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36 pedestrians (the focus of this research), but can similarly be pedestrians make significantly more risky decisions than sighted applied to represent other special pedestrian populations such pedestrians at unsignalized crosswalks at channelized right- as children, wheelchair users, or the elderly. turn lanes. A study of blind pedestrians crossing at a two-lane Another caveat of the utilization measure is that it can be roundabout (Ashmead et al. 2005) found that the experi- used to describe potentially risky behavior. For example, assum- menter sometimes had to physically restrain the study partic- ing that the defined threshold for a crossable gap is appropri- ipant from crossing to avoid a potential collision. The overall ate, any utilization of a non-crossable gap has the potential of observed intervention rate was a clear indication of the risky increasing the rate of crossable gap utilization to something nature of the studied two-lane roundabout crossing. The met- greater than 100%. This type of check also serves to validate ric of intervention rates was also previously used at crossing that the crossable gap threshold was defined reasonably. studies at single-lane roundabouts (NIH 2010). Delay Criterion Field Evaluation Approach The first two criteria describe traffic conditions (gap avail- The four crosswalk usability criteria were evaluated in con- ability), driver behavior (yielding rate), and pedestrian behav- trolled studies with blind volunteers as described in Chapter 3. ior (utilization). However, they ignore the temporal aspect of The crossing trials were monitored by a field observer and the pedestrianvehicle interaction. Assuming that a pedestrian were further videotaped for supplemental data extraction eventually utilizes a crossing opportunity, the third criterion in the office. The analysis approach worked on the basis of describes how much delay was experienced prior to that cross- time-stamped events that describe traffic conditions, driver ing initiation. The Highway Capacity Manual (TRB 2000) uses behavior, and pedestrian behavior. The following section delay to define levels of service for pedestrian crossings. From defines pedestrianvehicle interaction events as used in this an engineering perspective, it is thus intuitive that an inordi- project, followed by a section of variable definitions and nate amount of delay would make a crossing inaccessible. In performance measures used in the analysis. the HCM, a (sighted) pedestrian delay over 45 s at an unsignal- ized intersection corresponds to level of service (LOS) F, which Event Definitions is the worst category on an A through F scale. At a signalized crossing the corresponding LOS F threshold is 60 s, acknowl- The NCHRP Project 3-78A analysis used a performance edging that pedestrians may be more willing to accept higher evaluation framework that described the availability of cross- delays if they have confidence that the signal will eventually ing opportunities, the rate of utilization of these opportuni- provide them with an opportunity to cross. The HCM further ties, and the delay and risk associated with the crossings. For emphasizes that the likelihood of risk-taking behavior (by a single-lane crossing, the yield and gap events are uniquely sighted pedestrians) is very high at these levels of delay. defined by the vehicle state in the conflict lane. However, at a The usability of a crosswalk is improved with a reduction two-lane crossing the analysis needs to consider the vehicle in criterion 3. It is expected that increasing levels in criteria 1 state in both lanes. and 2 will result in an improvement (reduction) of pedestrian For a single conflicting lane, a pedestrianvehicle event is delay. Similarly, a low availability of crossing opportunities defined as the interaction of one pedestrian and one vehicle. For and/or a low utilization rate will increase delay. For pedes- each participant, the total number of events is therefore equiv- trians who are blind, experience with increasingly high delay alent to the number of vehicles encountered during the cross- at a particular crossing may lead people to avoid the cross- ing attempt(s). For a two-lane crossing, a pedestrianvehicle ing altogether, making it, in effect, inaccessible. Similarly, it event will be defined as the interaction of one pedestrian can be argued that high delays (caused by low levels in cri- and one vehicle in the lane nearest to where the pedestrian is teria 1 and 2) may lead to an increased propensity to make waiting. The vehicle state in the far lane is considered and risky decisions, as is hypothesized in the HCM. In this research, will be discussed in more detail below. the study trials were capped at a time duration of 2 min. For all types of crossings, a pedestrianvehicle event has one of five outcomes: Safety Criterion 1. Rolling yield (RY): Pedestrian encounters a driver who The fourth criterion describes pedestrian safety at a cross- has slowed down for the pedestrian, but has not come to a walk. Even if pedestrians encounter crossing opportunities full stop. and utilize them within an acceptable amount of time, the site 2. Stopped yield (STY): Pedestrian encounters a driver who remains inaccessible if these crossings occur in dangerous has come to a stop, defined as moving at a speed less than situations. Research (Schroeder et al. 2006) found that blind approximately 3 mph.