I was looking for the answer to this question. It brought me here, but the closest question was [closed]. The most that I am aware of was when driver Pierre Levegh's race car killed 83 spectators at Le Mans: June 11th, 1955. Was wondering if this was the most or if there might be more. There were 84 deaths in all, including the driver. Technically it could be considered a 2 car accident, as his car launched off of another car. Maybe question should be 2 car accident?
As gleaned from my research, it seems that the road accident with the highest death toll in history was the Salang Tunnel fire, having occurred on November 3rd, 1982. Here is the link for the Wikipedia article: https://en.wikipedia.org/wiki/Salang_tunnel_fire. If the Soviet bulletin is to be believed (giving the one who posed the initial question the deference in this instance), than the undesirable title of deadliest road accident ever is ceded to the Los Alfaques disaster; link here: https://en.wikipedia.org/wiki/Los_Alfaques_Disaster
Consider the ruthless profiteer John D. Rockefeller's epigram: "I always tried to turn every disaster into an opportunity." I suppose in an infecund way you did :)