The idea of autonomous vehicles, or cars that drive themselves, has become so popular that almost four dozen cities around the world are experimenting with them.

Bloomberg Philanthropies and the Aspen Institute have compiled a list of all the cities testing or developing AVs by interviewing and surveying public officials as well as media reports and public documents. The list includes 47 cities that are hosting pilot programs or have committed to doing so. An additional 22 cities have begun considering the regulatory, planning, and governance implications of AVs but have not yet agreed to trial them. The list will be updated continuously as Bloomberg and Aspen learn about new initiatives.

The project also administered an online survey, to which 38 cities responded. A quarter of the surveyed cities began their AV projects within the last year. Most foresee AVs providing the “last mile” of transportation to people’s homes after they depart existing buses, rails, or other public transportation. Many also anticipate AVs to replace taxis and mass transit.

Of the 30 cities who responded to questions about the obstacles they face developing autonomous vehicle projects, a lack of money and expertise ranked highest. A few cities also lack clarity or consensus on specific actions for moving forward, and a handful face regulatory hurdles from higher levels of government.