60' accounts for a 30' right of way, plus a 30' front building setback line. So MBL probably doesn't stand for Mean BUILDING line.
Road right of ways are rarely controlled by the location of the physical centerline. I don't know the exact circumstances in this case, but usually the road is defined by existing monumentation, deeds, recorded plat maps, etc. Gravel roads unless very well maintained are notorious for moving around over time.