The study looked at how transportation and income are related in EU-15 countries from 1970 to 2008. They found that the connection between transportation and income goes both ways. This means that as income goes up, transportation also increases, and vice versa. They also noticed that countries with lower incomes had different patterns. Portugal, Greece, and Italy showed less clear connections between transportation and income. The researchers think that this two-way relationship only happens after an economy has finished developing.