To understand what the NHTSA was trying to say, imagine the following scenario: It’s a foggy day, and the driver of an SUV is traveling along a road at the posted speed limit of 40 miles per hour. The limit then drops to 25 as the road approaches a town—but the road’s lanes do not narrow (which would naturally compel a driver to apply the brakes), and the lone sign announcing the lower speed limit is partially obstructed. Oblivious to the change, the driver keeps traveling at 40. As he enters the town, a pedestrian crosses the road at an intersection without a stoplight. The driver strikes the pedestrian.
By the federal government’s definition, the “critical reason” for this hypothetical crash—the last event in the causal chain—is the error made by the driver who was speeding at the time of the collision. Almost certainly, the police will hold him responsible. But that overlooks many other factors: The foggy weather obscured the driver’s vision;, flawed traffic engineering failed to compel him to slow down as he approached the intersection; the SUV’s weight made the force of the impact much greater than a sedan’s would have been.
The authors of the 2015 NHTSA report were aware of such contributing factors. But their disclaimer that the “critical reason” for a crash is not the same as the “cause” has been largely ignored. Even a page on the agency’s own website whittles the message down to “94% of serious crashes are due to human error.”