Millions of Britons are expected to take to the roads on what is expected to be the busiest day of the Christmas getaway. Airports and ports are also full of people travelling to spend the festive period overseas.In America, snowfall in the rockies, strong winds in the West and soaking rain in the South caused problems for holiday travellers on the first official day of winter, forecasters said.Gusty Santa Ana winds are expected  through Friday morning across Southern California into the start of the holiday weekend.In Britain, a spokesman for the airports authority said up to 18 million cars would be on the roads over the 24-hour period from Thursday night to Friday evening.“There will be a delayed rush hour today,” he said.Getaway traffic is likely to be heavy on major routes, including the M1 in Buckinghamshire and Bedfordshire, the M4 between London and Newbury in Berkshire, the M25 around London, and the M4 and M5 in the West Country.The National Weather Service in America said the winds, generated by a strong high-pressure system would continue but no “red flag” warnings for fire danger are forecast.