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36 chapter five Intersections Overview Alignment Design of and warrants for auxiliary lanes were common FHWA's Signalized Intersections: Informational Guide topics of interest during the past decade. Researchers also (Rodegerdts et al. 2004) states that "the approach to a sig- revisited effects of and treatments for skewed approaches, nalized intersection should promote awareness of an inter- channelization, and other intersection configuration elements. section by providing the required stopping sight distance in Consideration of bicycles and pedestrians at intersections was advance of the intersection." The document recommends the promoted through a variety of initiatives, particularly related following guidelines to meet drivers' and cyclists' expecta- to pedestrians with disabilities. The subject that captured the tions as they approach intersections: attention of many, however, was modern roundabouts; the design, installation, and operation of these alternatives "Avoid approach grades to an intersection of greater to signalization were the subject of much research, and lead than 6%. On higher design speed facilities (50 mph and to the publication of two FHWA Guides during this time. In greater), a maximum grade of 3% should be considered. addition, other innovative intersection designs appeared in Avoid locating intersections along a horizontal curve of research and on highway networks; many of these new inter- the intersecting road. sections are designed with the purpose of improving capacity Strive for an intersection platform (including sidewalks) by changing the manner in which left-turn movements are with cross slope not exceeding 2%, as needed for accommodated. Finally, access management near intersections accessibility." also garnered a great deal of interest, as researchers looked for ways to minimize impacts of adjacent driveways and side Approach curvature is a geometric design treatment that streets. can be used at high-speed intersection approaches to force a reduction in vehicle speed through the introduction of hori- zontal deflection, as described in NCHRP Report 613 (Ray Intersection Configuration et al. 2008). As shown in Figure 9, approach curvature con- sists of successive curves with progressively smaller radii. Kindler et al. (2004) described the development of an expert Research and applications of approach curvature previously system for diagnostic review of at-grade intersections on focused on roundabouts. However, the report states that this rural two-lane highways. This system, the Intersection Diag- geometric design treatment has potential to be applied to nostic Review Module (IDRM), was developed as a compo- conventional intersections as well. nent of the Interactive Highway Safety Design Model to aid designers in assessing the safety consequences of geometric The use of approach curvature at downhill approaches was design decisions, particularly for combinations of geomet- discouraged. The report authors' experience with approach ric features. IDRM was developed to allow such problems curvature suggested that this geometric treatment can be to be identified and evaluated in an automated and orga- used in conjunction with reduced speed limit signs or advi- nized fashion. IDRM identifies concerns by "using models sory speed signs. The length and curve geometry can be of the criticality of specific geometric design situations. determined from the upstream segment operating speed These include existing geometric design models--such as and the target speed and appropriate design vehicle for the sight distance models--as well as newly developed models. intersection. IDRM uses 21 specific models to address 15 high-priority issues related to [the] intersection as a whole and [to indi- vidual] approach" legs. "IDRM makes no attempt to select a Effect of Skew particular treatment as appropriate to the intersection. After further investigation, the IDRM user may select a particular Son et al. (2002) developed a methodology for calculating sight treatment as appropriate [on the basis of the] available evi- distance available to drivers at skewed unsignalized intersec- dence and engineering judgment, or the user may conclude tions. The methodology considered that the sight distance that no treatment is necessary and that the project should be may vary depending on the driving positions of the drivers built as designed." and the different lines of sight given to drivers by different