As four-lane North Glebe Rd (St Rte 120) descends to cross the Potomac River at Chain Bridge the posted limit decreases to 20 mph before it is really necessary for most modern vehicles and just before the road bends to provide an excellent vantage point for police. While there may be some justification for buses or trucks, or under bad conditions, generally the highway is clear and the slow-down seems to just benefit the county treasury since there is no way to see trap.