Whether you love the idea or hate it, the self-driving car is likely coming soon, and it's going to have a huge impact. I wrote about why the Google car could change everything, and noted that since cars were parked 90 percent of the time, it was likely that we might need 90 percent fewer of them and they would always be on the move doing something. 

Now a new study from the University of Michigan Transportation Research Institute looks at the question in detail: how many will be needed and how will they be used? Report authors Brandon Schoettle and Michael Sivak crunched data from a recent National Household Travel Survey (NHTS), looking at detailed information about every trip that drivers in a household make. The key information that they wanted to know was what trips did not overlap? If a self-driving car can return home on its own after delivering its occupant instead of being parked at the destination, then it can serve someone else in the house.

For example, if Ward Cleaver’s self-driving car takes him to work and then returns home, June can use it for shopping or the car can take Beaver and Wally to the malt shop — and it's likely that these functions all happen at different times. 

The authors of the report calculate that instead of households having an average of 2.1 cars, this could be reduced 43 percent to 1.2 cars per household. That’s huge. The flip side of the coin is that the car is going to be driving a lot further — 75 percent more, from 11,661 miles per year to 20,406. When I multiply the numbers, I find that the 2.1 cars travel 24,448 miles per year and the 1.2 cars travel 24,487 — just slightly more but that doesn’t include the deadhead miles traveled by the self-driving car to return home empty. Report co-author Brandon Schoettle tells Bloomberg Business:

It could be that sharing the vehicle ends up increasing the mileage because of all these connecting trips. The net effect is probably going to be an increase in mileage, and in general the more miles you drive the more fuel you burn.
However, as Dilbert cartoonist Scott Adams notes in this great new cartoon, Robot Read News, robots are better drivers than meat-sacks. No doubt they will use less fuel per mile. And while there will be the same number of cars on the road, there will be very few that are going in circles looking for parking. It's also likely that robot-driven cars will have smaller engines or be electric. (Robots don’t feel the need to do 0 to 60 in four seconds.)

The authors admit in their conclusion that there are many unknowns; that’s an understatement. But where I thought most self-driving cars would be shared and there would be far fewer of them, this study makes clear that in the traditional "Leave It To Beaver" suburban family that now has more than two cars, it will be busy and families will probably own it. And, the roads will still be full of them. 

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Lloyd Alter ( @lloydalter ) writes about smart (and dumb) tech with a side of design and a dash of boomer angst.