Sometime in the 1990's, Caltrans built a section of roadway in the median of I-15 (where the HOT lanes are now) in northern San Diego County with I believe a row of magnets embedded in the concrete. I can't find the video for it on YouTube, but specially modified cars would detect these magnets and use them as guidance. I'm not sure if the cars actually drove themselves or if it was simply an enhanced form of cruise control.
While self-driving road trains may become a reality thanks to Google, there are a lot of legal and technical issues that need to be worked out. For example, if one of the cars in the road train has a flat tire or mechanical issue, how will the driver be able to safely regain control and pull off to one side without causing a multi-car pileup?
Personally, I think a more feasible system is a moto-rail system like those used Europe, in which you can load your car onto a regular train for long distance travel. No need to make a grueling multi-hour drive, better for the environment, and less traffic on the roads. We already have a form of this called the Auto Train between Virginia and northern Florida.
