Anybody who drives in Arizona at this time of year will be familiar with the annual migration of “snowbirds” — retired people seeking to escape the sub-zero temperatures of the north for the warmer desert winter. As many as 300,000 snowbirds flock here each year, so if you haven’t seen them before, there’s a good chance that you will encounter one of them when you find yourself stuck behind a caravan with Nebraska licence plates doing 40mph on a single-lane highway.