drivers of commercial vehicles). courtesy vehicles, and public transit vehicles. Again, these data
Typically, these data are obtained from surveys of airline pas- are typically obtained from surveys of airline passengers.
sengers (for single-occupancy vehicles, such as private vehicles,
taxicabs, and limousines) or from visual observations for Peak-hour factors. Airport roadway traffic is not uni-
multiparty vehicles, such as courtesy vehicles, buses, and vans. formly distributed over a typical peak hour or other peak
The average occupancy of private vehicles operating on air- period. At small airports in particular, much larger volumes of
ports is higher than the average occupancy of private vehicles traffic may occur during one 15-minute period than during
operating on public streets (particularly during commute the preceding or subsequent 15-minute period. Peak-hour
hours) because vehicles on airports are typically transporting a (adjustment) factors are used to translate nonuniform flows
group of airline passengers rather than just a single occupant. into equivalent hourly flows to allow for the analyses of road-
ways exhibiting such nonuniform peaks. (This translation is
On-airport traffic circulation patterns. The locations on required because roadway capacities are defined and analyses
an airport where motorists begin or end their trips and the of roadway operations are performed using vehicle volume
paths they follow vary according to their choice of travel mode per hour.) These peak-hour factors can be determined from
(and parking facilities), and the on-airport roadway network airport roadway traffic surveys or indirectly from analyses of
configuration. Airline passengers follow numerous travel paths airline schedules. Traffic volumes generated by airline passen-
on an airport. For example, a private vehicle driver may enter gers can be estimated by the following:
an airport and then do one or more of the following:
· Multiplying the number of originating (or terminating)
· Go directly to the enplaning (or deplaning) curbside area airline passengers during the peak 60-minute period times
and then immediately exit the airport (e.g., a motorist drop- the percentage of passengers selecting each travel mode,
ping off an airline passenger who does not park), or recircu- adjusted using lead (or lag) times, and
late and return to the curbside (e.g., a motorist attempting · Dividing each volume by the corresponding vehicle occu-
to pick up a passenger and who was not allowed to remain pancy, taking care not to double count the same passen-
stopped at the curbside). gers (e.g., those in courtesy vehicles transporting parking
· Go first to a cell phone waiting area then proceed to the patrons). Exceptions are required for vehicles that may oper-
deplaning curbside to pick up an arriving airline passenger ate on a scheduled basis rather than in direct response to pas-
and then immediately exit the airport. senger demand (e.g., courtesy vehicles and scheduled buses).
· Go directly to a parking facility and park for the trip's dura-
tion (e.g., a long-term parking patron). Regression equations that correlate vehicle trips generated
· Go directly to the curbside area, drop off passenger(s), and to airline passengers to acres of airport property or other meas-
then continue to a parking facility and park for the trip's ures are provided in Intermodal Ground Access to Airports: A
duration (e.g., a long-term parking patron). Planning Guide, the ITE Trip Generation Handbook, and other
· Go directly to a parking facility, accompany a passenger into reference documents. Traffic volume estimates at commercial-
the terminal (or greet an arriving passenger at the baggage service airports developed using such equations are not con-
claim area), and then exit the airport (e.g., a short-term sidered reliable because of the significant differences in the
parking patron). characteristics of each airport, including differences in airline
· After landing at the airport, a passenger could go directly to activity peaking patterns and volumes; airline passenger demo-
a parking facility, retrieve his/her vehicle (which has been graphics (e.g., trip purpose, place of residency, travel mode
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Table 3-2. Typical vehicle circulation patterns--originating
airline passengers.
Travel mode Circulation pattern Percentage
Private vehicles
Drop off at curb, then exit 31%
Drop off at curb, then park--Hourly, remain 9
Drop off at curb, then park--Hourly, then exit 4
Drop off at curb, then park--Daily Parking 7
Drop off at curb, then park--Economy Parking 4
Direct to park--Hourly, remain for duration 4
Direct to park--Hourly, exit immediately 14
Direct to park--Daily 14
Direct to park--Economy 9
Direct to off-airport 4
100%
Rental Cars
Direct to rental car return 73%
Drop off at curb, then rental car return 23
Direct to off-airport 4
100%
Taxicabs
Drop off, then exit 83%
Drop off, then hold area 17
100%
Source: LeighFisher, July 2009, based on data gathered at Los Angeles International,
Salt Lake City International, Tampa International, and other airports.
preferences); passenger circulation patterns on and off the air- in the terminal building, return to their vehicle, drive to the
port; airport layouts; the availability of parking, public transit, curbside area to pick up the passenger, and then exit the air-
and commercial ground transportation services; and other fac- port, or (4) drop off passengers, park, and then return to the
tors influencing traffic volumes. terminal to accompany the passengers to/from the gate (e.g.,
a passenger with special needs, such as an unaccompanied
minor or a disabled passenger). The latter pattern (drop off
Traffic Generated by Visitors
and then park) has become less prevalent since 2001, because
The volume of traffic generated by visitors accompanying visitors are prohibited from accompanying an enplaning pas-
departing airline passengers (i.e., well-wishers) and arriving senger to an aircraft gate or greeting a deplaning passenger at
airline passengers (i.e., meeters and greeters) can be deter- a gate.
mined by establishing the average number of visitors accom- Similar to the travel times for airline passengers, visitor
panying each airline passenger or group of airline passengers. travel times shift from the scheduled aircraft departure and
The number of visitors accompanying a passenger is a func- arrival times. (See Figure 3-1.) By far, most visitors travel to
tion of airline passenger trip destination/purpose and the and from an airport in private vehicles. They rarely (i.e., less
demographics of the local community. For example, a greater than 5%) use public transportation or other travel modes.
number of visitors is expected to accompany airline passen-
gers traveling overseas for leisure purposes than those accom-
Traffic Generated by Employees
panying business passengers traveling on domestic flights. In
some cities, passengers are greeted by a large extended fam- Estimating the volume of traffic generated by airport
ily group, rather than one or two persons. Typically, visitors employees requires the following inputs.
either (1) use only the curbside areas, (2) park (for a short
period) while they accompany the airline passenger group Volume of employees and their work schedules. On an
to/from the terminal building, (3) park (for a short period) in average day, more than 10,000 people work at many large-hub
a parking lot (or cell phone lot) and, having met the passenger airports and more than 1,000 people work at typical medium-
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Figure 3-1. Sample airport visitor lead and lag time.
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hub airports (see Table 3-3). These people are employed by the operate 24 hours per day. Typically, the arrival and departure
numerous employers located on an airport, as follows: hours of employees at an airport do not coincide with regional
commute hours or with an airport's peak enplaning or deplan-
· The airport operator, including third-party contractors ing hours. For instance, major shift changes for airline employ-
working for the airport operator or sponsor, if different ees often occur between 5 A.M. and 6 A.M. and between 2 P.M.
(e.g., janitorial, parking operators, and bus operators), and 3 P.M. Another complicating factor is the presence of flight
providing services that have been outsourced; crews, who may only travel to/from the airport a few days per
· The airlines, including flight crew, aircraft maintenance, month. The trips made by flight crews at an origin-destination
and other employees who may not be working in the ter- (O&D) airport are sporadic, but while on an assignment, they
minal building; become like passengers at destination airports--requiring
· Concessionaires and other terminal building tenants, such courtesy vehicle service or flight crew transportation services
as rental car companies and the operators of newsstands, (i.e., chartered vans).
restaurants, and other retail establishments; Generally, employers are required to report the total num-
· Government agencies, including (at U.S. airports) the ber of their employees requiring security badges, but do not
FAA, TSA, Customs and Border Protection, Immigration report the number of employees working on each shift, the
and Customs Enforcement, U.S. Postal Service, and (at starting/ending times of each shift, or the travel modes used
some airports) the military; by their employees. Other than at airports with transportation
· Air cargo shippers and forwarders; management programs or ride-share promotional programs,
· Fixed-base operators; and few airport operators have accurate data indicating the num-
· Construction contractors, including construction workers ber of individuals working at the airport at any given time of
and subcontractors. day or the travel modes used by these individuals.
Surveys of the employers located on an airport are neces-
Airport-based employees, particularly those employed by sary to determine the number of people working on the air-
the airlines and cargo handlers, work unusual hours, because port, their work schedules, travel modes, and circulation
all commercial airports operate 365 days per year, and many patterns. Without such data (or traffic surveys conducted at
Table 3-3. Number of employees at selected airports.
Estimated
Total Parking average daily
Airport Hub size employees (a) permits employees (b)
Boston-Logan International Large -- -- 14,600
Bush Intercontinental/Houston Large -- -- 14,406
Chicago O'Hare International Large -- -- 40,000
Dallas/Fort Worth International Large 28,654 -- --
Denver International Large -- -- 17,400
Fort Lauderdale-Hollywood Large 14,000 -- 4,700
International
John F. Kennedy International Large 20,000 7,920 --
Lambert-St. Louis International Large -- -- 19,000
Las Vegas McCarran International Large -- -- 8,000
Los Angeles International Large -- -- 40,000
Phoenix Sky Harbor International Large 22,000 16,019 8,000
Salt Lake City International Large -- -- 13,026
San Diego International Large -- -- 3,000
San Francisco International Large 12,500 -- --
Seattle-Tacoma International Large -- -- 11,375
Tampa International Large 6,000 -- --
John Wayne (Orange County, CA) Medium -- -- 1,000
Mineta San Jose International Medium 4,750 -- --
Oakland International Medium -- -- 10,500
Omaha Eppley Airfield Medium -- -- 2,500
Portland International Medium 14,500 -- 5,000
Sacramento International Medium -- -- 1,500
(a) Includes badged and unbadged.
(b) Number of people working at the airport on an average day.
Source: LeighFisher, based upon information provided by individual airport operators.
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the entry/exit to employee parking lots), it is difficult to deter- Future employment and employee work schedules.
mine the number and pattern of employee vehicle trips. Forecasts of employment and employee trips tend to be impre-
cise because reliable estimates of future employment generally
Employee travel mode choices. As noted, little data are are not available and changes in future employment do not
available describing the travel modes used by employees on correlate well with changes in airline passenger numbers. His-
an airport. Data presented in ACRP Report 4: Ground Access torically, planners have estimated future employment assum-
to Major Airports by Public Transportation (2008), indicate ing that the rate of growth in employment represents the
that, at 14 airports for which data were available, about 98% average of the rate of growth in airline passenger and aircraft
of all employees working on the airport arrive and depart in operations numbers. However, anecdotal information suggests
private vehicles (with the exception of Boston-Logan, Chicago that this assumption is no longer correct because the airlines
O'Hare, and Denver International Airports). appear to be reducing their numbers of employees in order to
Employee reliance on private vehicles is a result of improve productivity levels and reduce costs. For example, the
(1) employees working nontraditional hours that do not co- increasing share of passengers who obtain their boarding
incide with the operations or the schedules of public trans- passes via the Internet or check their bags using electronic tick-
portation, (2) employees residing in locations not well served eting kiosks has reduced the need for ticket counter agents.
by public transportation (i.e., outside the central business It is suggested that additional research is required to develop
district), (3) employees working in locations outside of the methods for estimating the volume of traffic generated by
terminal area that are not well served by public transporta- employees on airports.
tion, and (4) the availability of free or very-low-cost employee
parking on airport property. Sample results. Using the steps presented above, the
One indicator of the number of vehicles driven by employ- employee trip generation rates presented in Table 3-4 were
ees on an airport is the number of parking permits or iden- developed as part of the Los Angeles International Airport
tification badges issued by the airport operator to these Master Plan Update. These data are presented as an example of
individuals. For example, in 1996, it was determined that 61% how employee trip generation rates can vary for a day or over
of the employees who were issued security badges at Los Ange- specific hours, and this example is not intended as a suggested
les International Airport had also been issued parking permits. proxy for another application.
The surveys indicated that, on a typical day, 29% of all employ-
ees were absent due to staff schedules, vacation, illness, or
Traffic Generated by Air Cargo
working away from the office. Of those employees traveling to
work on a typical day, it was determined that 64% drove alone, Air cargo (including airmail) traffic includes the trucks
33% participated in a ride-share program, and 3% rode public transporting the cargo, the private vehicles driven by the
transit, biked, or walked. The average vehicle occupancy for employees in the air cargo terminals, and customer trips. This
those individuals traveling to work at Los Angeles International traffic is generated by air cargo facilities (cargo terminals)
was 1.38 employees per vehicle. Because most of the large located away from the passenger terminal area, freight con-
employers operate multiple shifts, about 25% of the daily solidators or forwarders, and small package deliveries made
employee-generated vehicle trips occurred during a single directly to the terminal area.
hour. These data are similar to those reported at Boston- It is recommended that the volumes of trips generated by
Logan International Airport, where about 40% of all employ- trucks, delivery vans, and air cargo employees be estimated
ees are absent on a given weekday and about 25% of those separately. Employee vehicle trips are the largest component
working on a given day arrive between 6 A.M. and 10 A.M. of the traffic generated by an air cargo facility (over 70% of
the total traffic volume, according to surveys conducted at
Employee circulation patterns. The use of regional access Memphis and Los Angeles International Airports and other
roads and airport access roads by on-airport employees can be locations).
estimated by determining the minimum time path or mini- The volumes of truck and delivery van trips generated by
mum cost path between their places of residence and place of an air cargo facility (i.e., the trip generation rate) are unique
employment. Place of residence data, summarized at a zip-code to an individual airport and not transferable to other airports.
level, can be obtained from parking permit applications or from The two measures (or dependent variables) related to air
databases of airport-issued security badges. The minimum cargo that are most readily available--air cargo tonnage and
travel routes between these locations and points of access to the size of air cargo buildings--are not reliable indicators of
the airport can be determined using regional planning mod- the volume of cargo-related truck or total vehicle trips, largely
els or by planners familiar with the regional highway network. because there are many different forms of air cargo service,
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Table 3-4. Example of vehicle trips per employee working at
Los Angeles International Airport.
Employee trip generation rate (vehicle trips per employee)
Morning peak Airport peak Afternoon peak
Daily (8 A.M. to 9 A.M.) (11 A.M. to 12 P.M.) (5 P.M. to 6 P.M.)
Inbound 0.59 0.15 0.03 0.01
Outbound 0.59 0.01 0.03 0.15
Source: Leigh Fisher Associates, January 1996, using Los Angeles World Airports'
ride-share database representing a typical weekday, Los Angeles
International Airport Master Plan--Phase I, On-Airport Existing
Transportation Conditions.
including integrated cargo handlers, all-cargo or heavy freight As noted, air cargo is transported by a wide variety of cargo
carriers, as well as import, export, and shipments that require shippers, each having different trip generation rates. Little, if
special handling (e.g., flowers or fresh fish). Each form of air any, research has been published, or documented, on air cargo
cargo may generate a different number of truck trips, operate trip generation. Additional research is required to develop
at different truck arrival/departure times, and use different methods for estimating the volume of traffic generated by air
vehicle sizes. cargo terminals at airports and the employees working in these
For example, a local overnight delivery service operation terminals.
might have multiple tractor-trailers picking up and drop-
ping off containers, as well as dozens of local single-unit deliv-
Traffic Generated by Service and Delivery Vehicles
ery vehicles distributing packages locally. Conversely, a large
import/export freight operation may only generate a few Service and delivery vehicles include those vehicles (1) bring-
tractor-trailer trips. Thus, although airport operators have ing goods and materials (other than air cargo) to/from termi-
reliable statistics on air cargo tonnage transported, tonnage is nal building loading docks, consolidated warehouses, and other
not a reliable indicator of the volume of truck trips because the sites on an airport, (2) transporting individuals performing air-
volume of trips is a function of the type of cargo service and port maintenance and construction, (3) being used by airport
freight activity, not cargo tonnage (or the size of the air cargo police, fire, and emergency response staff, and (4) making trips
terminal). not directly generated by airport passengers, employees, or
air cargo. At most airports, little to no data are available on the
Sample results. Although not considered applicable to all current volume of service, delivery vehicle trips, or the activi-
airports, the data in Table 3-5, developed for Los Angeles Inter- ties generating these trips (i.e., the extent of goods and material
national Airport, present the estimated vehicle trips generated deliveries, trash removal, emergency responses, or construction
by different cargo facilities (including employee trips) per ton deliveries and traffic).
of air cargo. Generally, no data are available to guide estimates of the
Data from Chicago O'Hare International Airport, circa future volume of service/delivery vehicle trips, or the extent of
2004, indicate that a general-purpose cargo facility generated future activities generating these trips. Additional research is
about 0.13 daily truck trips per 1,000 annual cargo tons. required on this topic.
Table 3-5. Estimated airport cargo trips per daily cargo tonnage at
Los Angeles International Airport.
Daily Facility peak hour Commuter peak hour
trips (in Morning Afternoon Morning Afternoon
Cargo shipper and out) In Out In Out In Out In Out
International airline 25.2 0.39 0.13 0.19 0.29 0.23 0.13 0.16 0.16
Domestic airline 6.9 0.21 0.20 0.30 0.18 0.17 0.08 0.17 0.13
Overnight delivery service 3.0 0.30 0.24 0.77 0.27 0.30 0.03 0.55 0.26
Source: Leigh Fisher Associates, January 1996. Los Angeles International Airport Master Plan--
Phase I, On-Airport Existing Transportation Conditions.
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Traffic Generated by Other Airport Land Uses times on these routes as forecast by regional travel models or
other sources.
Other land uses commonly found at public airports include
general aviation/FBO facilities and military bases. At most
commercial-service airports, these other land uses do not gen- Off-Airport Origin and Destination Points
erate significant volumes of traffic during the peak hours for the (Trip Distribution)
airport or regional highway network. When the analysis is
Some non-hub and small-hub airports have single entry/exit
focused on the airport terminal area and primary airport access
points. At these airports, all vehicles enter and exit via one
roadways, the traffic volumes generated by these land uses are
roadway. The regional approach and departure vehicle distri-
often ignored or considered to be "background" traffic and
butions may be required to determine the proportion of left-
combined with that of service/delivery vehicles.
turn, right-turn, and trough traffic at the intersection of the
Traffic volumes generated by general aviation are a function
airport roadway with the regional highway network.
of the number of general aviation aircraft operations, and the Many airports have multiple entrance/exit points--one
type of aircraft (business jets, air taxis, or small propeller serving the terminal area and separate entrances/exits for air-
aircraft). Traffic volumes generated by military bases vary craft maintenance centers, general aviation terminals, military
according to the type of base and its function. Traffic volumes bases, or other land uses. Although the volume of traffic using
generated by nonaviation land uses that are not related to each entrance/exit can often be determined by the land use(s)
airport or aviation activity (e.g., industrial parks or large served by the specific entrance/exit, large airports may have
retail centers) can be estimated using the ITE Trip Genera- multiple connections to the regional roadway system, where
tion Manual. the use of each is determined by regional travel patterns (or a
combination of regional travel patterns and the on-airport
Traffic Generated by Nonairport Vehicles destination).
Using Airport Roadways For these large airports with multiple connections to the
regional roadway system, it is necessary to know the routes
Vehicles not related to the airport or airport land uses drivers follow when traveling to and from the airport in order
may use airport roadways as a shortcut to bypass congestion to analyze (1) the intersections or junctions of the airport
or delays on the regional roadway network. This traffic, access roadways and regional roadway network, (2) traffic
commonly referred to as cut-through traffic, adds to airport volumes on airport roadways associated with specific connec-
roadway requirements and contributes to airport roadway tions to the regional roadway network, and (3) the effect of
congestion. Cut-through traffic occurs at airports having multi- airport traffic on the regional roadway network. The routes
ple entrance and exit points (e.g., Dallas/Fort Worth, Phoenix drivers follow are a function of where they enter airport prop-
Sky Harbor, and Washington Dulles International Airports, erty and their on-airport destinations. These locations (or the
and Bush Intercontinental Airport/Houston) and where the distribution of these locations) are a function of airline pas-
roadway network configuration allows nonairport traffic to senger trip purpose, place of residency, regional land use pat-
share the airport roadways with airport-generated traffic. Most terns, the regional highway network, existing and forecast
airport operators discourage such cut-through traffic. roadway congestion/travel times, the availability of public
Determining the volume or proportion of existing cut- transit, and other factors.
through traffic may require recording and matching the license At airports having multiple entry/exit points serving the ter-
plates or electronic toll tags of vehicles entering and exiting minal area (or other major land use), drivers typically select
the airport at all major airport entry and exit points (i.e., a the most convenient entry/exit point, which generally implies
license plate matching survey or toll tag survey). It is not the point that minimizes travel time. It is possible to esti-
possible to identify cut-through traffic volumes using simple mate the proportion (and thereby the volume) of vehicles using
traffic volume counts. each entry and exit point by determining (1) the actual loca-
Estimating the volume of future cut-through traffic requires tions where motorists (including airline passengers, visitors,
an understanding of future regional land uses and expected and employees) begin their trips to the airport (or end their
regional traffic patterns/travel times. The volume of nonairport trips from the airport) or the distribution of these locations,
traffic using airport roadways is a function of the volume of and (2) the most logical routes used by motorists from each of
traffic on the regional roadways, and the travel-time savings these origin or destination points.
these vehicles would experience if they were able to use airport At many airports, fewer than 30% of all trips begin/end in
roadways as a shortcut. These time savings can be determined the downtown area, with the remainder arriving from or
by comparing the travel times via airport roadways and on going to places of residency and employment distributed
alternative routes, knowing the forecast congestion and travel throughout the region. A planner familiar with the regional
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highway network can determine the most likely routes from slow moving traffic at curbsides or other areas of potential
the primary regional origin and destination points. In addition, congestion.
these data (or trip distributions) can be obtained from surveys Generally, at an airport, most motorists follow the guide
of airline passengers or, when such data are not available, from signs directing them to the major on-airport destinations.
the local metropolitan planning organization, which can pro- Furthermore, most motorists will follow the prescribed routes
vide information on future distributions of places of residence even if they become congested, and typically deviate to a dif-
and employment, a description of the future regional trans- ferent route only if directed to do so by a traffic control offi-
portation network, and the likely travel paths or approach/ cer. Most employees and service vehicle drivers follow the
departure distributions. quickest route, unless they are prohibited from using specific
roads, or tolls or fees are associated with the use of specific
routes.
Assigning Traffic Volumes to the Roadway
The travel paths of originating airline passengers can be
Network (Trip Assignment)
determined using the information presented in Table 3-1
Assigning the traffic volumes generated by airline passen- (revised for the specific characteristics of the airline passengers
gers, visitors, employees, air cargo, and service/delivery vehi- and airport being analyzed), and the travel paths of terminat-
cles to the on-airport roadway network requires information ing airline passengers can be determined using similar infor-
as to (1) where these vehicles enter or exit the airport, (2) their mation. As noted, care must be taken when assigning trips
final and interim destination or origination points on the made by passengers who use multiple travel modes (e.g., those
airport, and (3) the routes or paths available to these vehicles. who park in a remote parking lot and also use a courtesy vehi-
cle) or multiple legs (e.g., those who go to the curb and then
· Airport entry and exit points. The methodology for deter- to parking).
mining traffic volumes entering and exiting an airport at For example, assuming that 100 vehicle trips per hour are
specific locations is provided earlier in this chapter (see Esti- generated by originating airline passengers at an airport; 65%
mating Traffic Volumes [Trip Generation]). of these trips are generated by private vehicles; 30% of those
· Origin and destination points on the airport. The method- private vehicles go to the curb and then go to parking, where
ology for determining the volumes of trips associated with they remain for their trip duration; and 80% arrive from the
specific on-airport origins and destinations is also provided east and 20% arrive from the west, these assumptions result in
in the previous section on Estimating Traffic Volumes (Trip 20 vehicle trips by private vehicles using both the curb and
Generation). daily parking (100 × 65% × 30%), of which 16 vehicles enter
· Travel paths. Typically, on a regional roadway network from the east and 4 enter from the west.
motorists can select from several alternative travel paths. The trip assignment process for airport roadways requires
Thus, a sophisticated traffic assignment procedure is (1) repeating this calculation for every combination of travel
required to allocate these vehicle trips among the available mode, circulation path, and regional approach/departure
travel paths (i.e., to assign the vehicle trips to the regional path, (2) assigning these vehicle trips to the corresponding
roadway network) and, if desired, allocate trips to alterna- roadway links, and (3) finally determining the sum of all
tive routes, as primary routes become congested and travel vehicle trips assigned to each roadway link. The sum of the
times decrease. In comparison, on an airport, there is gen- vehicle trips on each roadway link represents the estimated
erally only a single logical travel path available for airline traffic volume on that link. Travel forecasting software or
passengers and visitors, employees, and air cargo vehicles. spreadsheet analyses are frequently used to perform this
Thus, the traffic assignment process is much simpler at repetitive process, particularly when traffic forecasts are
airports. being prepared for large airport roadway networks. The use
of these methods allows planners to readily test the implica-
At most airports, there is only one travel path available tions of alternative assumptions regarding mode choice,
between the airport entry and exit points and the primary travel paths, or airline passenger activity patterns, as well as
origin/destination points. For example, at most airports, saving time and effort.
there is only one route connecting the airport entrance/exit
and the terminal curbside areas, public parking areas, or
Challenges with Estimating
rental car ready/return areas.
Roadway Traffic Volumes
Exceptions include those airports having several entrances/
exits used by airline passengers, or having multiple terminal As noted, several challenges are associated with estimating
buildings served by separate roadways. Some large airports roadway traffic volumes--either existing or future--using
provide internal bypass roads allowing motorists to avoid the traditional four-step travel forecasting techniques. Key
