Head races are time trial / processional races that take place from autumn (fall) to early spring (depending on local conditions). Boats begin with a rolling start at intervals of 10 – 20 seconds, and are timed over a set distance. Head courses usually vary in length from 2,000 metres (1.24 mi) to 12,000 metres (7.46 mi), though there are longer races such as the Boston Rowing Marathon and shorter such as Pairs Head. The oldest, and arguably most famous, head race is the Head of the River Race, founded by Steve Fairbairn in 1926 which takes place each March on the river Thames in London, United Kingdom. Head racing was exported to the United States in the 1950s, and the Head of the Charles Regatta held each October on the Charles River in Boston, Massachusetts, USA is now the largest rowing event in the world. These processional races are known as Head Races, because, as with bumps racing, the fastest crew is awarded the title Head of the River (as in “head of the class”). It was not deemed feasible to run bumps racing on the Tideway, so a timed format was adopted and soon caught on. Time trials are sometimes used to determine who competes in an event where there is a limited number of entries, for example the qualifying races for Henley Royal Regatta, and rowing on and getting on for the Oxford and Cambridge Bumps races respectively.