Figure 4 - 4: Geographic Traffic Distribution
(Of the two samples, Terminal Airspaces, “G” is typical of the core area of Europe and “H” of the geographic periphery of Europe. Because of the marked predominance of traffic distribution to the south/south-west of Terminal Airspace ‘H’, this model could fit the northern geographic periphery of Europe. Inversely, were the major traffic flow to/from the North, the model would probably fit that of the southern geographic periphery of Europe. The same can be said of dominant east or west flows).
Sorting the geographic traffic distribution by origin and destination so as to identify the raw demand3 is only necessary when (i) doubt exists that the current En-Route ATS route network is not sufficiently refined thus making it lightly that some aircraft are not on the most direct route or, (ii) in the case of futuristic design projects for new airports where part of the exercise is trying to develop an entire airspace organisation on a clean sheet. The diagrammatic representation of raw demand is not nearly as clean as that of entry/exit point.
3 this is usually the same as market demand.
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Figure 4 - 5: Geographic Distribution – Raw Demand
Given that the thicker lines in the above diagram represent routes of heavier (raw) demand, it is possible to ascertain – by comparing the location of existing Terminal entry/exit point [black circles above] in relation to these lines –whether these points have been placed effectively.
In those instances where En Route airspace designers alter their route network within the greater EUR ARN so as to minimise the differences between the raw demand ‘tracks’ and actual traffic routeings, it is not necessary for Terminal Airspace design planners to undertake the ‘raw demand’ exercise – providing that En-Route or Terminal Airspace design is undertaken collaboratively4 as a matter of course.
The significance of the proper identification of the predominant traffic flow(s) becomes evident when undertaking the route design process described in Part C Chapter 5. This is because the designer should strive to meet all the Guidelines of route design as regards the major traffic flows. Thus where a ‘conflict’ arises between the interests of a major flow and minor flow, the interests of the major flow should prevail.
Comment: Why should the traffic sample be analysed when ATC knows the traffic distribution? Many designers are surprised to discover errors in the way they perceive their major/minor traffic flows. This is particularly true when dealing with traffic samples based on forecast traffic where it may be incorrectly assumed that traffic increases will be proportionate to each entry/exit point.
4.3.1.3 Using Forecast Traffic Samples
Forecasting air traffic provides its own challenges: the more futuristic the forecast, the greater the likelihood of error creeping into some of the assumptions. Complex by definition, traffic forecasts attempts to determine whether and to what extent the traffic will change (increase or decrease) by examining the triggers that may bring about these changes. Whilst some triggering events can be forecast with reasonable accuracy, others cannot be easily foreseen.
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