Part-time shoulder use added along basic freeway segments should have pavement markings that guide drivers from the adjacent general purpose lane to the shoulder, but also maintain continuity of the general purpose lane. Therefore, the desirable locations for lane drops described above are also desirable locations to start part-time shoulder use.
However, starting part-time shoulder use requires a series of signs to indicate use restrictions and the point at which use of the shoulder becomes allowable. There are fewer considerations associated with adding a general purpose lane in comparison to dropping a general purpose lane because it requires no action on the part of the driver.
Likewise, dropping any type of lane in or immediately beyond an area with extensive, complex signing or other features contributing to high driver workload should be avoided if possible. Horizontal curves, crest vertical curves, and overpasses may limit a driver’s visibility of a downstream roadway, and dropping any type of lane-including part-time shoulder use-within or immediately beyond these features, should be avoided if possible. If the beginning or end of static or dynamic part-time shoulder use is along a basic freeway segment, then it is desirable for it to be located such that it is highly visible and comprehended by approaching drivers. Part-time shoulder use can begin and end along basic segments or at ramps. Logical termini should be established during project scoping and preliminary design consistent with National Environmental Policy Act (NEPA) guidance. The process for obtaining a design exception is covered in CHAPTER 9. The Basic Freeway Segment section below presents design criteria that part-time shoulder use may affect and require design exceptions. If the facility is an Interstate Highway, design elements in AASHTO’s A Policy on Design Standards - Interstate System (38) also apply. Part-time shoulder use may introduce design elements that are below the minimum criteria specified in AASHTO’s A Policy on Geometric Design of Highways and Streets (Green Book) (37) and design exceptions may be required. This section presents geometric design considerations for each of these segment types, as well as design considerations for arterials with part-time shoulder use and emergency turnout areas. There is little quantitative research on design elements of part-time shoulder use, and much will be learned in this area in the coming years as new facilities are built and existing facilities age.įrom a geometric design perspective, freeway part-time shoulder use can be subdivided into three types of segments: This chapter presents design considerations for part-time shoulder use in the context of existing design practices for facilities without part-time shoulder use and the experiences of states that have implemented part-time shoulder use. Part-time shoulder use is typically implemented on older freeways in constrained environments, so the design project in which a designing for operations process is used may be a reconstruction project rather than a new facility. In the context of part-time shoulder use, a designing for operations process incorporates design features needed for part-time shoulder use-full depth pavement, sufficiently wide shoulders and lateral offsets, space for turnouts, and so forth-into the design of a facility where there is a reasonable likelihood that part-time shoulder use would someday be implemented.
Special signing and pavement markings are necessary for part-time shoulder use, and dynamic signs are often used.ĭesigning for operations is “the collaborative and systematic consideration of management and operations during transportation project design and development” (36). Preexisting pavement and drainage should be assessed before part-time shoulder use is implemented, and improvements or modifications may be necessary.
However, the same elements of roadway design-geometry, pavement, drainage, and signing and pavement marking-are applicable to both full-time lanes and lanes designated for part-time shoulder use and require some level of attention prior to implementation. Part-time shoulder use typically utilizes an existing shoulder therefore, the design of a part-time shoulder use implementation project is typically less involved than the design of a full-time added lane. Table of Contents Use of Freeway Shoulders for Travel - Guide for Planning, Evaluating, and Designing Part-Time Shoulder Use as a Traffic Management Strategy Chapter 7.