The autobus forms on mountain stages when non-climbers fall off the back of peloton during the climb. Gradually some riders fall off the back of the peloton and form a large group at the end of the race. These riders are generally sprinters or domestiques unconcerned about their finishing positions in the mountain stages. Their primary concern is beating the elimination time to ensure their survival in a multi-stage race such as the Tour de France.
If the autobus arrives outside the elimination time, the organisers may extend the elimination time if the autobus is large enough. However if a rider is dropped from the autobus in this situation they will almost certainly be eliminated unless they have suffered an accident, in which case they may be exempted.
Due to the common interest in survival the mood in the autobus has a greater sense of camaraderie than in the main peloton. Riders often assist one another with food and drinks despite riding for separate teams.