The single worst auto racing accident of all time occured in the 1955 24 Hours of LeMans.  An incredible 83 spectators, plus one driver were killed.  Another 120 people were injured.  Click through for the original Pathe News (British) newsreel report.

An accident on a straightaway caused Pierre Levegh’s Mercedes Benz significant damage at speeds near 150 mph.  The already damaged car hit an earthen embankment and came apart.  The hood, the front axle, and the engine all broke apart and tore through the crowd on hand.  The race organizers decided to continue the race in fear that the departing throng would clog the streets and keep ambulances from delivering the injured to local hospitals. 

Hours later, Mercedes decided to pull their remaining team from the race in a show of respect for the dead and injured.   

Thankfully, the auto racing industry has improved both driver and spectator safety exponentially.  Still, any time vehicles are reaching speeds of 150+ MPH, bad things can happen.  Here’s hoping a tragedy of this magnitude never repeats itself.