With the transformation of the two lane road into one, accidents become more likely. Make the transition smoother with a proper warning before half a mile. Signs work.
Standard: 01 The ONE LANE ROAD (W20-4) sign shall be used only in advance of that point where motor vehicle traffic in both directions must use a common single lane (see Section 6C.10). It shall have the legend ONE LANE ROAD, XX FEET, XX MILES, or AHEAD.