roads that should be widened to 24 feet to serve their traffic properly. There are also about 875 miles of two-lane roads that should be converted to four-lane divided highways; about 1,350 miles now surfaced with three or more lanes undivided which should be rebuilt as multiple-lane divided roads; and there are more than 450 miles of divided highways on which the traffic lanes are presently of inadequate width.
Shoulder width
Except in mountainous areas, the defined standards call for shoulder width of 10 feet, measured to the intersection of shoulder and side- slope planes. This is equivalent to an effective shoulder width of about 8 feet. In mountainous areas the standards call for an effective shoulder not less than 4 feet wide. Shoulders effectively 8 feet wide are needed to provide an extra emergency lane into which to turn to avoid a collision. They are needed also as an area in which to stop for the changing of tires or the making of other emergency repairs, and for other similar purposes related to highway safety and the avoidance of obstruction of free traffic flow. More than 71 percent of the rural mileage of the system, 22,684 miles, is without shoulders as wide as 8 feet. On this mileage, parked vehicles must necessarily encroach to a greater or less extent on a traffic lane.
Nearly 6,700 miles have no shoulder as wide as 4 feet, the minimum allowed by the standards in mountainous areas. Since only 1,949 miles of the rural system are in areas classed as mountainous, it is apparent that much of this gross shoulder inadequacy has no such justification of exceptional terrain.
