Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 88


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 87
87 First, while the tested single-lane roundabouts were found channelized turn lane. At both types of facilities, crosswalks to be manageable by most pedestrians who are blind, it is are typically located at least one vehicle length away from the expected that a higher-speed design will have a severe adverse circulating (downstream) lane to ensure (1) storage for one effect on accessibility. Following the same argument, larger vehicle downstream of the crosswalk and (2) a separation two-lane roundabouts with higher design speeds and two or between the driver's cognitive tasks of interacting with pedes- more lanes can be expected to be less accessible than the tested trians and the circulating traffic (or downstream CTL traffic). two-lane roundabout in Golden. However, this placement of the crosswalk often results in Second, if an existing site features high speeds through a vehicle queues across the crosswalk. Some interventions (espe- combination of roadway geometry and driver behavior, it is cially at the CTL) occurred in response to vehicles in queues hypothesized that a higher-impact treatment would be needed that began moving as the blind pedestrian approached the side to make the site accessible. Low-impact treatments like the of the vehicle, not realizing it was there. While this research ones tested at the high-speed CTL location are apparently not did not perform any field studies on alternative crosswalk sufficient to make a high-speed crossing accessible. It is hypoth- locations, a simulation-based analysis of roundabout signals esized that a traffic calming treatment like the raised crosswalk demonstrated advantages in vehicular delay of the zigzag or or a red signal display would be more appropriate at high- distal crosswalk location. The greater exit-lane separation speed locations. However, more research on these treatments enhanced these two objectives (more storage and better sepa- is necessary to solidify this claim, mainly due to the observed ration of cognitive tasks for the driver), but is associated with multiple threat incidents at the RCW in the posttest. two tradeoffs that may negatively affect pedestrians: (1) with increasing separation, the speed-reducing effect of the round- about is reduced, and (2) the pedestrian travel distance is Crossing Geometry increased if measured from the main intersection. However, In all crossings observed in this research, vehicles approached it is unclear at this point whether an alternate crosswalk ge- from only one direction at a time. Both roundabouts and ometry has any (positive or negative) impact on the crossing channelized turn lanes feature splitter islands that provide the ability of pedestrians who are blind. benefit of refuge for pedestrians before they complete the second part of the crossing. It is important to emphasize that Inter-Participant Variability this form of traffic pattern is a potential benefit of both types of intersection compared to intersections with one-stage The analysis results showed a very large variability between crossings across two-way traffic. different blind participants. It needs to be emphasized that at Despite the advantages of one-way approaching traffic, all of the crossings, there were some individuals who were able identifying gaps and yields with curved vehicle trajectories to cross with a reasonable amount of delay and without expe- and elevated levels of ambient and vehicular noise contribute riencing any interventions. Others experienced multiple inter- to crossing challenges at these types of crosswalks for pedestri- ventions at the same crossing, and others had some trials that ans who are blind. In particular, prior research (e.g., Ashmead were terminated because a 2-min time-out limit was reached. et al. 2005, Guth et al. 2005) has noted that the exit portion Appendix A provides more detail on the inter-participant vari- of a roundabout crossing is more difficult to cross due to diffi- ability and shows the distribution of key performance measures culties discerning exiting traffic from circulating traffic and also across participants. This variability causes challenges for the due to less frequent yielding behavior. From a driver behavioral analysis, the interpretation of the results, and ultimately the perspective, pedestrians may be considered more of a nuisance decision to consider a site accessible with or without treatment. when exiting the roundabout. Upon entering a roundabout, The results in this report emphasize average performance vehicle trajectories are characterized by deceleration (to be able and associated variability (i.e., standard deviation) when eval- to safely navigate the circle) and the potential to have to come uating pretestposttest treatment effects. Consequently, any to a stop (to yield to traffic in the circulating lane). On the con- differences that are statistically significant are to be considered trary, the exiting driver accelerates and psychologically has noteworthy in light of high standard deviations. For example, cleared the delay-causing intersection. the two-lane roundabout treatments seemed to have improved In this research, the hypothesized difference between enter- crossing effectiveness and safety for all participants to the point ing and exiting traffic was evident in a lower likelihood of driver of reducing delay and eliminating interventions (at the given yielding at the exit. Interestingly, this difference did not result sample size). On the other hand, the CTL treatments did not in a consistent and notable increase in delay or safety at the help all participants and left many with significant challenges exit portion of the crosswalk across sites. in crossing at the location. The inter-participant variability also Another aspect of crossing geometry is the relative place- means that sites that may be accessible for some travelers pose ment of the crosswalk in relation to the roundabout or the severe challenges for others. For example, some participants