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

3

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

4

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

5

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

6

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

exogenously: Annual Passengers AP, Annual Operations AO, Design Hourly Pas-

sengers HP, and Design Hourly Operations HO. These last two could signify peak

hour volumes, or some other hourly volume such as the thirtieth busiest hour or

the peak hour of the average day of the peak month. The relationship between

these four traffic figures is illustrated in Figure 4. It can be given by the

following formulae:

HP = AP. x

HO = AO. y

HP = HO. m

AP = AO. n

It is clear from the requirements of consistency that for the rectangle of

Figure 4 to close, the following relationships must hold:

(x/y) = (m/n)

Note that m and n are important planning parameters; m more so than n. The

number of passengers per operation during the design hour, m, is an important

determinant of landside facility sizing, and of the balance between landside

7

and airside capacity. Attempts are often made to forecast this indicator

exogenously by projecting and adapting historic trends. It is noted that the

assumptions regarding m are closely related to those regarding the fleet mix

and the prevailing load factor during the design hour. Let ACH be the

aircraft size during the design hour, and LFH the corresponding load factor.

Then

m = ACH.LFH

which means that assumptions regarding m should be consistent with assump-

tions regarding the load factor and the fleet mix. Such assumptions cannot

be made by the master planner without consultation with the airline, for

example, whose planning is based on similar assumptions of fleet mix and load

factors. It is also important to note that projections of m must be made

separately from n which is usually derived simply by dividing the exogenously

given forecasts of AP and AO (n = AP/AO). The reasons for this is that m is

usually larger than n, indicating that the number of passengers per operation

during the design hour is larger than that averaged over the whole year. The

same applies to the projection of the design hour factors x and y. While both

of these factors will vary over time, basically declining as the traffic

volume increases at the airport, it is common to expect that x will always

exceed y. While airline schedules may offer more flights during periods of

high passenger flows, it is usual for load factors to increase during those

periods and hence for the operation's design hour factors to be lower than

those for passengers.

When constructing such an "Assumptions Rectangle" for an international

airport, it is important to distinguish between domestic and international

traffic, and between origin-destination and transit traffic. Even if sim-

ilar peaking characteristics can be assumed similar for both, it is likely

that seating factors m and n are different due to the different fleet mixes

8

used in domestic and international operations. This is illustrated by showing

an example "Assumptions Rectangle" construction done as a part of the

elaboration of the master plan for the Viracopos International Airport serving

the Sao Paulo region in Brazil. The rectangle shown in the following

illustration is for the year 2000 and is based on the following exogenous

forecasts:

AP = 6.488 million 0-D plus 1.5 million international transit

AO = 119,035

HP = 152 domestic plus 2130 international 0-D and 526 international transit

ACH = 133 seats domestic 248 seats international

LFH = 13% domestic

60% international

In order to complete the assumptions rectangle, it is necessary to com-

pute the hourly operations. This is done on the basis of the information

given regarding average aircraft size ACH, the load factors LFH, and the

hourly passenger flows HP.

Domestic Operations = (Domestic HP). (Domestic ACH.LFH) = (152)/(133 x 0.13) = 9 operations

International Operations = (2130 + 526)/(248 x 0.60) = 18 operations

Total Hourly Operations HP = 9 + 18 = 27.

The given information can be used to construct the rectangle and to

calculate the corresponding values of m and n for total traffic, and m' and n'

for traffic excluding the transit flows.

A check of the consistency can be simply made by comparing (x/y) with

(m/n) or (m'/n') in this case all having a value of 1.59. It is noted that

the separation of transit flows in this case is essential since these flows

9

would count in the calculation of aircraft operations and hence in runway

system planning, but not in landside flows and facility planning. In other

words, while HO = 27 is a planning figure for the airfield system, HP' = 2270

would be the corresponding passenger flow figure for terminal and landside

planning.

Another aspect of consistency in assumptions is the need to adjust some

of the parameters. For example, the rectangle in Figure 5 can be constructed

for different forecast years, or levels of traffic volume. For each, dif-

ferent assumptions regarding m, n, x, and y need be made. Fleet mixes usually

change over time resulting in different aircraft sizes, and so do the peaking

characteristics resulting in different design hour factors x and y. It is

customary to observe that as the overall traffic volume increases at an

airport, the peak hour factor declines. This is expected since for an airport

with a single operation, the peak hour factor will have a maximum value of

unity, and for an airport with constant traffic flow throughout the whole

year, the peak hour factor will have a minimum value of (1/365 x 24 =

0.00011). The phenomenon is illustrated in Figure 6 where typical trends in

design hourly traffic DHT and design hour factor DHF are shown. Such trends

would apply similarly whether traffic is measured by passengers or by opera-

tions. The adjustments of the design hour factor are essential for achieving

consistency in the master plan, and for avoiding imbalance between the various

components of the airport system. We illustrate this with an example from the

Viracopos International Airport master plan. In that master plan, passenger

terminal units are designed for a load of 2000 hourly passengers each. In

preparing a staging plan for the airport toward the ultimate capacity of the

site, it was necessary to seek a balance between the ultimate capacity of the

airfield area, with three parallel runways, and the terminal

10

building area with space available for eight terminal units. To do this, it

was necessary to project the annual capacity of the terminal system with dif-

ferent numbers of terminal units, and then to compare that with the corre-

sponding operations capacity of the runway system as it evolves from the

single runway currently in use to the two and consequently three runway con-

figuration. Figure 7 illustrates how the capacity projection of the terminal

system is done. As the number of terminal units is increased, the peak

hourly flow capacity is correspondingly increased in increments of 2000 pas-

sengers. These hourly flow capacities correspond to increasingly higher an-

nual capacities since the peak hour factor is assumed to decrease. Thus,

while two terminal units giving a capacity of 4000 peak hour passengers will

correspond to an annual capacity of ten million passengers, four terminal

units with an hourly capacity of 8000 passengers correspond to an annual

capacity of twenty-seven million passengers. The following table illustrates

this projection.

Table 2

Traffic Projections for Viracopos Airport

Year Annual Passengers AP'

Design Hour Factor X

Hourly Passengers HP'

Terminal Units

1985 1.1 M 0.04% 440 1

1990 1.4 0.04% 560 1

1995 5.0 0.04% 2000 1

2000 6.4 0.035% 2240 2

2005 8.0 0.035% 2800 2

2010 24.0 0.030% 7200 4

2015 28 0.030% 8400 5

2020 33 0.030% 10000 5

11

Note that if the system capacity were obtained by simply multiplying the

number of terminal units by five million annual passengers, then a totally

different staging plan would be obtained, and one that is likely to lead to an

oversizing of the passenger terminal system. Figure 7 illustrates this by

comparing the forecast capacity with the flows obtained by this simple

multiplication, according to which seven terminal units would be needed by the

time the annual traffic volume reached thirty-three million versus five

according to the project capacity. The implications on the airport layout

plan are significant. In this particular case, the number of runways is

limited by the physical characteristics of the site to three. The approach to

the layout plan is to allocate only as much space to the terminal system as

would be needed to provide a passenger capacity corresponding to what can be

handled by three runways. Even taking into consideration the uncertainties

regarding future aircraft size and runway capacity flow rates, it would still

make a difference whether one allocated space for a maximum of six or of eight

terminal units, since this means a total land area of 9.4 or 6.8 sq. km.,

respectively.

Summary and Conclusions

Achieving consistency in the assumptions used in airport master planning

is more important than attempting forecasting accuracy. Long range forecasts

of airport traffic are not likely to be accurate. A flexible airport master

plan can provide robustness against forecasting errors. However, errors in

some of the planning assumptions cannot be corrected against and may result in

lasting suboptimalities in the development of the airport system. The attempt

to achieve consistency in making planning assumptions is an attempt to avoid

some of these pitfalls in airport master planning.

Finally, it is recognized that airport master planning continues to be

based on assumptions rather than accurate predictions of the future. This,

12

however, should not preclude making these assumptions realistic. This can be

done by, first, seeking consensus on the assumptions among all the parties

involved in the planning process, and second, maintaining consistency among

all the planning assumptions made.

13

Fig. 1 COMPARISON OF ACTUAL AND FORECAST AIRPORT TRAFFIC

14

Fig. 2 COMPARISON OF ACTUAL AND FORECAST AIRPORT OPERATIONS

15

Fig. 3 COMPARISON OF ACTUAL AND FORECAST PASSENGERS PER OPERATION

16

Fig. 4 ASSUMPTIONS RECTANGLE OF AIRPORT TRAFFIC

17

Fig. 5 ASSUMPTIONS RECTANGLE FOR VIRACOPOS AIRPORT

YEAR = 2000

18

Fig. 6 TRENDS IN DESIGN HOUR FACTOR

19

Fig. 7 FORECASTING TERMINAL SYSTEM CAPACITY