The effect of a 10% fare increase on both routes (for instance caused by a rise in a national passenger tax) would be to reduce the total traffic in the market by 8% (the weighted average of the route net effects), which is exactly what is implied by the aggregate market elasticity of -0.8. Using the weighted average elasticity of -1.4, on the other hand, would incorrectly imply a 14% decrease in aggregate air travel.
This example considers the impact on outbound leisure passengers, where a rise in passenger tax will affect all destination choices. That is not the case for inbound tourists. The choice facing US residents in travelling to destination A, say the UK, or destination B, say Italy will be significantly affected by national passenger taxes. For example, the recent doubling of the UK passenger departure tax added roughly 4% to the cost of travel. This will have had a relatively inelastic impact (-3.2%) on UK residents departing on overseas holidays, for the reasons set out above. However, it will have led to an elastic impact (-5.6%) on the choice of US residents travelling to the UK. Many will have been diverted to holiday in, say, Italy. In total, this demand response would significantly limit the effectiveness of national passenger taxes as a way of managing demand or limiting the rise of greenhouse gas emissions from air travel.