nchrp research report 948

The implementation project focuses on NCHRP Report 948  Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges .  The implementation plan is divided into three principal categories that would directly promote the implementation and technology transfer of the research products: (1) training and outreach efforts, (2) pilot applications and case studies, and (3) integration with other guidance documents.

NCHRP Report 948 had the objective to develop a guide for transportation practitioners to improve and integrate pedestrian and bicycle safety considerations at alternative intersections and interchanges (A.I.I.) through planning, design, and operational treatments. The implementation project's objective is to share and disseminate the results of the research with public agencies, and to provide hands-on technology transfer assistance to these agencies.

IMPLEMENTATION OUTCOMES

• Safer Intersections for Pedestrians and Bicyclists | October 25, 2022 | Link to Webinar Page | Slides

Summary Report

• Final summary report is available here:  https://onlinepubs.trb.org/onlinepubs/nchrp/20-44(35)/ImplementationSummaryReport.pdf
• Design Flags Calculator is available here:  https://onlinepubs.trb.org/onlinepubs/nchrp/20-44(35)/DesignFlagsCalculator.xlsx
• Video capture of the web training is available here: https://vimeo.com/918484372

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NCHRP Report 948: 20 Design Flags to Evaluate Pedestrian and Bicyclist Safety at Alternative and Other Intersections

Recorded LTAP Webinar

This webinar will provide an overview of NCHRP Report 948 and a methodology for evaluating and quantifying pedestrian and bicyclist safety at alternative intersections, as well as conventional intersections the new designs may be compared against. The “ 20 Design Flags ” method provides a performance-based approach to evaluating design elements of existing or proposed intersection designs. The resulting assessment score then allows designers to make modifications to intersection form, intersection control, or specific design elements to enhance intersection safety in the project development process. The webinar will include an overview of the 20-Flag method, a discussion of how the method can be integrated into agency processes and intersection control evaluation (ICE), and tangible examples of applying the method to real-world intersection.

Learning Objectives (1)   Inform participants about the NCHRP Report 948 method. (2)   Describe how the method can be used throughout the project development process. (3)   Illustrate the method using real-world case studies.

Agenda ✔ Introduction and motivation for the method ✔ Overview of alternative intersections and pedestrian and bicyclist considerations ✔ Presentation of the 20-Flag Methodology ✔ Case study examples for the method ✔ Discussion of integration in project development and ICE process ✔ Q&A

Bastian Schroeder, PhD, PE is a Senior Principal Engineer for Kittelson based in Wilmington, NC and serves as the firm’s Director of Research and Innovation. As a recognized leader in the transportation industry, he helps guide transportation emerging trends and technologies. Bastian has a passion for developing solutions to complex problems across all areas of transportation with a focus on advancing agency processes and integrating research into standard practices. His areas of expertise span traffic operations, safety, and design, and his broad multidisciplinary interests include multi-resolution modeling, big data analysis and visualization, and designing intersections and streets for people walking and biking. He is active in the Institute of Transportation Engineers (ITE) and TRB. He is the chair of the TRB Committee on Highway Capacity and Quality of Service and a member of the TRB Simulation Committee. Outside of his professional career, Bastian is an avid runner, cyclist, and triathlete who enjoys spending as much time as possible outdoors, particularly taking his family camping and hiking where no cell signal has gone before.

Mike Alston, RSP grew up in North Carolina and earned his bachelor’s degree in civil engineering from North Carolina State University before defecting to the San Francisco Bay Area, where he has lived and worked since 2010. He received master’s degrees in transportation engineering and in city & regional planning from UC Berkeley and joined Kittelson in 2017. He is a certified road safety professional. As a plangineer, Mike is excited about diving into technical details in service of planning outcomes and better, more socially equitable communities. He has done so on a variety of project types, including safety projects throughout Northern California, urban and rural transportation impact analyses, and preliminary engineering projects. Mike worked on a road diet feasibility and assessment process for the City of Oakland, including implementing Highway Capacity Manual methods to create custom service volume tables. He enjoys working with large datasets and specializes in GIS mapping and analysis, safety analysis and prioritization, and operations analysis. When not watching the sun set over Mt. Tamalpais from his office, Mike can be found riding his bike for exercise, for transportation, or to go camping.

To earn Professional Development Hours (PDHs) or a Certificate of Completion for each recorded webinar, you must view the entire webinar. After viewing, please fill out the web form at the link below to request your certificate. The Florida LTAP Center will follow-up within 2-3 weeks.

This webinar will award 1.5 PDHs .

Click the link below to view the recorded webinars

Guide for Roundabouts (2023)

Chapter: appendix a - design performance check techniques.

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

A-1   Contents A-1 A.1 Geometric Speed Check Techniques A-3 A.1.1 Hand-Sketch Method A-3 A.1.2 CAD-Based Methods A-6 A.1.3 Speed–Radius Graphs A-8 A.2 Sight Distance and Visibility A-12 A.3 Vehicle Path Alignment A-14 A.4 Design Vehicle A-15 A.5 Bicycle and Pedestrian Design Flags A-18 A.6 Pedestrian Crossing Assessment A-19 A.6.1 Possible Method A-19 A.6.2 Possible Application A-22 A.7 Pedestrian Wayfinding Assessment A-24 A.8 References This appendix details a variety of design performance check techniques that can facilitate the check process discussed in Chapter 9: Geometric Design Process and Performance Checks. The techniques in this appendix are representative but not exhaustive of all possible techniques. Practitioners must sometimes modify performance check techniques to meet a specific configura- tion; any modifications need to be compatible with the design principles in Chapter 9. A.1 Geometric Speed Check Techniques This section presents a variety of geometric speed check techniques, including both hand- drawn and CAD-based methods. Each method varies in detail but can produce results that are consistent with the principles presented in Chapter 9: Geometric Design Process and Perfor- mance Checks. The geometric speed check method is an easy, efficient, and effective way to check speeds for hand-sketched concepts or red-lined revision markups of early CAD-based designs. CAD-based methods offer increased precision in computing estimated speeds. The geometric speed model is based on a generalized vehicle dimension and assumed driver behavior. Both geometric speed techniques use the same model assumptions; because of these assumptions, computed speeds should be assessed only to the nearest 1 mph or 1 km/h. A P P E N D I X Design Performance Check Techniques

A-2 Guide for Roundabouts The geometric speed check method combines individual alignment elements: tangents, curves, and spirals. Each curve has an associated speed based on its radii and assumed side friction and superelevation. Drivers navigate a curvilinear horizontal alignment by viewing the roadway ahead and adjusting their speed and position based on the combination of alignment elements, in effect considering an alignment as a unit rather than as its individual components. Drivers naturally follow a spiral path between curves and tangents, which can be captured using hand- drawn and CAD-based techniques. Exhibit A.1 depicts the concept of alignment elements and an alignment unit. Exhibit A.2 represents two curvilinear paths: one alignment that can be characterized as a flowing alignment and another that can be characterized as an inconsistent alignment. The flowing align- ment path represents a smooth curvilinear alignment resulting in similar speeds, associated lateral forces, and similar driver comfort between alignment elements. The inconsistent align- ment represents a driving path that may be possible to drive but that is not likely to be the fastest or smoothest alignment possible within the given geometry. Regardless of the method, practitioners need to review each developed fastest path align- ment objectively to assess if it represents the model’s intent. The desired fastest path alignment Alignment Unit Alignment Elements Exhibit A.1. Alignment elements. Flowing Alignment Inconsistent Alignment Exhibit A.2. Flowing and inconsistent alignment.

Design Performance Check Techniques A-3   is the smoothest, flattest path possible for a single vehicle in the absence of other traffic and ignoring all lane markings. A.1.1 Hand-Sketch Method A possible method for conducting a geometric speed check using hand-sketch techniques includes the following steps: 1. Draw the roundabout or plot a CAD drawing at a scale that allows the roundabout and its approaches to fit onto a common size of paper (typically 11 in. × 17 in./A3 or smaller). For most roundabouts, a scale of 1 in. = 50 ft or 1:500 is most useful. 2. Place a vellum or other trace paper over the roundabout concept and secure it with tape. Place a registration mark on the trace paper. 3. Scale appropriate offsets from curbs and stripes as appropriate using the recommendations in Chapter 9: Geometric Design Process and Performance Checks. Sometimes, it is helpful to place a few small pencil dots along the possible path. 4. Start 125 ft to 200 ft (40 m to 60 m) upstream from the roundabout entrance based on a loca- tion on the approach not affected by the roundabout entry. Lightly sketch an entry path. 5. Similarly, work backward from a point approximately 125 ft to 200 ft (40 m to 60 m) down- stream of the roundabout exit and lightly sketch upstream toward the circulatory roadway. 6. Using an offset of 5 ft (1.5 m) from the central island, sketch a light line upstream and down- stream from the circulatory roadway, aiming toward the entry alignment and the exit align- ment. This forms the initial circulating alignment. 7. Lightly sketch the entry alignment toward the circulating alignment and from the circulating alignment toward the entry until the paths meet. Include tangents or gradual transitions between reversing curves. 8. Lightly sketch the exit alignment upstream toward the circulating alignment and from the cir- culating alignment toward the exit alignment until the paths meet. Include tangents or gradual transitions between reversing curves. 9. Lightly pass over the fastest path alignment to darken the pencil sketch, smoothing the drawn path as needed to create a balanced and flowing alignment. It is common to erase small portions of the sketched path and resketch portions to improve the fastest path. 10. Measure the radius using a template. Look up the speed to the nearest 1 mph or 1 km/h cor- responding to the measured radius using the speed–radius graphs (see Section A.1.3) based on positive or negative superelevation for the location of the curve being measured. Record the speeds and compare them to the target performance. 11. Conduct the evaluations for other through and turning movements. 12. If the estimated speeds are adequate for all movements, continue refining the roundabout. If speeds on some movements are too fast, make geometric changes as needed to reduce speeds and repeat the assessment. A.1.2 CAD-Based Methods Several states provide guidance for CAD-based methods. Many of these methods apply spline curves that generate smooth spiral curves like those obtained using freehand methods. Strategi- cally placed points along the spline curve result in a path dictated by the roundabout’s geometric elements. Best-fit circular curves are then used to measure the controlling curves along the spiral path to identify R1 through R5 radii for each approach. The general approach for each CAD-based method is to create construction lines offset from curbs and edge lines that represent the center of the passenger car. It is common to consider the approaching and departing evaluation distance to be 165 ft (50 m), but the distance may be shorter

A-4 Guide for Roundabouts or longer depending on the roundabout’s approach and departure geometry. Practitioners must consider each roundabout movement individually and employ the CAD-based method to best reflect the geometric speed exercise’s intent to create a smooth and consistent pathway reflecting likely driver behavior. Exhibit A.3 illustrates how to construct the fastest vehicle paths at a single-lane roundabout. Exhibit A.4a and Exhibit A.4b illustrate how to construct the fastest vehicle paths at a multilane roundabout. Each path should be reviewed to assess if the CAD-drawn path reflects likely driver behavior. The CAD-drawn path may not always represent the probable actual path. Exhibit A.4b shows the potential difference between the “probable actual path” and the “CAD-drawn path.” The actual exiting speeds between these two paths might not result in substantive predicted speed performance differences. There may be differences in CAD commands depending on the platform. However, the pro- cess and intent are the same between methods and software applications. CAD-based geometric speed checks usually include the following steps. 1. Copy curb offsets from the face of the curb or painted lines using values associated with the cross-section feature. See Chapter 9: Geometric Design Process and Performance Checks for additional information about the offset dimensions. 2. Establish the upstream limit line and downstream limit line for each roadway approach and departure. This is commonly 165 ft (50 m). The actual value depends on the roundabout’s approach and departure geometry and may be closer to or farther from the roundabout to best represent a smooth and consistent path. SOURCE: Adapted from Georgia Department of Transportation (1). Exhibit A.3. Fastest vehicle paths for a single-lane roundabout.

Design Performance Check Techniques A-5   SOURCE: Adapted from Georgia Department of Transportation (1). Exhibit A.4a. Fastest vehicle paths for a multilane roundabout. SOURCE: Adapted from Georgia Department of Transportation (1). Exhibit A.4b. Fastest vehicle path through a multilane roundabout with CAD-drawn path.

A-6 Guide for Roundabouts 3. Draw the spline curve between the upstream movement and the offsets. This is typically accom- plished by snapping three points that occur outside the upstream limit line, on the upstream limit line, and on the curb offset line. a. In some configurations, the left or right curb line could be the control depending on the logical vehicle driving path. b. In some configurations, often at exits, the logical vehicle driving path is outside the offset line, and the fastest path may not be affected by the curb offset line. c. In some right turns, the fastest paths could be offsets from the right (inside) of the turning radii or from the left (outside) of the turning radii that are controlled by the splitter island, truck apron, and exiting splitter island. 4. Review and revise the spline lines to be sure they are outside required offsets or located as needed to represent a fastest path. The beginning or end of the spline may need to be pulled farther away from the roundabout (up or downstream) to create a realistic fastest path. 5. Measure the radius values. Arc lengths for any circular curve should extend from 65 ft to 80 ft (20 m to 25 m). If they are shorter than that, the path should be modified to achieve these lengths. Achieving these lengths may require adjusting the spline lines to be sure the fastest path reflects driver behavior. 6. Conduct the evaluations for other through and turning movements. 7. If the estimated speeds are adequate for all movements, continue refining the roundabout. If speeds on some movements are too fast, make geometric changes as needed to reduce speeds and repeat the assessment. A.1.3 Speed–Radius Graphs Chapter 9: Geometric Design Process and Performance Checks explains the speed–radius graphs used for estimating speeds when conducting geometric speed checks. Exhibit A.5 through Exhibit A.8 provide larger versions of the graphs in Chapter 9 to allow for easier use when con- ducting checks. SOURCE: Based on AASHTO Green Book, Equation 3-7 and side friction factors assumed for design (AASHTO Figure 3-4) (2). 0 5 10 15 20 25 30 35 0 100 200 300 400 Sp ee d (m ph ) Radius (ft) e=+0.02 e=-0.02 Exhibit A.5. Speed–radius relationship, US customary up to 400 ft.

Design Performance Check Techniques A-7   SOURCE: Based on AASHTO Green Book, Equation 3-7, and side friction factors assumed for design (AASHTO Figure 3-4) (2). 0 5 10 15 20 25 30 35 40 45 50 0 100 200 300 400 500 600 700 800 Sp ee d (m ph ) Radius (ft) e=+0.02 e=-0.02 Exhibit A.6. Speed–radius relationship, US customary up to 800 ft. SOURCE: Based on AASHTO Green Book, Equation 3-7, and side friction factors assumed for design (AASHTO Figure 3-4) (2). 0 5 10 15 20 25 30 35 40 45 50 55 60 0 10 20 30 40 50 60 70 80 90 100 110 120 Sp ee d (k m /h ) Radius (m) e=+0.02 e=-0.02 Exhibit A.7. Speed–radius relationship, metric up to 120 m.

A-8 Guide for Roundabouts A.2 Sight Distance and Visibility Chapter 9: Geometric Design Process and Performance Checks discusses stopping and inter- section sight distance. Practitioners will use geometric speed check values to establish stopping and intersection sight distance. A possible method for conducting stopping sight distance evalua- tions, illustrated in Exhibit A.9 through Exhibit A.13, includes the following steps: 1. Assess the stopping sight distance on the approach by considering the approach speed and establishing the appropriate sight distance corresponding to that speed. 2. Measure the distance along the roadway approach path to the pedestrian waiting area or to the entrance line as appropriate. From the vehicle positioned at the distance along the traveled SOURCE: Based on AASHTO Green Book, Equation 3-7, and side friction factors assumed for design (AASHTO Figure 3-4) (2). 0 10 20 30 40 50 60 70 80 0 50 100 150 200 250 Sp ee d (k m /h ) Radius (m) e=+0.02 e=-0.02 Exhibit A.8. Speed–radius relationship, metric up to 250 m. SOURCE: Adapted from Georgia Department of Transportation (1). Exhibit A.9. Stopping sight distance to the pedestrian crossing and entrance line on the approach.

SOURCE: Adapted from Georgia Department of Transportation (1). Exhibit A.10. Stopping sight distance for a right-turn bypass lane. SOURCE: Adapted from Georgia Department of Transportation (1). Exhibit A.11. Stopping sight distance for approach curvature. SOURCE: Adapted from Georgia Department of Transportation (1). Exhibit A.12. Stopping sight distance on a circulatory roadway.

A-10 Guide for Roundabouts approach, the sightline to the waiting area can be established. For roundabouts with a sepa- rated right-turn lane, the stopping sight distance should be provided to the pedestrian waiting area and entrance line. 3. Measure the distance along the roadway approach path to the entrance line. From the vehicle positioned at the distance along the traveled approach, establish the sightline to the entrance line. 4. Verify that there are no sight distance obstructions within the inscribed sightline. 5. Assess the stopping sight distance along the circulatory roadway using the computed R4 speed. Measure the distance along the circulatory roadway with an offset of 5 ft (1.5 m) from the central island curb. Then, establish a sightline to a forward position on the circulating path. Note that this sightline can be projected as if the driver circulated the entire roundabout to provide stopping distance at the central island. For noncircular roundabouts, practitioners can use the various geometric speed check speeds and establish sightlines similarly. 6. Assess the stopping sight distance to the pedestrian waiting area on the exit by locating the vehicle at the entrance line and establishing the sightline to the pedestrian waiting area. Even if a crosswalk is not provided, it is prudent not to preclude a future crossing, so the sight dis- tance to the exit should be established. 7. Verify that there are no sight distance obstructions within the inscribed sightline on the cen- tral island. Exhibit A.14 illustrates a possible intersection sight distance method that includes the fol- lowing steps: 1. Consider a vehicle waiting at the entry and the two potential conflicts on the circulatory roadway and the immediate upstream entry. 2. Consider sight distance in advance of the entry: The length of the approach leg of the sight triangle and of the conflicting branch for the immediate upstream entry, b1, should be limited to 50 ft (15 m). The length of the conflicting branch on the circulatory roadway, b2, is calculated as previously described. If the combination of sight distance along the approach leg and the immediate upstream entry leg of the sight triangle exceeds these recommendations, it may be advisable to add landscaping that restricts sight distance to the minimum requirements. SOURCE: Adapted from Georgia Department of Transportation (1). Exhibit A.13. Sight distance to a crosswalk on exit.

Design Performance Check Techniques A-11   3. Use Equation A.1 through Equation A.4 to compute the intersection sight distance for the two branches, b1 and b2. Exhibit A.14 shows the lengths of the two conflicting branches. US Customary Metric Equation A.1 Equation A.2 Equation A.3 Equation A.4 where = length of entering branch of sight triangle, ft = length of circulating branch of sight triangle, ft = speed of vehicles from upstream entry for the conflicting through movement, assumed to be average of and , mph = speed of circulating vehicles, assumed to be , mph = design headway, s, assumed to be 5.0 s where = length of entering branch of sight triangle, m = length of circulating branch of sight triangle, m = speed of vehicles from upstream entry for the conflicting through movement, assumed to be average of and , km/h = speed of circulating vehicles, assumed to be , km/h = design headway, s, assumed to be 5.0 s Exhibit A.15 shows the computed length of the conflicting leg of an intersection sight trian- gle using an assumed value of design headway, tg, of 5.0 s. This design headway is based on the amount of time required for a vehicle to safely enter the conflicting stream. This is an assumed value based on judgment and experience, originally developed using observational data for critical headways from NCHRP Report 572 and more recent observational data from FHWA research (3, 4). Some agencies use smaller values for design headway or other alternatives for locations with restricted sight distance. Exhibit A.14. Intersection sight distance.

A-12 Guide for Roundabouts View angles use the intersection sight distance values from this section. A possible method for conducting view angle evaluations includes the following steps: 1. Determine the vehicle location at the yield line. For multilane entries, each lane should be checked. View angles must also be checked for right-turn bypass lanes. 2. Establish the sightline assuming there is a vehicle at the yield line and a vehicle upstream at the location needed for intersection sight distance (distance b1). 3. Measure the angle between the alignment of the vehicle at the yield line and the alignment of the sightline. Exhibit A.16 depicts an approach with an intersection angle less than 75 degrees. A.3 Vehicle Path Alignment Chapter 9: Geometric Design Process and Performance Checks discusses vehicle path alignment evaluations, which are specific to multilane roundabouts. The natural vehicle paths are the paths approaching vehicles will take through the roundabout geometry, guided by their speed and Conflicting Approach Speed (mph) Computed Distance (ft) Conflicting Approach Speed (km/h) Computed Distance (m) 10 73.4 20 27.8 15 110.1 25 34.8 20 146.8 30 41.7 25 183.5 35 48.7 30 220.2 40 55.6 NOTE: Computed distances are based on a critical headway of 5.0 s. Exhibit A.15. Computed length of a conflicting leg of an intersection sight triangle. SOURCE: Adapted from Tian et al. and NCHRP Report 672 (5, 6). Exhibit A.16. Example design with a severe angle of visibility to the left.

Design Performance Check Techniques A-13   orientation in the presence of other vehicles. The key consideration in evaluating vehicle path align- ment is that drivers cannot change the direction or speed of their vehicle instantaneously. A possible method for vehicle path alignment evaluations includes the following steps: 1. Ensure that the roundabout entry directs vehicles to their intended lanes in the circulatory roadway. 2. Assess if the roundabout approach and entry channelization (e.g., splitter islands or traffic islands for right-turn lanes) allow transitions (tangents) between reverse curves (i.e., no back- to-back reverse curves). 3. Verify that consecutive vehicle path curves have a relatively similar radius that supports consistent speeds. 4. Assess if the vehicles circulating the roundabout are being directed to their intended exit lanes. 5. Verify that there is a tangent between the circulating lanes and the exit curve. 6. Assess if the exit curve provides speeds that are comparable to or larger than the circulating curve. 7. Inspect roundabouts (especially noncircular forms) for circulating speeds that are faster than exit speeds and require drivers to decelerate. However, if entry speeds are kept low, the added speed associated with a noncircular configuration may be 2 mph to 3 mph (3 km/h to 5 km/h) and have few adverse effects. Exhibit A.17 shows a hand sketch of paths to assess vehicle alignment at a roundabout entry and exit. It also shows how to assess how effectively the geometry guides entering vehicles to their correct circulating lanes and how vehicles exiting the circulatory roadway are guided to their exit lanes. Exhibit A.18 presents a CAD-based example of assessing entry and exit configurations. The entry includes a tangent portion that guides the driver in the right lane along that bearing to the right circulating lane. This tangent also helps guide the left lane to the left circulating lane. It is common to include 2 ft to 5 ft (0.6 m to 1.5 m) of tangent on the left side of the left lane, with a SOURCE: Kittelson & Associates, Inc. Exhibit A.17. Vehicle path alignment entering and exiting the roundabout.

A-14 Guide for Roundabouts forward bearing that is tangential to the left edge of the left circulating lane. The exhibit also shows a tangent from the circulating lanes to the exit. This tangent provides a transition between reverse curves and guides the driver to the exit, where their path follows the exit curvature. Lines A–B and B–C need to be in the range of 40 ft to 60 ft (12 m to 18 m). A.4 Design Vehicle Chapter 9: Geometric Design Process and Performance Checks discusses design vehicle perfor- mance and checks. Software is commonly used to conduct design vehicle checks. However, some agencies continue to use truck templates to support early concept design development. A possible method for conducting a design vehicle evaluation includes the following steps: 1. Establish and document the design vehicle, which is the primary design check for trucks. Ensure that the design vehicle can travel through the roundabout between curbs, with some movements possibly using a truck apron for trailer off-tracking. 2. Establish and document what larger vehicle must be accommodated. This is based on serving a less frequent but larger control vehicle (or check vehicle). The check vehicle is an anticipated but infrequent user of the roundabout that simply needs to “get through.” The check vehicle may require design features, such as additional truck aprons along the exterior, hardened surfaces beyond the curb, passageways through splitter islands or the central island, removable signs, or other treatments. A check vehicle may be required to allow a truck driver to drive their cab onto the truck apron to complete some movements. 3. For multilane roundabouts, establish and document if design vehicles may straddle lanes (use the entire curb-to-curb width for entering, circulating, and exiting plus the truck apron as needed) or be required to stay in-lane. 4. Conduct design vehicle performance checks: a. For the design vehicle, AASHTO recommends providing 1 ft to 2 ft (0.3 m to 0.6 m) of shy distance between the vehicle path (the traveled way) and the curb (2). Buses are to be accommodated within the circulatory roadway without tracking over the truck apron. b. Swept paths should be prepared for each turning movement. Frequently, right-turn move- ments are critical for truck movements, particularly at single-lane roundabouts. c. A smooth vehicle path should reflect a driver’s realistic travel path. The cab of a tractor trailer design vehicle is typically assumed to stay within the travel lanes and not mount curbs, with truck aprons supporting off-tracking of only the trailer. SOURCE: Adapted from Georgia Department of Transportation (1). Exhibit A.18. Vehicle path alignment using CAD.

Design Performance Check Techniques A-15   d. When conducting the design vehicle check, avoid using crawl speed and going wheel-lock to wheel-lock, as these are unrealistic operational assumptions. Understand and show the dif- ference between the tire locations relative to the curb and the truck envelope itself, which may extend above and beyond the curb. Exhibit A.19 presents the typical swept paths for an oversize/overweight (OSOW) vehicle making a through movement. A.5 Bicycle and Pedestrian Design Flags This section considers the design flag procedure in NCHRP Research Report 948: Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges (7). Of the 20 design flags (Exhibit A.20 through Exhibit A.22) identified in NCHRP Research Report 948 and summarized in Chapter 9: Geometric Design Process and Performance Checks, many can apply to roundabouts. Each flag can have a comfort aspect, a safety performance aspect, or both. A design review can proceed through the set of design flags and identify if any comfort or safety flags are present. These flags can provide a metric for comparing alternatives in the ICE activities, and they can also support preliminary design, including identifying potential design modifica- tions to reduce or eliminate the flag. A possible method for applying the design flags includes the following steps: 1. Review a design for each of the flags and identify if there are any comfort-related or safety- related flags. Practitioners should evaluate these flags for each of the bicyclist or pedestrian movements through the intersection, depending on the nature of the design flag. 2. If possible, modify the design to address any identified flags, or identify the modification that would be necessary for the concept to advance for further design refinement. SOURCE: Adapted from Georgia Department of Transportation (1). Exhibit A.19. Through movement swept path of an OSOW vehicle.

A-16 Guide for Roundabouts Design Flag Comfort Flag Safety Flag Notes Motor vehicle right turns na na The typical roundabout design of setting bicycle and pedestrian crossings at least one vehicle length away from the circulatory roadway addresses this flag. This flag is more common at other intersection forms. Uncomfortable/tight walking environment Pedestrian facilities less than 5 ft (1.5 m) of effective width. Pedestrian facilities that do not meet ADA requirements, which creates significant out-of- direction travel and exposure for people who are blind or have low vision. na Nonintuitive motor vehicle movements na na The typical roundabout design of setting bicycle and pedestrian crossings at least one vehicle length away from the circulatory roadway addresses this flag. This flag is more common at other alternative intersection forms. Crossing yield or uncontrolled vehicle paths Pedestrian crossings that result in high pedestrian delay. Pedestrian crossings that do not satisfy pedestrian crossing assessment for people who are blind or have low vision. na Indirect paths Pedestrian crossings that are farther than one to two vehicle lengths from the circulatory roadway if unsignalized or farther than 80 ft (25 m) from the circulatory roadway if staggered and signalized. Pedestrian crossings that are omitted from a leg of a roundabout, forcing other routing around the roundabout or to adjacent intersections. na Executing unusual movements na na The typical roundabout design of setting bicycle and pedestrian crossings at least one vehicle length away from the circulatory roadway addresses this flag. This flag is more common at other alternative intersection forms. Multilane crossings Pedestrian crossings with splitter islands that are too narrow for refuge can confuse pedestrians who are blind or have low vision, who may mistake a narrow island for a place where they can stop. Multilane pedestrian crossings that do not have supplemental vertical deflection or active traffic control devices (signals, pedestrian hybrid beacons, or rectangular rapid-flashing beacons). Multilane crossings at roundabouts are shorter than multilane crossings at other intersection forms, but multilane crossings are typically signalized at other intersection forms. Long red times na Can occur if pedestrian crossings are signalized and coordinated with a long background cycle length. This does not generally apply to roundabouts, except possibly when signalized crossings are implemented. This flag is more common at signalized intersections of various forms. NOTE: na = not applicable. SOURCE: Adapted from NCHRP Research Report 948 (7). Exhibit A.20. Summary of design flags for pedestrian and bicycle intersection assessment, part 1 of 3.

Design Performance Check Techniques A-17   Design Flag Comfort Flag Safety Flag Notes Undefined crossing at intersections na na Pedestrian crossings are typically marked at roundabouts. Motor vehicle left turns na na The typical roundabout design of setting bicycle and pedestrian crossings at least one vehicle length away from the circulatory roadway addresses this flag. This is more common at other intersection forms. Driveways and side streets at or near intersection Driveways and side streets within or in proximity to the roundabout may adversely affect wayfinding for pedestrians with vision disabilities. Driveways may introduce conflicts with both pedestrians and bicyclists. At roundabouts with high vehicular volumes or where ambient noise is high, it may not be possible for pedestrians who are blind or have low vision to hear vehicles entering from a driveway or side street. Geometric speed control may be needed to ensure yielding by vehicles entering from a driveway or side street close to a circulatory roadway. Sight distance and auditory distance for gap acceptance movements na Inadequate sight distance between drivers and pedestrians. Inadequate auditory distance, especially in noisy environments, for pedestrians who are blind or have low vision to make gap or yield judgments. In noisy environments, it may be difficult to hear vehicles well enough to make safe gap or yield judgments at even single-lane roundabouts. Grade change na Inadequate sight distance between drivers and bicyclists or between drivers and pedestrians. These are most often caused by grade breaks or vertical curves adjacent to intersections. Riding or walking in mixed traffic Roundabouts with a shared bicycle-pedestrian facility around the perimeter. Multilane roundabouts without any type of bicycle facility that is separated from motor vehicles around the perimeter. This is typically more of an issue with on- street bicycle facilities on high-speed or high-volume roads. Separated bicycle and pedestrian facilities are more comfortable for people riding and walking. Pedestrians, especially those who are elderly or who have disabilities, may be unable to hear or see bicycles or to quickly move out of the path of bicyclists who assume that pedestrians will move out of their way. Bicycle clearance times na na This does not apply to roundabouts and is more common with larger signalized intersections. NOTE: na = not applicable. SOURCE: Adapted from NCHRP Research Report 948 (7). Exhibit A.21. Summary of design flags for pedestrian and bicycle intersection assessment, part 2 of 3.

A-18 Guide for Roundabouts Design Flag Comfort Flag Safety Flag Notes Lane change across motor vehicle travel lane(s) na Left-turning bicyclists at multilane roundabouts without a separated bicycle or shared bicycle-pedestrian facility around the perimeter; nonyielding, right-turn bypass lanes, where through bicyclists using the travel lanes must cross bypass lane to continue. na Channelized lanes na na The typical roundabout design does not have bicyclists traveling next to motor vehicles in long channelized lanes. This is more common at other intersection forms. Turning motorists crossing bicycle path na na The typical roundabout design of setting bicycle and pedestrian crossings at least one vehicle length away from the circulatory roadway addresses this flag. This is more common at other intersection forms. Riding between travel lanes, lane additions, or lane merges Right-turn bypass lanes where a parallel acceleration lane or deceleration lane is next to a bicycle lane. na na Off-tracking trucks in multilane curves na na The typical roundabout design of not using bicycle lanes at entry or exit or in the circulatory roadway addresses this flag. This is more common at other intersection forms. NOTE: na = not applicable. SOURCE: Adapted from NCHRP Research Report 948 (7). Exhibit A.22. Summary of design flags for pedestrian and bicycle intersection assessment, part 3 of 3. 3. If a concept continues to have flags (which it might, depending on the alternative), the flags can be tallied to determine the total number of comfort-related flags and safety-related flags. NCHRP Research Report 948 provides examples of forms for this process. 4. If desired, use a qualitative rating or ranking to compare alternatives based on the number of comfort-related or safety-related design flags, recognizing that these design flags are only part of the overall evaluation process. If a concept has safety-related design flags that cannot be addressed through design modifications, the concept could be flawed enough to eliminate from subsequent evaluations. If a concept has comfort-related design flags, the quantity and nature of these flags may help differentiate alternatives. A.6 Pedestrian Crossing Assessment This section summarizes the key assessment models for crossing delay and expected level of risk as presented in NCHRP Research Report 834: Crossing Solutions at Roundabouts and Chan- nelized Turn Lanes for Pedestrians with Vision Disabilities: A Guidebook and its successor project, NCHRP Project 03-78c, “Training and Technology Transfer for Accessibility Guidelines for Roundabouts and Channelized Turn Lanes,” which amended the assessment procedure in NCHRP Research Report 834 (8, 10, 11). Further detail can be found in those documents,

Design Performance Check Techniques A-19   including sample checklists and worksheets. Crossing sight distance has been integrated into the sight distance procedure presented in Chapter 9: Geometric Design Process and Perfor- mance Checks. A.6.1 Possible Method A possible method for conducting a pedestrian crossing assessment may include the following steps: 1. Assess vehicle path speeds using one of the techniques described in Section A.1. 2. Assess sight distance using one of the techniques described in Section A.2. 3. Assess pedestrian delay and crossing risk using the models presented in this section, supported by NCHRP Research Report 834 as amended by NCHRP Project 03-78c (10). 4. Assess the sufficiency of these calculated performance measures to determine an appropriate crossing treatment, if any. Section A.6.2 illustrates a possible application. The worksheets and models in Exhibit A.23 and Exhibit A.24 were developed for NCHRP Project 03-78c in US Customary units only. Appropriate conversions need to be used for inputs and outputs. A.6.2 Possible Application This section presents an example of a possible application based on unpublished work pre- pared for the Montana Department of Transportation and the Pennsylvania Department of Transportation (10, 11). This example of a possible application should not be misconstrued as a standard, as it requires deciding what constitutes an acceptable level of risk, which is beyond the scope of this document and the Transportation Research Board. Exhibit A.25 shows results from this possible application. The three models used for this possible application are as follows: • Intervention model. This model predicts high risk at two-lane exits for fastest path speeds exceeding 25 mph. The research defines an intervention as an event when a pedestrian who is blind or has low vision makes a crossing decision that would have resulted in a certified orienta- tion and mobility specialist stopping the person from crossing, such as stepping in front of an oncoming vehicle that the person who is blind or has low vision did not detect. This is based on an assumed maximum acceptable crossing risk of 4 percent, which is comparable to most single-lane roundabouts studied under NCHRP Research Report 834. Using this threshold, two- lane entries show acceptable risk up to 40 mph based on the NCHRP Project 03-78c models and, therefore, do not suggest the need to evaluate a risk-based treatment for entries unless ambient noise level is high. The intervention model results in the region denoted high risk in Exhibit A.25. • Delay model. This model predicts high delay at a combination of high speeds (results in low yielding) and high volumes (results in low gap availability). The maximum accept- able pedestrian delay was set at 30 seconds, based on the Highway Capacity Manual observation (HCM Exhibit 20-3) that for delays exceeding 30 seconds per pedestrian, the delay approaches a tolerance level with the risk-taking behavior likely (12). Everything above the trend line in Exhibit A.25 is considered high delay. • Yielding model. This model predicts yielding as a function of speed and other variables. The high yield (more than 50 percent) is shown in Exhibit A.25. In low-speed environments, low yielding is typically only a concern in combination with high volume, which results in high delay. As a result, the yielding model results are shown for reference only and are not used in the guidance development.

SOURCE: NCHRP Project 03-78c (9). Exhibit A.23. Pedestrian crossing assessment, part 1 of 2.

Exhibit A.24. Pedestrian crossing assessment, part 2 of 2. SOURCE: NCHRP Project 03-78c (9).

A-22 Guide for Roundabouts For each of the dierent regions, the treatment selection is as follows: • High Risk 1 High Delay. is condition suggests using a trac control signal or a pedestrian hybrid beacon (PHB). • High Risk 1 Low Delay. is condition suggests using at least a raised crosswalk (RCW) and may require more advanced treatments, like a trac control signal or a PHB. A rectangular rapid-ashing beacon (RRFB) is not suggested because vehicle speeds are too high. • Low Risk 1 High Delay. is condition suggests at least an RRFB and may require more advanced treatments, like a trac control signal or a PHB. An RCW is not suggested because vehicle volumes are too high. • Low Risk 1 Low Delay. is condition suggests no additional treatments are needed, but additional treatments could certainly be considered. is exhibit is based on assumed thresholds for acceptable performance and is an example of possible guidance. is should not be misconstrued as a recommended standard or guidance in this Guide or by the Transportation Research Board. Exhibit A.25 suggests that under certain vehicle speed and vehicle volume conditions, it may be possible to provide equivalent accessibility using treatments other than active regulatory devices, such as a trac control signal or a PHB. As noted in Chapter 12, to meet pedestrian and driver expectations, the same crossing treatment should be used on the entry and exit of a roundabout leg. A.7 Pedestrian Waynding Assessment Exhibit A.26 provides a checklist for pedestrian waynding performance. e checklist has been adapted from Appendix C of the NCHRP Project 03-78c Final Report, which amends NCHRP Research Report 834 (9, 8). e checklist references NCHRP Research Report 834 for design details, many of which have been superseded by content in this Guide. e checklist also references US DOT regulations related to the ADA (42 USC 12131-12134) and the Manual on Uniform Trac Control Devices (MUTCD) as well as proposed Public Right-of-Way Accessibility Guidelines (PROWAG) (13–15). SOURCE: Montana Department of Transportation and Pennsylvania Department of Transportation (10, 11). RRFB = rectangular rapid-flashing beacon; PHB = pedestrian hybrid beacon; RCW = raised crosswalk. 0 200 400 600 800 1000 1200 0 5 10 15 20 25 30 35 40 Ve h Vo lu m e (v eh /h /ln ) Untreated Vehicle Speed (mi/h) 2-lane Roundabout Entry High Yielding / Low Risk / Low Delay Low Yielding High Delay No Treatment RRFB No Treatment 0 200 400 600 800 1000 1200 0 5 10 15 20 25 30 35 40 Ve h Vo lu m e (v eh /h /ln ) Untreated Vehicle Speed (mi/h) 2-lane Roundabout Exit High Risk Low Yielding Too High Delay High Risk + High Delay RCW Signal or PHB No Treatment RRFB Exhibit A.25. Example of a possible assessment of treatments by risk, delay, and yielding for two-lane roundabouts in low-noise environments.

Design Performance Check Techniques A-23   SOURCE: Adapted from Kittelson & Associates, Inc., and Accessible Design for the Blind, Appendix C (9). Question Sources for Information Determining the Crossing Location Do sidewalks lead to the crosswalks? Chapter 10; NCHRP Research Report 834 (8). Is continuous, cane-detectable edge treatment or landscaping provided between the sidewalk and the curb? Chapter 10; NCHRP Research Report 834 (8). Required by proposed PROWAG at roundabouts (15). Are there detectable warning surfaces at the bottom of curb ramps or on the sidewalk at each end of raised crosswalks? Chapter 10; NCHRP Research Report 834 (8). Required by US DOT ADA regulations (13) and proposed PROWAG (15). If other ramps or driveways are nearby, are they adequately delineated and separated from the pedestrian crossing ramps? Chapter 10; NCHRP Research Report 834 (8). Are traffic control devices, including push buttons for signals and beacons, accessible? Chapter 12; NCHRP Research Report 834 (8); MUTCD (14). Required by proposed PROWAG (15). Aligning to Cross and Establishing a Correct Heading Is the curb ramp width the same as the crosswalk width? Chapter 10; NCHRP Research Report 834 (8). Is the curb ramp slope aligned with the crossing? Chapter 10; NCHRP Research Report 834 (8). Are ramp edges aligned with the crossing? Chapter 10; NCHRP Research Report 834 (8). Is the detectable warning aligned with the slope of the curb ramp? Chapter 10; NCHRP Research Report 834 (8). If push buttons for traffic control devices are provided, are they accessible and in the correct location? Chapter 12; NCHRP Research Report 834 (8); MUTCD (14). Is there a sufficiently level landing for turning at either the top of perpendicular curb ramps or the bottom of parallel ramps? Required by US DOT ADA regulations (13) and proposed PROWAG (15). Maintaining a Correct Heading while Crossing and Staying Within the Crosswalk Is the crossing configured at the shortest distance practical? Chapter 10; NCHRP Research Report 834 (8). Is the crossing aligned perpendicular to the curb and splitter edges? Chapter 10; NCHRP Research Report 834 (8). Are markings clearly visible? Chapter 12; NCHRP Research Report 834 (8); MUTCD (14). Crossing from Channelization Islands and Splitter Islands Are islands wide enough to provide safe refuge? Chapter 10; NCHRP Research Report 834 (8). Are there detectable warning surfaces at the bottom of curb ramps or on the sidewalk at each end of raised crosswalks (unless the islands are less than 6 ft, or 1.8 m, in width)? Chapter 10; NCHRP Research Report 834 (8). Required by US DOT ADA regulations (13) and proposed PROWAG (15). Are paths through islands clearly defined by grade difference or landscaping? Chapter 10; NCHRP Research Report 834 (8). If push buttons for traffic control devices on the island are provided, are they accessible and in the recommended location? Chapter 12; NCHRP Research Report 834 (8); MUTCD (14). Exhibit A.26. Pedestrian wayfinding checklist.

A-24 Guide for Roundabouts A possible method for conducting a pedestrian wayfinding assessment may include the follow- ing steps: 1. Evaluate each of the wayfinding questions for each quadrant of the roundabout. Examples of wayfinding assessments are in NCHRP Research Report 834. 2. Review a design using each of the wayfinding questions and identify any design gaps. These should be evaluated for each quadrant or leg of the roundabout. 3. If possible, modify the design to address any identified gaps, or identify the modification necessary for the concept to advance for further design refinement. 4. If desired, use a qualitative rating or ranking to compare alternatives based on how well the concept addresses wayfinding. These ratings or rankings may help differentiate alternatives. A.8 References 1. Roundabout Design Guide. Georgia Department of Transportation, Atlanta, 2021. 2. A Policy on Geometric Design of Highways and Streets, 7th ed. AASHTO, Washington, DC, 2018. 3. Rodegerdts, L., M. Blogg, E. Wemple, E. Myers, M. Kyte, M. P. Dixon, G. List, A. Flannery, R. Troutbeck, W. Brilon, N. Wu, B. N. Persaud, C. Lyon, D. L. Harkey, and D. Carter. NCHRP Report 572: Roundabouts in the United States. Transportation Research Board of the National Academies, Washington, DC, 2007. http:// dx.doi.org/10.17226/23216. 4. Staplin, L., K. Lococo, S. Byington, and D. Harkey. Highway Design Handbook for Older Drivers and Pedes- trians. Report FHWA.RD-01-103. FHWA, US Department of Transportation, May 2001. 5. Tian, Z. Z., F. Xu, L. A. Rodegerdts, W. E. Scarbrough, B. L. Ray, W. E. Bishop, T. C. Ferrara, and S. Mam. Roundabout Geometric Design Guidance. Report F/CA/RI-2006/13. Division of Research and Innovation, California Department of Transportation, Sacramento, 2007. 6. Rodegerdts, L., J. Bansen, C. Tiesler, J. Knudsen, E. Myers, M. Johnson, M. Moule, B. Persaud, C. Lyon, S. Hallmark, H. Isebrands, R. B. Crown, B. Guichet, and A. O’Brien. NCHRP Report 672: Roundabouts: An Informational Guide, 2nd ed. Transportation Research Board of the National Academies, Washington, DC, 2010. http://dx.doi.org/10.17226/22914. 7. Kittelson & Associates, Inc., Institute for Transportation Research and Education, Toole Design Group, Accessible Design for the Blind, and ATS Americas. NCHRP Research Report 948: Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Transportation Research Board, Washington, DC, 2020. http://dx.doi.org/10.17226/26072. 8. Schroeder, B., L. Rodegerdts, P. Jenior, E. Myers, C. Cunningham, K. Salamati, S. Searcy, S. O’Brien, J. Barlow, and B. L. Bentzen. NCHRP Research Report 834: Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities: A Guidebook. Transportation Research Board, Washington, DC, 2017. http://dx.doi.org/10.17226/24678. 9. Kittelson & Associates, Inc., and Accessible Design for the Blind. Guidelines for the Application of Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities. Unpublished final report. Transportation Research Board of the National Academies, Washington, DC, February 2020. 10. Kittelson & Associates, Inc. Recommended Treatment of Multilane Crosswalks at Roundabouts. Unpublished memorandum. Montana Department of Transportation, December 10, 2020. 11. Kittelson & Associates, Inc. Roundabouts: Safety and Operations Report. Unpublished report. Pennsylvania Department of Transportation, December 2017. 12. Highway Capacity Manual: A Guide to Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington DC, 2016. http://dx.doi.org/10.17226/24798. 13. FTA, US Department of Transportation. ADA Standards for Transportation Facilities. Website, 2006. https://www.transit.dot.gov/regulations-and-guidance/civil-rights-ada/ada-regulations. 14. Manual on Uniform Traffic Control Devices for Streets and Highways, 2009 ed., Including Revision 1, Dated May 2012; Revision 2, Dated May 2012; and Revision 3, Dated August 2022. FHWA, US Department of Transportation, 2022. http://mutcd.fhwa.dot.gov/. 15. (Proposed) Accessibility Guidelines for Pedestrian Facilities in the Public Right-of-Way. US Access Board, 2011. https://www.access-board.gov/prowag/. Accessed January 2, 2022.

Roundabout implementation in the United States has increased in the last decade, and practitioners have learned lessons in successfully applying roundabouts in various land use and transportation environments and contexts.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 1043: Guide for Roundabouts provides information and guidance on all aspects of roundabouts.

Supplemental to the report is NCHRP Web-Only Document 347: Background and Summary of a Guide for Roundabouts , which describes the research for and development of the guide.

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NCHRP Implementation Support Program. Implementation for NCHRP Research Report 948—Guide for Pedestrian and Bicycle Safety at Alternative Intersections and Interchanges

The implementation project focuses on NCHRP Report 948 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. The implementation plan is divided into three principal categories that would directly promote the implementation and technology transfer of the research products: (1) training and outreach efforts, (2) pilot applications and case studies, and (3) integration with other guidance documents. NCHRP Report 948 had the objective to develop a guide for transportation practitioners to improve and integrate pedestrian and bicycle safety considerations at alternative intersections and interchanges (A.I.I.) through planning, design, and operational treatments. The implementation project's objective is to share and disseminate the results of the research with public agencies, and to provide hands-on technology transfer assistance to these agencies.

• Record URL: http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=5046
• Status: Completed
• Funding: $249950

Project 20-44(35)

National Cooperative Highway Research Program

American Association of State Highway and Transportation Officials (AASHTO)

Federal Highway Administration

Wadsworth, Trey

Kittelson and Associates, Inc.

Schroeder, Bastian

• Start Date: 20220601
• Expected Completion Date: 20231201
• Actual Completion Date: 20231201

Subject/Index Terms

• TRT Terms: Cyclists ; Highway design ; Highway operations ; Highway planning ; Highway safety ; Interchanges and intersections ; Pedestrian safety ; Plan implementation ; Technology transfer
• Subject Areas: Design; Highways; Operations and Traffic Management; Pedestrians and Bicyclists; Planning and Forecasting; Safety and Human Factors;

Filing Info

• Accession Number: 01847463
• Record Type: Research project
• Source Agency: Transportation Research Board
• Contract Numbers: Project 20-44(35)
• Files: TRB, RIP
• Created Date: May 27 2022 12:50PM

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