Institute of Transportation Studies Working Paper UCB-ITS-WP-81-2 THE CONSISTENCY OF TRAFFIC FORECASTS FOR AIRPORT MASTER PLANNING Adib Kanafani Paper prepared for presentation to the Third World Airports Conference, Singapore, March 1981 University of California Berkeley, February 1981 ISSN 0192 4141
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Institute of Transportation Studies
Working Paper UCB-ITS-WP-81-2
THE CONSISTENCY OF TRAFFIC FORECASTS
FOR AIRPORT MASTER PLANNING
Adib Kanafani
Paper prepared for presentation to the Third World Airports Conference, Singapore, March 1981
University of California
Berkeley, February 1981
ISSN 0192 4141
2
THE CONSISTENCY OF TRAFFIC FORECASTS FOR AIRPORT MASTER PLANNING
Adib Kanafani
University of California, Berkeley
Introduction
The importance of traffic forecasting in the preparation of airport
master plans cannot be overemphasized. Traffic forecasts provide an in-
dispensable input for the preparation of facility designs, economic evalu-
ations, and investment programs for airport development. If it is recog-
nized that the purpose of airport master planning is to provide for an
orderly and rational development of an airport through to its ultimate
potential, then it is possible to argue that the purpose of traffic fore-
casting in this process is not to predict future volume, per se, but to
provide for a consistent set of assumptions regarding the parameters of
design and evaluation. The important inputs into the master planning pro-
cess are not necessarily the predictive statements of what the traffic
levels might, or will, be at some future date, but the planning statements
of what traffic levels the airport is to be planned for. The difference be-
tween these two is vital and often overlooked in airport master planning.
Aggregate traffic forecasts are almost certain to be inaccurate, particu-
larly as the forecast horizon becomes longer. The airport master plan
should be made robust to this possible discrepancy. It should be based on a
consistent set of assumptions regarding the constitution and the charac-
teristics of the traffic volumes for which the airport is being planned. It
is the characteristics of traffic, such as aircraft mix and peaking pat-
terns, that determine the ultimate potential capacity of a given airport
site. When this ultimate level will in fact materialize is a prediction
that cannot be made with certainty. Nor need it be. The development of an
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airport system can be made to adapt to actual traffic growth and evolution
with only short-term forecasting necessary, if sufficient flexibility is
built into the master plan.
Current State-of-the-Art
In current practice, it is customary for the airport sponsor to provide
to the master planner a set of forecasts of total traffic. These forecasts
are usually provided for passengers and for total aircraft operations, as
these are the sorts of forecast variables that are amenable to econometric
forecasting. The record on this type of forecasting is clear: most long-
term forecasts of passenger traffic and of aircraft operations have tended to
overestimate actual developments. Figures 1 and 2 show the results of
forecasts made in and around 1969 for 1980 traffic levels compared with the
actual volumes of 1978 and 1979. Figure 1 shows the comparisons for annual
passenger forecasts, and Figure 2 for annual aircraft operations. These
comparisons cover a range of airports including high volume ones such as
London Heathrow and medium volume ones such as Tampa, Florida. The trend is
self-explanatory: the forecasts in all cases have overestimated traffic.
Particularly, in the case of operations forecasts, there seems to be a tend-
ency for the overestimation of volume to increase with the level of traffic.
It should be noted that these figures are approximate and indicate that bar-
ring any dramatic traffic growth between 1979 and 1980, an overestimation of
traffic by the 1969 forecasts is demonstrable.
The impacts of such an overestimation should not be as critical as one
might believe at first. It usually results in a master plan in which the
development of the airport system is seen to occur at a faster pace than what
in fact occurs. Larger facilities may be justified on the basis of the high
forecasts, but then their construction is delayed because traffic does not
materialize as anticipated earlier. At worst, some facilities may be built
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earlier than the optimal year, with the result that resources are misallocated
for a period of time. Eventually, traffic grows and the available facilities
will be put to use.
The need for accuracy is much more critical when dealing with some of the
design and planning parameters that are either derived from the forecasts, or
based directly on independent assumptions. Errors in forecasting some of
these parameters may result in irreparable damage to the airport system plan.
As an example of this, consider the number of passengers per operation. The
forecasts for this parameter made in 1969 for 1980 are shown in Figure 3
compared with actual 1978/79 figures. Here the trend is also clear. There is
not consistent over- or underestimation, but in the majority of cases, the
forecast and the actual numbers are far apart. The type of planning error
that can result from this type of forecast discrepancy can be detrimental to
the efficient functioning of the resulting airport system. If the number of
passengers per operation is overestimated, then the terminal facilities and
the access facilities will be overdesigned in relation to the airfield and the
apron-gate subsystems. Additional traffic growth, whether rapid or slow,
will not correct this inconsistency, resulting in landside facilities that are
underutilized. If, on the other hand, the number of passengers per operation
is underestimated, then the opposite will occur, and landside facilities will
be overcrowded in relation to the airfield and apron/gate facilities. The
crowding of the passenger terminal facilities at Los Angeles (LAX), for
example, should not be surprising given the actual and the forecast passengers
per operations (86 and 55, respectively).
Traffic Assumption Needs in Master Planning
In airport master planning, numerous assumptions need be made regarding
the constitution and characteristics of forecast traffic volumes. Total
annual passenger volumes and aircraft operations are of only limited value
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as planning inputs by themselves, as most of the facility designs and evalua-
tions are based on considerably more detailed breakdowns of these volumes. It
is usually not possible to forecast these detailed characteristics, and hence
assumptions must be made regarding them. These assumptions should be arrived
at by a consensus of the various parties involved in airport master planning.
Many of them are based on observed historic characteristics which are modified
to account for changes in technology and in operating procedures.
The following is a listing of the traffic assumptions most often used
in planning, and the purpose for which they need to be made:
Table 1
Assumptions Needed in Airport Master Planning
Assumption
Annual Passengers
Annual Operations
Design Hourly Passengers
Design Hourly Operations
Fleet Mix
Use in Planning Economic evaluation.
Economic evaluation,and general sizing of airfield facilities.
For design of terminal facilities, and access system.
For design and layout of airfield system, and sizing of apron and air-craft gate positions.
For airfield design, apron and gate position design, and for integration of airside and landside facilities.
In addition to these basic inputs, assumptions have to be made regarding
parameters that provide for a consistent linkage between them. The following
are important characterizations of traffic about which assumptions must be
made: passengers per operation; load factor; proportion of transfer and
transit traffic; access mode choice; and incidence of passenger companions. In
addition, these assumptions have to be made separately at the annual level, and
at the design hourly level. For example, the annual number of passengers per
operation is different from that during the peak hour. The former may
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be useful in some economic analysis, but the latter is crucial for the de-
sign of the airport facilities and for integrating the plans of the airside
and the landside.
Consistency in Forecast Assumptions
The concern of this section is not with the assumptions made as a part of
the econometric forecasting process used to arrive at forecasts of total annual
passenger and operation volumes, but at the assumptions made in order to arrive
at the more specific parameters used in planning and design. It is usually
necessary to take a set of figures that have been forecast exogenously and use
them to derive these planning parameters.
In order to illustrate the concept of consistency between forecast assump-
tions, we take the case where the following four traffic figures are given