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13 CHAPTER 2 Synthesis of the Literature Facility Design of the splitter island serves to divide the pedestrian crossing task into two separate segments. Under low traffic volumes, a Current roundabout and CTL design criteria are presented pedestrian may be able to cross in a single movement. Under in documents such as the AASHTO Policy on Geometric higher traffic volumes, pedestrians may wait on the splitter Design of Highways and Streets (AASHTO 2004); the Inter- island until a crossable opportunity is detected on the second section Channelization Design Guide (Neuman 1985); the leg of the crossing. In either case, the pedestrian crossing task FHWA's Manual on Uniform Traffic Control Devices (2009); is typically focused to one direction at a time. Figure 1 shows a the AASHTO Guide for Planning, Design, and Operation of schematic drawing of typical roundabout crosswalk geometry. Pedestrian Facilities (2004); the FHWA's Pedestrian Facilities The actual alignment of the crosswalk can vary. Often Users Guide (Zegeer et al. 2002); the FHWA's Signalized Inter- there is no deviation in the orientation of the crosswalk, and sections: Informational Guide (2004); the FHWA's Round- the crosswalk proceeds straight from curb to curb. However, abouts: An Informational Guide (2000); an updated version some crosswalks are designed with a bend at the splitter island, of the FHWA roundabout guide available as NCHRP Report which may pose wayfinding challenges for blind travelers in the 672: Roundabouts: An Informational Guide, Second Edition absence of additional tactile cues. There are a few crosswalks (Rodegerdts et al. 2010); and the research results and synthesis that use an offset or zigzag design that deflects pedestrian traf- to come from NCHRP Project 3-72, "Lane Widths, Channelized fic onto an elongated splitter island before the second part of Right Turns, and Right-Turn Deceleration Lanes in Urban the crossing. The intent of this treatment is to reinforce two- and Suburban Areas" (Midwest Research Institute 2011), stage crossing behavior and, to some extent, increase the which will be available in 2011. These documents include pro- distance between the crosswalk and the circulating lane. visions for determining the placement of crosswalks, signage, For all crosswalks, pedestrian ramps at either curb are sup- and other aspects of roundabout and CTL design. A key issue is posed to be perpendicular to the curb/gutter line. Due to the that existing designs are intended to accommodate the major- radius of the curve, they may not be in line with the direction ity of pedestrians, who have normal vision. Current designs of travel on the crosswalk and therefore may cause alignment were not developed specifically to support unassisted cross- difficulties for blind pedestrians. Curb ramps built after 2001 ing of streets by pedestrians who are blind. are supposed to have truncated dome detectable warning sur- faces at the bottom of the ramp to alert the pedestrian who is blind that he or she has arrived at a streetsidewalk boundary. Geometric Design for Pedestrian Crossings In the United States, few pedestrian crosswalks at round- at Roundabouts abouts are signalized (either for pedestrian or traffic control Current practice in the United States (FHWA 2000) locates purposes). the pedestrian crosswalk approximately one car length back from the circulating lane, although this varies. The crosswalk The Geometric Design for Pedestrian Crossings is generally perpendicular to the travel lane and passes through at Channelized Turn Lanes an approach splitter island. This island is designed to sepa- rate opposing traffic streams, reduce wrong-way movements Channelized turn lanes are much more prevalent in the around the central island, and provide refuge to pedestrians United States than roundabouts. Despite their increased preva- before they cross the second leg of the approach. The presence lence, less attention has been given to the effects of treatments

OCR for page 13
14 This figure shows a schematic drawing of a roundabout with key features highlighted. The pedestrian crosswalk is placed approximately one car length back from the circulating lane. The entry- and exit-leg portions of the crosswalk are separated by a raised splitter island to provide pedestrian refuge. Figure 1. Typical roundabout design features and crosswalk geometry (source: FHWA 2000). designed to improve pedestrian safety and access. In essence, there are three typical locations for a pedestrian crosswalk associated with a CTL. The crosswalk may be located at the upstream (entering) side of the turn lane, at the downstream (or exiting) side, or at the midpoint, perpendicular to the tan- gent of the curve that defines the turn lane. Figure 2 shows a schematic drawing of a typical CTL with the crosswalk located at the midpoint. The intersection shown has a deceler- ation lane in the approach of the CTL. This feature may not be present in all designs and can vary in length. Alternate designs may also have an acceleration lane at the downstream end of the CTL (FHWA 2004). The midpoint crosswalk location presumably minimizes the crossing distance and is likely to coincide with the loca- tion of slowest vehicle speeds. When vehicles are stopped for pedestrians, this design provides some degree of storage upstream of the crosswalk before traffic in the through lane This figure shows a schematic drawing of a CTL with key is affected. features highlighted. The pedestrian crosswalk is placed Upstream crosswalk locations require pedestrians to dis- at the midpoint of the CTL and approximately one car length back from the downstream merge point.The CTL criminate between through vehicles and vehicles that intend to shows a deceleration lane that allows right-turning traffic turn into the channelized turn lane. This can be difficult, even to slow down for the turn away from through traffic at for sighted pedestrians, since there may be no indication of the intersection. driver intent until the vehicle is very close to the crosswalk. If it Figure 2. Typical CTL crosswalk geometry is not possible to discriminate through traffic from turning (source: FHWA 2004).