• Gross Domestic Product. GDP is used to measure
the effect of income on air travel. GDP estimates
are widely available, providing a variable that can
be consistently defined between regions and over
time. Within the US, GDP estimates are available at
the city pair level. Regression models using the IPS
data set used UK national GDP. Regression analysis
for all other regions used national GDP values, converted into US dollars. GDP appears in all model
specifications.