How about "Play Nice And Share Otherwise The Government Won't Let Anybody Use The Roads." It's the PNASOTGWLAUTR campaign. Think it'll catch on?
My serious comments: Bicycles allowed full use of lane is about the only saying that comes close to letting drivers know that they'll have to burn an extra calorie and turn the wheel 5 degrees to change lanes and pass. Share the Road sounds too much like "Cyclists move over" (honestly my interpretation back in high school and that of a few motorists I've encountered) and Bikes in Road sounds like "Look out, bicycle obstacle course up ahead."