from the CA vehicle code (thanks, craigslist forums!):

21959. It is unlawful for any person to ski or toboggan on or across any roadway in such a manner as to interfere with the movement of vehicles thereon. A person on skis proceeding on or across a highway at a pace no greater than a walk is not within the prohibition of this section and shall be considered to be a pedestrian with all the rights and duties thereof as prescribed in this code.