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35 stopping traffic with a red signal indication. The framework to cross) and inversely proportional to the pedestrian walking therefore is sensitive to different treatment objectives, such speed (slower walkers require more time to cross). It is fur- as increasing the propensity of drivers to yield, P(Yield), or ther reasonable to implement some additional buffer time to enhancing the ability of using crossing opportunities. account for some lost time before a crossing is initiated and some safety clearance time after the crossing is completed. The U.S. Highway Capacity Manual (HCM) (TRB 2000) defines Crosswalk Usability Criteria the critical gap for pedestrians as the crossing distance divided The crossing performance characteristics above provide by the walking speed, plus a safety buffer. This same concept a framework for quantifying pedestrian behavior from an has been applied in other research on pedestrian behavior observational study. For the purpose of establishing criteria (Yang et al. 2006; Rouphail, Hughes, and Chae 2005). For the for whether or not a crosswalk is accessible to and usable by a purpose of this analysis framework, an increase in the occur- pedestrian who is blind, four accessibility criteria have been rence of crossable gaps directly affects the crossing opportu- formulated. These criteria address different aspects of usability nity criterion. and together provide a comprehensive approach for quantify- The remaining critical definition in this criterion is the term ing crossing performance at a test location. The four crosswalk sufficient, which describes whether there are enough crossing usability criteria, from Schroeder et al. 2009, are: opportunities in the traffic stream. The determination of how many crossing opportunities are enough depends on the rate 1. Crossing opportunity criterion of utilization of opportunities (criterion 2) and ultimately on Are there sufficient crossing opportunities in the form acceptable levels of delay and risk (criteria 3 and 4). of yields or crossable gaps? 2. Opportunity utilization criterion Opportunity Utilization Criterion Are the crossing opportunities utilized by the pedestrian? 3. Delay criterion The second criterion quantifies the level of pedestrian uti- Is a crossing opportunity taken within a reasonable time? lization of the available crossing opportunities. The utilization 4. Safety criterion of crossable gaps is a function of the gap acceptance character- Do the crossing attempts involve a significant degree of istics of the pedestrian. It may further be influenced by back- collision risk? ground noise at the site. At roundabouts and channelized turn lanes in particular, the noise from background traffic may Each of the four criteria is discussed in more detail below. mask the auditory information at the crosswalk, affecting the ability of a blind pedestrian to identify a crossable gap or yield (Guth et al. 2005, Schroeder et al. 2006). Previous research Crossing Opportunity Criterion has shown that pedestrians with vision impairments often Crossing opportunities may take the form of yields or gaps. do not cross in front of yielding vehicles because they either A yield is defined as a driver reducing the speed of the vehicle cannot hear the car or they are not confident that the cross- or coming to a full stop to allow a pedestrian to cross the street. ing is safe despite the yield condition (Ashmead et al. 2005, Legislation in most U.S. states requires drivers to yield to Davis and Inman 2007). Multiple threat situations (FHWA pedestrians in the crosswalk, but the laws (and driver under- 2004) at two-lane approaches, where a vehicle in the near lane standing of the law) vary in terms of the requirement to yield visually and/or auditorily masks approaching vehicles in the to pedestrians at the crosswalk. A nationwide survey of yield- far lane, further complicate yield utilization. ing practices at unsignalized crosswalks (Fitzpatrick et al. It has been demonstrated in ongoing research funded by the 2006) identified a wide range of yielding behaviors and fur- NIH (2010) that sighted pedestrians can successfully iden- ther found varying levels of yield compliance for different tify and utilize most (if not all) crossing opportunities they pedestrian crossing treatments. Similar inconsistency was encounter. Clearly, individual differences remain, but with a found for yielding behavior at roundabout crossings across relatively conservative definition of what constitutes a crossing the United States (Rodegerdts et al. 2007). For the purpose opportunity (e.g., the crossable gap threshold), it is hypothe- of this analysis framework, an increase in yielding directly sized that a yield and gap utilization rate of 100% represents affects the crossing opportunity criterion. a reasonable benchmark for what may constitute crossing When crossing opportunities are presented in the form of behavior of a sighted pedestrian. While the first criterion is gaps between successive vehicles, a threshold needs to be intro- largely independent of the behavior (and any disability) of the duced to define what constitutes a crossable gap. The threshold pedestrian, this second criterion begins to distinguish between for separating crossable and non-crossable gaps is proportional pedestrian groups with different travel skill levels. This dis- to the crossing distance (longer distance requires more time tinction includes the difference between blind and sighted