sag wagon n. at a bicycling event, the vehicle which carries bicyclists who have withdrawn from the event (due to injury, bicycle malfunction, tiredness, etc.) (source: Double-Tongued Dictionary)
Further reading
When she was a child, Chris from Santa Rosa, California, misunderstood the definition of the word vehicle—with amusing results. This is part of a complete episode.
Related
originally known as a “voiture balai” French for “Broom Wagon” which ‘sweeps up’ cyclists who have ‘thrown in the towel’. This van follows the tail end of the motorvehicle entourage daily following the Tour De France and other major races.