It has just gone noon in El Paso, Texas, and a cacophony of car horns screams out in the blistering heat as drivers with frayed tempers wait in line to cross the border with Mexico.

Since the terrorist attacks of 9/11 in 2001, when US authorities chose security over speed in processing the long lines of cars and people crossing into the US, such scenes have become commonplace. But today, the queue is for people travelling south to Mexico – a journey that until recently took no time at all.