A new study conducted by AlixPartners, a consultancy that has automotive clients, estimates that there are already 500,000 fewer vehicles on the road due to changing attitudes about owning a vehicle and the rise of car sharing services.

By 2020, it expects 4 million people will be sharing cars, and that there will be 1.2 million fewer cars.

AlixPartners interviewed 1,000 licensed drivers in ten U.S. metro markets to understand what’s motivating people to use car-sharing services and what effect that’s having on vehicle sales. The survey covered Austin, Texas, Boston, Chicago, Miami, New York, Portland, Ore., San Diego, San Francisco-Oakland, Seattle and Washington, D.C.

Boston, where Zipcar was founded and first launched, showed the strongest awareness of car sharing among survey-takers. New York and D.C. participants were the two groups most likely to avoid ownership. Younger consumers and households with children were most likely to do without their own car.

Among people who have already shared a car, 51% said they’ve avoided purchasing or leasing, and that 45% expect to continue to so in the future. AlixPartners estimates that a single sharing vehicle equates to 32 personal vehicles that don’t get purchased.

To provide a comparison with major cities, the consultants also interviewed 1,000 people nationally. The second survey found that 34% have foregone purchasing or leasing a vehicle, while 21% expect to act similarly going forward.

Among survey-takers, cost and convenience rated highest as reasons for sharing over owning. Only 23% cited the environment as a primary factor.

AlixPartners expects car sharing to keep up its growth. It points, in particular, to single way services, which make one-way rides possible, and more smartphone adoption, which makes reservation, locking, and locating a vehicle easier. Driverless cars could also give the industry a bump once they begin to appear, the report says.