DENVER (AP) — Eight states have now signed up in an effort to encourage U.S. automakers to develop affordable vehicles that run on natural gas.

The governors of Colorado and Oklahoma announced in November that their states, along with Pennsylvania and Wyoming, intended to seek proposals to replace vehicles in their state fleets with ones that run on natural gas.

Since then, Maine, Utah, New Mexico and West Virginia also have signed on.

The idea is to drive demand for vehicles that run on alternative fuel — and for fueling stations that serve those vehicles.