Cars have been conceptualized on Rust's official website and within the community, but have not been implemented in Experimental Rust as of yet. There is no official word on whether or not cars will be introduced any time soon, or when so. The Rust development team proposed a modular system for the construction of vehicles, such as having interchangeable wheels or different uses (e.g. personal transport, combat purposes, resource transport, etc.)
Not much else is known.