To be exact I am talking about in ca. on county maintained roads. A friend and I were arguing discussing this. I thought it was ten feet to either side and underneath. I tried to do a search but came up empty.
Many kinds of roads--adjudicated by many agencies--both state and federal and private. Bottom line is 50'-100'--legally known as MUST NOT UNDERMINE ADJACENT AND SUBADJACENT SUPPORT FOR ANY BRIDGE,ROAD,OR PIER. Cal-Trans and counties highway dept online for exact verbage--John