The rapid growth of car-sharing programs has cut the number of vehicles on U.S. roads by more than half a million, according to new research by AlixPartners, a consultancy group with clients in the automotive industry. The trend will continue beyond 2020, the group projects, at which point 4 million people will be participating in car-sharing programs and 1.2 million fewer cars will be on the road. Of the 10 cities surveyed, residents of Boston, home of the Zipcar company, were most aware of car-sharing programs. Young people and, surprisingly, households with children were least likely to own their own cars, the survey said. Roughly half of the people who had tried car-sharing had already decided not to purchase or lease their own car, and did not plan to do so in the future. Rather than environmental concerns, nearly 60 percent of interviewees said cost and convenience led them to participate in car-sharing.