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34
CHAPTER 4
Analysis Framework
This chapter presents the analysis framework used in established that pedestrians who are blind have difficulty dis-
NCHRP Project 3-78A. It discusses performance character- cerning either of these events reliably. As a result, it is expected
istics of pedestrians that are used to quantify the availability that the rate of utilization of gap and yield crossing opportuni-
of crossing opportunities in the traffic stream, as well as the ties is less than 100%, which is described mathematically by
pedestrians' ability (or willingness) to successfully utilize these two additional probability terms:
opportunities. This analysis framework was initially described
in Schroeder and Rouphail (2007) and was later adopted in · P(GO|CG): the probability of utilizing an encountered cross-
Schroeder et al. (2009), which are included in Appendices I able gap and
and J, respectively. These characteristics are then tied to a set · P(GO|Y): the probability of utilizing an encountered yield.
of accessibility criteria that were formulated in Schroeder et al.
(2009). On the basis of these accessibility criteria, the chapter In the interpretation of these two probabilities, it is impor-
discusses analysis strategies for the collected field data, includ- tant to emphasize that it is often unclear from observational
ing definitions of pedestrianvehicle events and variables studies whether the crossing opportunities are missed (i.e., the
used in the analysis. The results of the analysis are presented pedestrian didn't hear the yielding vehicle) or rejected vol-
in Chapter 5. untarily (i.e., the pedestrian chose not to cross in front of the
yielding vehicle). For observational studies, the objective
description is that the opportunity was not utilized, which
Crossing Performance Characteristics
does not pass judgment about the reason for not crossing.
The analysis framework hypothesizes that the interaction This approach for describing pedestrian crossing behav-
of pedestrians and vehicles at unsignalized crosswalks can be ior is initially based on unsignalized crossings but can also be
described using principles of probability theory. Conceptu- applied to signalized crossings. At a signalized crossing, pedes-
ally, pedestrians encounter two types of crossing opportu- trians also encounter crossing opportunities in the form of
nities: (1) gaps in between vehicles of sufficient duration to gaps (no traffic) and yields (cars stopped at the signal), but
allow for a safe crossing, and (2) drivers yielding the right of with the caveat that these crossing opportunities tend to coin-
way to pedestrians. The likelihood of occurrence of these events cide with a particular phase in the signal cycle (i.e., the "Walk"
is described by the following probabilities: phase). When applying this approach to a signalized crossing,
the analyst should therefore make note of differences in these
· P(CG): the probability of encountering a crossable gap (CG) probability terms at different times in the cycle. Even at sig-
in the traffic stream and nals, some drivers may not yield (e.g., may run a red light) and
· P(Yield): the probability of encountering a yield in the (blind) pedestrians may miss crossing opportunities, especially
traffic stream. when no audible signal is provided. The probability terms
described above therefore provide a universal evaluation
The definitions for what constitutes a yield or a crossable framework of pedestrianvehicle interaction at crosswalks that
gap are given later in this chapter. Given the complex auditory can be applied independently of the presence of a signal.
environment at roundabouts and CTLs and the lack of reliable Similarly, the above framework allows the analyst to quan-
cues to help identify these events, prior research (e.g., Guth tify the impact of any other crossing treatments that are
et al. 2005, Ashmead et al. 2005, Schroeder et al. 2006) has intended to facilitate pedestrian crossings but fall short of
