The largest human migration takes place in China each year during the Chinese Lunar New Year festival period. Hundreds of millions of people travel by cars, buses, trains, and airplanes to head back home during the Spring Festival. The great migration usually lasts for 40 days and places great pressure on the transport system across the country. For example, during the Spring Festival of 2017, 3 billion trips were made, among which 2.5 billion were by road.