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.
9/21/17 - Experimental Rust cars have been revealed in wipe week. Currently only available to admins and must be spawned from the console using the command "spawn sedan". Cars only support one passenger and collisions are still being worked on. It can be destroyed using rockets, how ever, it will not show an animation. It will simply disappear.