Explanatory VariablesThe econometric research estimated regression models that included a variety of explanatory variables to search for the best fit: AveragePrice.• Average air travel price was used to measure the price of air travel, reflecting average route prices over the period reported. The average price variable appears in all model specifications.GrossDomesticProduct.• 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