If you are new to triathlons or not familiar with the sport, you might be confused by what is the order is of a triathlon. Traditionally, a triathlon consists of three events done in a continuous manner, a swimming section, a cycling section and a running portion of the race.
Swim-bike-run is the traditional order of a triathlon
Most triathlons order their events in a swim/bike/run format, although the first triathlon in Mission Beach in San Diego had the swim portion last. One main reason given for having the swim portion of the triathlon first is for safety reasons. Starting the race with the swim portion ensures that the triathletes are not already
Triathlons that don’t follow swim-bike-run format
While most triathlons follow the swim/bike/run order for their triathlon races, some races that have to use an indoor pool for their swim portion may reorder the race into to run/bike/swim order for the race in order to spread out the triathletes on the run and bike course before they reach the pool and swim portion of the race. This reverse order triathlon allows a more steady or spread-out number of triathletes entering the pool since they have already been spread out on the run and bike based on their running and cycling abilites.
Having the bike or cycling portion of the triathlon second allows triathletes a little bit of recovery right after finishing the swim where they can pedal in an easier gear while either taking on food and hydration or simply warming up from a cold water swim. The bike portion of a triathlon also usually covers a longer distance than the run portion and requires more road closures and police/traffic control than the run portion of a triathlon, so having the bike portion second after the swim allows the race director to get the triathletes on and off the bike course earlier in the day before road closures can significantly impact local traffic. Road safety is another reason for having the bike second, especially with longer-distance triathlons such as Ironman-distance races that can take up to 17 hours to complete. If the cycling portion of the triathlon was last, then many of the triathletes would be completing the bike portion of the triathlon in the dark.