Contributions of other participants and factors. Sivak and Schoettle noted that not all crashes are caused by drivers. While an autonomous vehicle could compensate for some crashes caused by other traffic participants (they use the example of a drunk stepping into the roadway), and could in principle respond more quickly than a human, it might not be able to stop in time in all situations because of braking limitations.

Further, a small percentage of crashes are caused by vehicle failures. The UMTRI team sugests that there is no reason to expect that such “other” vehicle failures would be less frequent on self-driving vehicles than on conventional vehicles.

Indeed, given the complexity of the sensing hardware and of the information-processing software, it is reasonable to expect that, overall, vehicular factors would likely occur more frequently on self-driving vehicles than on conventional vehicles.

—Sivak and Schoettle

Certain roadway (e.g., floods and downed power lines) and environmental factors will also cause problems.

Information. Fully realizing autonomous driving will require detailed mapping, including locations of streetlights, stop signs, crosswalks, lane markings, and all other crucial elements. Further, this information will need to be updated in real time.

The role of experience. Fatalities per distance driven is heaving influenced by the age of the driver, and is represented by a U-shaped function with the lowest rates for middle-aged drivers.

To the extent that not all predictive knowledge gained through experience could exhaustively be programmed into a computer (or even quantified), it is not clear a priori whether computational speed, constant vigilance, and lack of distractibility of self-driving vehicles would trump the predictive experience of middle-aged drivers.

—Sivak and Schoettle

Transition period. Turning over the vehicle parc in the US takes a long time—the current average age is 11.4 years. There will be at least a several-decade-long period during which conventional and autonomous vehicles will need to interact.

One main concern during this transition period is that drivers of conventional vehicles would have certain expectations about the likely actions of other vehicles (depending on factors such as the location of the interaction, the type of the other vehicle, and the age and gender of the driver of the other vehicle, etc.). … Furthermore, in many current situations, interacting drivers of conventional vehicles make eye contact and proceed according to the feedback received from other drivers. Such feedback would be absent in interactions with self-driving vehicles. The degree of the importance of both driver expectations and feedback from other drivers, and the consequent effects on the safety of a traffic system containing both conventional and self-driving vehicles, remain to be ascertained.

—Sivak and Schoettle

From their discussion, Sivak and Schoettle drew four conclusions:

• The expectation of zero fatalities with self-driving vehicles is not realistic.

• It is not a foregone conclusion that a self-driving vehicle would ever perform more safely than an experienced, middle-aged driver.

• During the transition period when conventional and self-driving vehicles would share the road, safety might actually worsen, at least for the conventional vehicles.

Resources

• Michael Sivak, Brandon Schoettle (2015) “Road Safety With Self-Driving Vehicles: General Limitations And Road Sharing With Conventional Vehicles” UMTRI-2015-2 (UMTRI publications found here when released.)