Autonomous vehicle will have lower rates of severe crashes: VTTI

11 January, 2016

Published: January 11, 2015 | Arlington, VA

In a recently published report, VIRGINIA TECH TRANSPORTATION INSTITUTE compared the crash results of conventionally driven vehicles and self-driving vehicles. This would seem to be an easy question to answer: simply compare current published crash rates with the data on self-driving cars. A deeper look at the available data and collection methodologies, however, reveals that such a simple comparison is problematic.

Two factors complicate the national crash data. First, states have different requirements concerning what incidents are reported as crashes. Second, many crashes go unreported. Estimates of unreported rates of crashes have ranged from as little as 15.4 percent to as much as 59.7 percent (Blincoe et al., 2015; M. Davis & Co, 2015). The result is that the current national crash rate is essentially a low estimate of the actual crash rate.

Legal requirements for self-driving cars further complicate matters. In California (arguably the jurisdiction covering most automated vehicles), every crash involving a self-driving car, regardless of how minor, must be reported. Thus, we have a situation in which we are attempting to analyze self-driving car data, which has a full record of all crashes, relative to the current vehicle fleet, which has an incomplete record of crashes. The comparison is, as the old saying goes, apples to oranges.

The study, “Automated Vehicle Crash Rate Comparison Using Naturalistic Data” assessed driving risk for the United States nationally and for the Google Self-Driving Car project. Driving safety on public roads was examined in three ways. The total crash rates for the Self-Driving Car and the national population were compared to (1) rates reported to the police, (2) crash rates for different types of roadways, and (3) scenarios that give rise to unreported crashes. First, crash rates from the Google Self-Driving Car project per million miles driven, broken down by severity level were calculated.


When compared to national crash rate estimates that control for unreported crashes (4.2 per million miles), the crash rates for the Self-Driving Car operating in autonomous mode when adjusted for crash severity (3.2 per million miles; Level 1 and Level 2 crashes) are lower. These findings reverse an initial assumption that the national crash rate (1.9 per million miles) would be lower than the Self-Driving Car crash rate in autonomous mode (8.7 per million miles) as they do not control for severity of crash or reporting requirements. Additionally, the observed crash rates in the SHRP 2 NDS, at all levels of severity, were higher than the Self-Driving Car rates. See fig

As self-driving cars continue to be tested and increase their exposure, the uncertainty in their event rates will decrease. Current data suggest that self-driving cars may have low rates of more-severe crashes (Level 1 and Level 2 crashes) when compared to national rates or to rates from naturalistic data sets, but there is currently too much uncertainty in self-driving rates to draw this conclusion with strong confidence. However, the data also suggest that less-severe events (i.e., Level 3 crashes) may happen at a significantly lower rate for self-driving cars than in naturalistic settings.


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