The Advanced Snow Plow system that Caltrans used as a test on Interstate 80 near Donner Summit had four miles with embedded magnets. This cost about $25,000/mile in '98.

The full description of the system can be read at https://path.berkeley.edu/sites/default/files/advanced_snowp...

Alaska uses GPS for their precision plowing - https://www.truckinginfo.com/329914/how-alaska-dot-uses-gps-... . Note that to get the 2" precision you need high quality GPS receivers.

> The trucks have two GPS receivers mounted atop the cab. These receivers cost about $10,000 each, Shankwitz says. "That's probably why this hasn't been deployed in many other areas; it's just too expensive and most applications do not require that level of accuracy."

> The two-centimeter accuracy actually comes from a third receiver -- a high-precision, stationary ground-based receiver perched atop a microwave communications tower in nearby Valdez. It's accurate to within millimeters and it acts as reference receiver for the plow-mounted systems.

I don't see this being standard on self driving cars.

That said, ground penetrating radar is being looked at and appears to be a lower price point. https://phys.org/news/2016-06-vehicles-high-precision-advers...