231.4 Shoulder Width

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231.4.1 Introduction

A shoulder is the portion of the roadway contiguous to the traveled way that accommodates stopped vehicles, emergency use, and provides lateral support of the subbase, base and pavement. Shoulders may be paved (with concrete or asphalt) or unpaved (with aggregate or soil). This article describes only the geometric aspects of shoulders. For surfacing, refer to Shoulder Surface.

Desirably, a vehicle stopped on the shoulder should clear the edge of the traveled way by at least 1 ft., and preferably by 2 ft. This preference has led to the preferred use of a 10 ft. shoulder on major roadways. A shoulder at least 2 ft. wide is encouraged on minor roadways.

On urban roadways, the shoulder is located inside a curb. Surfaced areas behind curbs located on urban roadways may be perceived as a sidewalk and thus subject to ADA requirements. Therefore, a surfaced area is not to be provided behind a mountable curb.

A minimum shy distance at least 2 ft. wide is often used to increase roadside safety when roadside barriers, walls or other vertical elements are present in the roadway. A shoulder that is provided should be wide enough to ensure the vertical element is offset at least 2 ft. from the edge of the traveled way to provide the minimum 2 ft. wide shy distance. Chapter 5 of AASHTO's Roadside Design Guide provides design guidelines for selecting and designing a roadside barrier system. Chapter 3 of AASHTO's Roadside Design Guide discusses the clear zone concept that should also be consulted when considering roadside barrier systems.

Regardless of the width, a shoulder functions best when it is continuous. The full benefits of a shoulder are not realized unless it provides a driver with refuge at any point along the traveled way. A continuous shoulder provides a sense of security so all drivers making emergency stops will leave the traveled way. Although continuous shoulders are preferred, narrow shoulders and intermittent shoulders are still superior to no shoulders at all.

231.4.2 Pavement Resurfacing and Rehabilitation Projects

Each district has an asset management plan specific to their area and the needs within the district. When a pavement resurfacing or rehabilitation project arises, the district will make shouldering improvement decisions that are consistent with the assumptions in their district’s asset management plan as well as their district’s safety plan.

In addition, new paved shoulders may be included as part of rehabilitation projects if safety funding is available for this part of the project cost and if a reasonable expectation of severe crashes exists. A benefit cost (B/C) analysis should be completed to ensure the location being considered for shoulders will provide a good return on investment. This analysis should compare the expected reduction in fatal and serious injury crashes over the life of the treatment to the costs of the safety improvement. For segments with an AADT of 4,500 or greater, a systemic B/C of 12:1 has already been calculated by the Highway Safety and Traffic Division. For segments with and AADT between 2,500 and 4,500, a systemic B/C of 8:1 has already been calculated. For segments with less than 2,500 AADT, a separate B/C analysis will need to be performed. A B/C analysis for 1,000 to 2,500 AADT should be performed by Highway Safety and Traffic to allow for systemic analysis.

231.4.2.1 Rural Major Routes

For rural major routes, rehabilitation projects shall provide a 4-ft. minimum, paved shoulder. A design exception shall be submitted when providing a 4-ft. paved shoulder is not feasible or practical. Always consider the context of the surrounding route. New construction projects should provide shoulders with a width of 4 feet to 10 feet.

231.4.2.2 Rural Minor Routes

The shoulder on rural minor roadways serves as structural support for the pavement and as additional width for the traveled way. This permits drivers meeting or passing other vehicles to drive on the edge of the roadway without leaving the surfaced area. Roads with a narrow traveled way, narrow shoulders and significant traffic tend to provide a poor level of service, have a higher crash rate, and need frequent and costly maintenance.

If a benefit/cost ratio study identifies the need for the construction of new shoulders or the addition of shoulders as part of a project:

• For rural minor roads with a demonstrated history of severe lane departure crashes or with characteristics identified as part of a systemic safety improvement, rehabilitation projects should provide a minimum 2-ft. paved shoulder and edgeline rumble strip. A benefit cost analysis should be completed to ensure the location being considered for shoulders will provide a good return on investment. This analysis should compare the expected reduction in fatal and serious injury crashes over the life of the treatment to the costs of the safety improvement. For segments with an AADT of 4,500 or greater, a systemic B/C of 12:1 has already been calculated by the Highway Safety and Traffic Division. For segments with and AADT between 2,500 and 4,500, a systemic B/C of 8:1 has already been calculated. For segments with less than 2,500 AADT, a separate B/C analysis will need to be performed. Always consider the context of the surrounding route. Adjacent segments not meeting this criteria may also be good candidates for paved shoulders, particularly if bookended by segments that do.
• New construction projects should provide 4-ft. shoulders.
• Rural minor routes classified as low-volume (less than 400 AADT) should not receive paved shoulders.

231.4.3 Shoulder Widening Projects

Shoulders are the portion of the roadway contiguous with the traveled way that accommodates stopped vehicles, emergency use, and lateral support of subbase, base, and surface courses. In some cases, the shoulder can accommodate bicyclists. Shoulders typically consist of the following types: earth (sod), aggregate, asphalt, or concrete product.

EPG 231.4.3 Shoulder Widening Projects should be considered for shoulder widening adjacent to an existing roadway only. Since shoulder widening is not the same as the construction of new Type A2 or A3 shoulders, see EPG 231.4.1 Introduction for recommended shoulder widths. For new travel lane and shoulder construction, see EPG 231.3 Lane Width and EPG 231.4.2 Pavement Resurfacing and Rehabilitation Projects.

231.4.3.1 Conceptual Stage

A Conceptual Study Report should be completed for all shoulder widening projects. As part of the conceptual stage, all National Environmental Policy Act (NEPA) documentation for environmental and historic preservation clearances must be addressed (see EPG 127 MoDOT and the Environment). All project documentation shall be stored in eProjects.

231.4.3.2 Design

Typically, shoulder widening projects are constructed in conjunction with a resurfacing/overlay project. This allows the newly widened shoulder to be overlaid with the same surface material as the travelway lanes. Edgeline rumble strips are a safety enhancement that should be considered with shoulder widening construction. (Refer to EPG 626 Rumble Strips for guidance.)

The standard shoulder widening design should include a bituminous base (paid in tons), placed on a compacted subgrade and capped with an asphalt surface that is placed monolithic with the roadway resurfacing. An optional concrete typical section should be included in the plans showing a concrete base in lieu of the bituminous base and capped with an asphalt surface that is placed monolithic with the roadway resurfacing. Include the Special Provision Optional Shoulder (JSP-13-03), which details how a no-cost change order will be issued to accommodate a contractor’s choice to use a concrete base. Placement of an aggregate base under the bituminous base is not typically necessary; however, if the existing subgrade is determined to be unstable, a 4-inch aggregate base should be included with both the asphalt and concrete option. Typically, the thickness of the bituminous base is increased by 2 inches in lieu of a 4-inch aggregate base.

The Type A2 and A3 standards are only appropriate for shoulders constructed with new pavements and should not be used for shoulder widening projects. Use of pay items in square yards should also be avoided for shoulder widening projects.

To address excavation, grading and backfilling, Special Provision Shoulder Grading NJSP-15-27A should be included in the contract documents, along with the following pay item:

212-99.00 Misc. Shoulder Grading.

Other shoulder widening pay items are:

• 402-05.20 Tons Bituminous Pavement Mixture PG64-22 (Surface Leveling) over
• 401-30.00 Tons Bituminous Pavement Mixture PG64-22 (Base) over
• 304-05.04 SY Type 5 Aggregate for Base (4 in. thick) - Optional

OR

• 401-12.09 Tons Bituminous Pavement Mixture PG64-22 (BP-1) over
• 401-30.00 Tons Bituminous Pavement Mixture PG64-22 (Base) over
• 304-05.04 SY Type 5 Aggregate for Base (4 in. thick) - Optional.

231.4.3.3 Additional Design Considerations

• Drainage pipes and box culverts may need to be extended.
• Slopes may be warped within reason to remain within existing right of way.
• A Safety Edge^SM shall be constructed at the edge of each shoulder that is 4 feet wide or narrower.