Study: One-way car-sharing reduces VMT, GHG emissions, and vehicle ownership

By Robbie Webber

A recent study done by researchers at University of California-Berkeley has answered several questions many have had since car-sharing began: Does car sharing increase or decrease driving? What is the effect on overall VMT, vehicle ownership, and GHG emissions among car-sharing members?

The researchers used data from car2go in five North American cities—Calgary, San Diego, Seattle, Vancouver, and Washington, D.C.—and found that vehicle ownership was reduced, either by members selling their vehicles or not purchasing a vehicle. Although only a small percentage of members sold or delayed purchasing vehicles, the impacts added up. For each car2go vehicle in the study cities, membership sold one to three vehicles and delayed or did not purchase four to nine vehicles. On average across the study cities, each car2go vehicle removed seven to 11 private vehicles from the roads.

In addition, overall vehicle miles traveled and greenhouse gas emissions were reduced among members. Low estimates of GHG emissions reduction ranged from four to seven metric tons of CO2 equivalent per year per car2go vehicle, while the high estimates calculated 10-14 metric tons per year per vehicle.

Another question examined by the researchers was the impact of car-sharing membership on use of other modes. In general, car-sharing was found to complement instead of replace use of transit and active transportation modes. In some cities transit use among members was reduced, while in others it was increased, and the same was found to be true for active transportation modes. In no city were the changes among members found to be consistent or significant. Only taxi and ridesourcing/transportation network company (e.g., Uber) use was found to be reduced consistently.

The researchers used data only from car2go, a company that provides one-way car-sharing, allowing members to pick up and drop off a car at different locations instead of requiring a round trip and return of the car where it was picked up. This model is popular in urban areas that have a variety of convenient transportation options, allowing members to trip chain using a variety of modes. Members might take transit to work, walk, or use bike-sharing to dinner or shopping, and then use a car to return home with bulky purchases or when transit is less frequent.

car2go is the largest car-sharing operation in the world as well as the dominant one-way car-sharing company. Use of the data from one source allowed the researchers to compare fine-grained information across study cities.

Although this research was specific to one company and one model, principal researcher Susan Shaheen has found a reduction of vehicle ownership among members of round-trip car-sharing organizations as well. She notes that, although car-sharing tends to be most popular in denser urban areas with existing transportation alternatives, in the future, car-sharing could also have a role in helping connect people to transit in less dense suburban areas.

Robbie Webber is a Senior Associate at SSTI.