Driverless Cars Won't Make Roadways Perfectly Safe While self-driving cars could eventually improve driving conditions overall, the expectation of zero fatalities is not realistic, a new report says.

One of the most popular selling points for the self-driving car movement is that if we can get tired, distracted drivers out from behind the wheels of cars, the roadways will be exponentially safer.

That may be true, if you leapfrog generations into a future where there are no longer any drivers at all and where the technology of driverless cars has become more sophisticated. Until then, there's a lot of muddy water, according to a recent report from the University of Michigan Transportation Research Institute, an organization that studies transportation safety and sustainability.

In a utopian world, autonomous autos could indeed be more safe than cars that depend on drivers. "The safety performance of self-driving vehicles could, in principle, be made perfect," the report says. That said. The report also concludes, "the expectation of zero fatalities with self-driving vehicles is not realistic."

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To be sure, there is a crisis of safety on our roadways. There were 32,850 deaths in 2013 in the U.S. (the latest year the data is available for) as a result of accidents on the roads, according to data from the U.S. Department of Transportation sited in the report. Across the globe, 1.24 million people died in 2013 as a result of crashes, according to the the World Health Organization.

But as drastic as those death-counts are, not all of them are a result of accidents made by drivers, the report says. Accidents on roadways can be a result of careless pedestrians, malfunctioning automobile parts, roadway damage, weather conditions and the like. While smart, driverless cars would certainly be able to prevent accidents in some of those cases, they would not be able to predict for and prevent all accidents in all cases, the report says.

For example, sensing technology in today's driverless cars does not work in fog, snow or especially heavy rain.

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In general, whether or not a self-driving car is a safety improvement over a car driven by a human depends on the age of that human, the report says. For example, a self-driving car would likely be safer than a young, inexperienced driver as well as an older driver with diminished reflexes. The safest drivers, according to the report, are middle aged because experience allows them to predict the behavior of other drivers and they haven't started to lose any response time in their reflexes.

"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 ... whether computational speed, constant vigilance and lack of distractibility of self-driving vehicles would trump the predictive experience of middle-aged drivers," the report says.

Eventually, if all cars on the road are as safe as those otherwise operated by middle-age drivers, that sounds like a significant safety improvement. But during the transition period -- when some cars have drivers and some don't -- the danger on roadways could even potentially increase, the report finds.