Statistically Significant: The History and Criticism of NHTSA’s “94% Driver Error” Figure

Recently, Partner Mike Nelson shared an article about the safety benefits of ADAS features on his LinkedIn. The post unexpectedly generated some debate, most pointedly over the article’s statement that “NHTSA warns that 94% of serious crashes are directly linked to human error”—with one commenter calling the statistic “propaganda” and another taking the position that the statistic was “too low.”

We took a closer look. The 94% statistic originated in a 2015 NHTSA Traffic Safety Facts publication entitled “Critical Reasons for Crashes Investigated in the National Motor Vehicle Crash Causation Survey.” There, NHTSA summarized that:

The National Motor Vehicle Crash Causation Survey (NMVCCS), conducted from 2005 to 2007, was aimed at collecting on-scene information about the events and associated factors leading up to crashes involving light vehicles. Several facets of crash occurrence were investigated during data collection, namely the precrash movement, critical pre-crash event, critical reason, and the associated factors. A weighted sample of 5,470 crashes was investigated over a period of two and a half years, which represents an estimated 2,189,000 crashes nationwide. About 4,031,000 vehicles, 3,945,000 drivers, and 1,982,000 passengers were estimated to have been involved in these crashes. The critical reason, which is the last event in the crash causal chain, was assigned to the driver in 94 percent (±2.2%)† of the crashes. In about 2 percent (±0.7%) of the crashes, the critical reason was assigned to a vehicle component’s failure or degradation, and in 2 percent (±1.3%) of crashes, it was attributed to the environment (slick roads, weather, etc.). Among an estimated 2,046,000 drivers who were assigned critical reasons, recognition errors accounted for about 41 percent (±2.1%), decision errors 33 percent (±3.7%), and performance errors 11 percent (±2.7%) of the crashes.

Although NHTSA caveated that the “critical reason” should not “be interpreted as the cause of the crash,” the 94% statistic has been repeatedly used incorrectly to imply that the vast majority of roadway collisions and accidents could have been avoided but for human driver error. Many autonomous vehicle advocates, like the author of the article Mike posted on LinkedIn, have used the 2015 “human error” statistic to promote the development of ADAS technologies as a solution. NHTSA even cited the statistic on its “Automated Vehicles for Safety” website page.

But the statistic has also been the subject of criticism, long before the debate we saw unfold on LinkedIn. Earlier this year, Jennifer Homendy, Chair of the National Transportation Safety Board (NTSB), offered her own criticism when she reportedly called NHTSA’s use of the 94% statistic on its website “dangerous” and said that it “has to change.” According to Homendy:

[The statistic] relieves everybody else of responsibility they have for improving safety, including DOT. . . . You can’t simultaneously say we’re focused on a ‘safe system’ approach—making sure everybody who shares responsibility for road safety is taking action to eliminate fatalities and serious injuries . . . and have a 94% number out there, which is not accurate.

In an interview with Streetsblog, Homendy stated that while the NTSB does consider the human driver when investigating vehicle crashes, there are other factors, including “the vehicle, the environment, the roadway conditions, the regulatory oversight. . . [B]y saying this 94 percent statistic over and over again, that becomes a way for people to think, ‘Well, we can’t really do anything about it. [Road users] just shouldn’t make terrible decisions.’”

Homendy also encouraged a shift in how we talk about road collisions: “In highway safety, we always say crash; other modes, like aviation, likes the word ‘accident.’ It’s hard. But I think that nothing is an accident. [These incidents] are all preventable, every single one of them. But everybody shares the responsibility to prevent them, not just one single individual.”

The 94% figure remained on NHTSA’s website from 2015 all the way through January 2022, when the agency finally removed the claim after Homendy publicly criticized its use.

Copyright Nelson Niehaus LLC

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