"Shoulders increase safety and highway capacity and provide a place for pedestrians and bicyclists when no sidewalks are provided.... The treatment of shoulders is important from a number of perspectives, including safety, the capacity of the highway section, impact on the surrounding environment, and both the initial capital outlay and ongoing maintenance and operating costs. The shoulder design should balance these factors. For example, a designer must consider the impact of the shoulder width and other roadside elements on the surrounding environment and, at the same time, how these dimensions will affect capacity."

Shoulders

Although the physical dimensions of automobiles and trucks limit the basic width of travel lanes, the treatment of that portion of the highway to the right of the actual traveled way, that is, the “roadway edge,” provides the designer with a greater degree of flexibility. This is true in both urban and rural areas, although different design elements are more appropriate in each location.

Shoulder widths typically vary from as little as 0.6 m (2 ft) on minor rural roads, where there is no surfacing, to about 3.6 m (12 ft) on major highways, where the entire shoulder may be stabilized or paved.

The treatment of shoulders is important from a number of perspectives, including safety, the capacity of the highway section, impact on the surrounding environment, and both the initial capital outlay and ongoing maintenance and operating costs. The shoulder design should balance these factors. For example, a designer must consider the impact of the shoulder width and other roadside elements on the surrounding environment and, at the same time, how these dimensions will affect capacity. Even with a maximum lane width of 3.6 m (12 ft), the absence of a shoulder or the presence of an obstruction at the edge of the travel lane can result in a reduction in capacity of as much as 30 percent, compared to an area where shoulder or clear zone exists that is a minimum 1.8 m (6 ft) wide. On the other hand, significant environmental, scenic, or historic resources may be adversely affected by a widened shoulder.

Another consideration is the accommodation of pedestrians and non motorized vehicles. In many parts of the country, highway shoulders provide a separate traveled way for pedestrians, bicyclists, and others (when no sidewalks are provided).

In addition to the dimensions of shoulders, designers have choices to make about the materials used. Shoulders may be surfaced for either their full or partial widths.Some of the commonly used materials include gravel, shell, crushed rock, mineral or chemical additives, bituminous surface treatments, and various forms of asphaltic or concrete pavements.

In a number of States, particularly in the southern part of the country where snow removal is not an issue, grass or turf surfaces have been provided on top of compacted earth embankments. The advantages of grass shoulders are that they provide both a natural storm water detention system and are aesthetically pleasing. The disadvantages can be that they are often less safe than paved shoulders and force pedestrians and bicyclists to share the road with motorists, if no off-street facility is provided.

Shoulders represent an important element in roadway drainage systems by carrying surface runoff away from the travel lanes into either open or closed drainage systems. A variety of design treatments have been used to accommodate roadway drainage across shoulder areas. In rural and suburban areas, the most common technique allows surface runoff to cross over the shoulder and go directly into drainage ditches running parallel to the roadway edge.

In rural areas where significant physical and/or environmental constraints exist, more “urban” style solutions have been used. For example, along an older section of Maryland State Route 51, passing through the Green Ridge State Forest in Allegany County, steep, narrow cuts along the existing alinement severely limited the total roadway width. Asphalt curbing and a closed drainage system were constructed in conjunction with a recent pavement rehabilitation project. This allowed for a modest widening of the travelway and elimination of an area of steep and narrow ditches, without the need to engage in major earthwork.