I wouldn’t let them loose on the road, but it is fairly easy nowadays to build a rudimentary one. See for example http://www.donkeycar.com/, https://makezine.com/projects/build-autonomous-rc-car-raspbe..., https://zhengludwig.wordpress.com/projects/self-driving-rc-c....

The rest is a matter of scaling up, both in car size (easy) and in software (lots of work)