Airlines complete complex analytical techniques to estimate the predicted success of a market before launching routes. Macroeconomic and microeconomic factors will be critically assessed prior to launching a route. They will assess the existing traffic between countries, cities, and airports to determine the number of passengers travelling directly and indirectly.
If there are large numbers of passengers already flying indirectly between airports, then it may be suitable for a new direct route. Airports will have identified gaps in their route map and will work with airlines to launch routes. Flight search engines can provide useful data. They have data on the number of people searching for flights on particular routes.
However, demand data at this level does not necessarily equate to a profitable route. The airline needs to have the capacity in aircraft and staff to service new routes. There is also the opportunity cost of launching one new route over another to assess.