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21
letter-grade level of service is then determined from a hazard participants (in groups of 7 to 10) a tape of a series of signal-
index look-up table. ized intersections. The intersections portrayed on the tape
Zhang and Prevedouros [17] developed a model of were chosen in cities outside the local area to eliminate fa-
vehicle-to-vehicle and vehicle-to-pedestrian conflicts and miliarity by the subjects, but in a location nearby so that local
blended it with the existing HCM delay LOS criteria for sig- conditions were represented. The results of the study showed
nalized intersections to obtain an LOS model that combines that, on average, subjects' delay estimates were fairly accurate,
safety risk with traditional delay measures of LOS. Two delay but widely variable on an individual basis. The study also
and safety indices are computed--one for pedestrians, the showed that subjects perceived three or four levels and were
other for vehicles. Each index is computed as a weighted sum more tolerant of delays than suggested by the HCM. At least
of potential conflicts and delay. The weights are analyst spec- 15 factors emerged from the group discussions that subjects
ified. The two indices are then weighted by pedestrian and ve- identified as influential in their LOS ratings. These included
hicle volumes, respectively, to obtain a weighted average delay, traffic signal efficiency, arrows/lanes for turning vehi-
delay and safety index for the intersection. No surveys of cles, clear/legible signs and road markings, geometric design
traveler perception were performed. This paper was oriented of intersection, leading left-turn phasing scheme, visual
toward methodological approaches rather than traveler clutter/distractions, size of intersection, pavement quality,
perception. queue length, traffic mix, location, scenery/aesthetics, and
Several recent studies of intersection LOS have also cast presence of pedestrians.
some doubt on the HCM's methods. Zhang and Prevedouros
(2004) [18] investigated motorists' perceptions of LOS at case
Use of Fuzzy Logic for LOS Modeling
study signalized intersections and found that, although the
HCM 2000 predicts that permitted left-turn phases provide a Recent research has begun to use "fuzzy logic" to identify
higher level of service, users ranked protected left-turn delay thresholds for rating the level of service of signalized
phased intersections higher. This finding suggests that users intersections.
may be including the perceived safety benefits of protected Fang and Pecheux [22] conducted a video laboratory of
phasing at these case study locations in their assessments of 98 subjects assessing the quality of service on 24 signalized in-
LOS, in addition to delay. In a follow-up study, Zhang and tersection approaches. Cluster analysis (employing fuzzy
Prevedouros (2005) [19] surveyed users' perceptions of ser- thresholds) revealed that their subjects' quality of service as-
vice quality at intersections and found that users consider sessments did not distinguish between the delays at HCM
multiple factors beyond delay (as calculated by the HCM LOS A or B. The LOS ratings of their subjects, however, did
2000), including signal efficiency, left-turn treatment, and distinguish two classes of delay for delays at HCM LOS F.
pavement conditions. Delay scored relatively low among im- Zhang and Prevedouros [23] conducted a web-based
portant factors. Drivers prefer to make left turns under pro- stated preference survey of 1,300 volunteers. Their survey
tected left-turn signals, especially at large intersections. Safety identified delay, pavement markings, presence of exclusive
was stated to be 3 to 6 times more important than delay, de- left-turn lanes, and protected left-turn phases, as factors sig-
pending on the type of conflict. nificantly affecting the perceived level of service at a signal-
The importance of safety in determining the level of service ized intersection. Fuzzy inference was used to identify a
offered by an intersection is reflected in a study by Li et al. distribution of LOS responses for a given physical condition.
(2004) [20]. Li et al. used a "gray system" theory-based A percent confidence level was then reported for each LOS
method to rank and evaluate the operational and safety letter grade.
performance of signalized intersections in mixed traffic con- Lee, Kim, and Pietrucha [24] exposed 27 subjects to video
ditions. The degree of saturation, average stopped delay, clips of 12 signalized intersections. Subjects were asked to
queue length, conflict ratio, and separation ratio are all used (1) rate their intersection experience as "poor," "acceptable," or
as parameters. Results of application in the urban area of "good" and (2) describe the relative importance of six criteria
Changsha, China, show that the method can be used to con- to their rating of the intersections. The six criteria evaluated
duct a comprehensive (safety and operations) performance were delay, gaps in cross street traffic while waiting, efficiency
under mixed traffic conditions. of traffic signal operation, visibility of signal, signing/markings,
Pecheux, Pietrucha, and Jovanis [21] addressed users' per- and physical features of the intersection.
ception of level of service at signalized intersections. The re-
search objectives were to examine delay distributions, assess
Rural Road Research
the accuracy of delay estimates, determine if current levels
of service are appropriate, and identify factors affecting per- Nakamura, Suzuki, and Ryu [25] conducted a field driving
ceptions. The research used a video laboratory to show 100 survey on a rural motorway section under uncongested traffic
