Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities: A Guidebook (2017)

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6C H A P T E R 2 Design Process This chapter presents an overall design process for roundabouts and CTLs that fully integrates accessibility. The design process is iterative in nature, as a design may need to be revised through- out its development to achieve a desirable performance. If changes are made to the initial design, these changes may affect performance measures differently. As an example, a raised crosswalk may increase the rate at which drivers yield to pedestrians but it may also decrease the vehicular capacity of the affected lanes. Similarly, a reduction in curve radius may help reduce speeds at the crosswalk, but may also affect the adequate accommodation of the design vehicle. These trade-offs are very similar to others faced by designers in balancing operational per- formance, safety performance, and costs, to name just three factors. Designers should therefore develop a good understanding of the trade-offs of different geometric configurations and acces- sibility treatments to minimize the amount of iterations necessary to arrive at an acceptable solution, and to ensure that the needs of all users are reflected in the design. The design process employed in this guidebook is a performance-based process. A performance- based process recognizes that each project is unique. This approach has been recognized in a number of national documents, including Flexibility in Highway Design (FHWA, 2012), and are integrated into the roundabout design process in NCHRP Report 672 (Rodegerdts et al., 2010). The FHWA document discusses the need for a balanced design. This balance needs to include the accommodation of all road users, including pedestrians of differing abilities. Performance-based checks provide a systematic way for designers to achieve acceptable performance while being tailored to the unique features and constraints of a given project. 2.1 Roundabouts For roundabouts, NCHRP Report 672 provides a comprehensive process for designing a round- about, reproduced here in Figure 2-1. A goal of this guidebook is to expand the “performance checks” portion of the design process (highlighted in Figure 2-1) to include accessibility-related checks. A key aspect of this figure—and one of the key philosophies presented in NCHRP Report 672— is the use of performance checks and the resulting iteration that occurs in the design. For exam- ple, the entries and exits of a roundabout should be narrow enough and tight enough to limit fastest path speeds, yet wide enough and with flat enough curvature to accommodate design vehicles. It is challenging for designers to provide appropriate speed control and accommodate design vehicles for all movements on the first attempt. Rather, to meet these performance mea- sures and others, iteration and refinement of an initial design are often needed.

Note: The chapter and section numbers in this figure refer to NCHRP Report 672. Figure 2-1. Roundabout design process (Exhibit 6-1 from NCHRP Report 672). The flowchart in Figure 2-1 shows a multistep roundabout design process with the following components: three data input steps and eight overall design process steps. The data input steps are: (1) operational analysis, (2) identify lane numbers and arrangements, and (3) external input from other studies. The steps in the overall design process are: (4) identify initial design elements, (5) single-lane roundabouts, (6) multilane roundabouts, (7) mini-roundabouts, (8) performance checks, (9) design details, (10) other design details, and (11) applications. The eighth step—performance checks—is highlighted. The specific performance checks listed in the figure are fastest path, natural path, design vehicle, and sight distance and visibility. This guidebook adds two new checks for accessibility performance—crossing assessment and wayfinding assessment. After performance checks, designers either advance to the next step (design details) or iterate and return to step four to modify the design.

8 Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities: A Guidebook The performance checks outlined in NCHRP Report 672 can be characterized as follows, with the means to conduct each check given in parentheses: • Achieve acceptable operational performance (lane numbers and arrangements check), • Achieve acceptable safety performance (fastest path, path alignment, and sight distance/ visibility checks), • Accommodate the design vehicle (design vehicle check), and • Accommodate non-motorized users (pedestrians and bicyclist feature check). This project expands the non-motorized user and sight distance/visibility checks to include accessibility-related issues as follows: • Pedestrian wayfinding task (Chapter 6 of this guidebook) and • Pedestrian crossing task and pedestrian-related aspects of sight distance, delay, and risk (Chapter 7 of this guidebook). Integration of these additional checks into the design process is intended to elevate accessibil- ity to be a normal and integral part of the roundabout design process. 2.2 Channelized Turn Lanes As stated in A Policy on Geometric Design of Highways and Streets (AASHTO, 2011), referred to as the Green Book commonly and in this report, the primary reasons for installing CTLs are as follows: 1. To increase vehicular capacity at intersections, 2. To reduce delay to drivers by allowing them to turn at higher speeds, 3. To reduce unnecessary stops, 4. To clearly define the appropriate path for right-turn maneuvers at skewed intersections or at intersections with high right-turn volumes, 5. To improve safety by separating the points at which crossing conflicts and right-turn merge conflicts occur, and 6. To permit the use of large curb return radii to accommodate turning vehicles, including large trucks, without unnecessarily increasing the intersection pavement area and the pedestrian crossing distance. When the decision to install a CTL has been made, the design practice generally relies on established techniques and agency preferences rather than an iterative, performance-based process as described previously for roundabouts. Many of these best practices are captured in NCHRP Web-Only Document 208, and summarized in Chapter 4 of this document. Figure 2-2 presents the state of practice for CTL design processes, although this process is not formalized as in the case of roundabouts. The key aspects of this figure are selection of control and design development according to established practices. Operational needs, land use and contextual environment, and agency pref- erences are all factors influencing the selection of control. The Green Book, agency guidance documents, and typical detail drawings are factors influencing the selection of island size, turn- ing roadway width and radius, and crosswalk location, to name a few of the elements of CTLs. In some cases, site-specific conditions necessitate variations of established practices. For example, if an intersection is skewed, the turning roadway width and radius required to accommodate a design vehicle may need to be determined with vehicle turning template software rather than the orthogonal intersection figures in the Green Book.

Design Process 9 Figure 2-2. Channelized turn lane design process. The flowchart in Figure 2-2 shows the following steps: 1) identify CTL as desired right-turn treatment, 2) choose control (yield, uncontrolled, or signal), and 3) develop design according to best practices or agency guidance (island design, radius of turning roadway, angle of intersection with cross-street, sight distance and visibility, design vehicle accommodation). After Step 3, there is a decision point in the chart. If the intersection is a typical intersection, a standard design can be used. If the intersection is a special case such as a skewed intersection or has site con- straints or an atypical design, a site-specific design is needed. The process converges and performance checks are used to assess sight distance and visibility and design vehicle accommodation, and some iteration may be needed before the design process is advanced. The accessibility checks proposed in this document—crossing assessment and wayfinding assessment—are added to the performance checks.