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120 Exhibit 29. O&M intervention statistics for single-lane roundabout crosswalk. P(Risky Crossing) DAV-CLT Avg. Min. Max. Std. Dev. Entry 0.8% 0.0% 8.3% 2.6% Exit 0.8% 0.0% 8.3% 2.6% Overall 0.8% 0.0% 8.3% 2.6% PS-RAL Entry 2.1% 0.0% 6.3% 3.1% Exit 5.8% 0.0% 25.0% 7.3% Overall 3.9% 0.0% 25.0% 5.8% correspond to LOS scores C and D, respectively. To recall, Discussion the HCM defines levels of service on a scale from A (best) to F (worst) in terms of average delay per person. Based on all the criteria considered in the comparison of the Finally, the analysis includes the rate of O&M interventions two sites, the higher-volume PS-RAL site is in fact more acces- that represent a measure of pedestrian risk during the crossings. sible than the DAV-CLT site from a delay perspective. This is The study participants were at all times accompanied by a primarily due to the high frequency of yields at the PS-RAL site certified O&M specialist who was directed to stop the partic- and a high yield utilization rate. As a result, blind pedestrians ipants if the crossing decision would have resulted in undue on average experience less than half the delay at PS-RAL com- risk to pedestrian and/or driver. The resulting rate of O&M pared to DAV-CLT. Similarly, the amount of unnecessary intervention is defined as follows. delay beyond the first crossing opportunity is about three times as high at the DAV-CLT site. These findings are some- Intervention Rate (%): The intervention rate is defined by what surprising, given that the availability of (long) crossing the number of times the O&M specialist intervened for a gaps at the PS-RAL is less than that at DAV-CLT. The cross- particular subject divided by the total number of lanes able gap utilization rates appear to be comparable for both sites. crossed for a particular condition. For example, one inter- However, from a risk perspective, the PS-RAL clearly shows vention over a set of eight lane crossings at the roundabout higher intervention rates and thus a more dangerous crossing entry corresponds to an intervention rate of 12.5%. situation. In light of these findings, it is evident that the question of Exhibit 29 shows the rate of experimenter interventions. roundabout accessibility is complex and cannot be reduced The intervention rates at PS-RAL are clearly higher than to a simple relationship to traffic volumes. While a low-volume DAV-CLT, and the exit lane crossing is especially risky with site may appear to be easily accessible, a higher-volume site an intervention rate of 5.8%. At the DAV-CLT site, one par- may result in higher accessibility if associated with a higher ticipant experienced a single intervention at the entry leg and rate of yielding that is being utilized. The greater accessibility of another one a single intervention at the exit leg (1 intervention the PS-RAL site is attributable to higher P(Yield) and P(GO|Y) in 12 crossing results in a rate of 8.3%). Since no other subjects probabilities. These two factors seemed to have a significant experienced any interventions, the resulting average inter- overall impact on reduced pedestrian delay, despite the fact that vention rate across 10 subjects was 0.8% for both the entry the site had higher volumes and consequently a lower availabil- and exit leg. ity of crossable gaps, P(Gap>Min). Given the higher propensity However, with repeated crossings even the 0.8% inter- to yield at PS-RAL, the associated higher volumes resulted in vention rate at DAV-CLT could result in a high likelihood of more frequent crossing opportunities per unit of time. a risky decision over time. Ashmead et al. (2005) discussed that The analysis in this section shows that the studied higher- the probability of a dangerous crossing decision is given by volume roundabout was in fact more accessible to blind pedes- 1 (1 pper crossing)n, where pper crossing is the observed intervention trians based on the multi-criteria established in this study. The rate and n the number of crossing attempts. Consequently, for hypothesis that the DAV-CLT roundabout is "easily accessible a pedestrian who crosses this roundabout twice a day, the prob- because of low volumes" could not be supported by the compar- ability of a dangerous decision after one month (10 crossings ison data from PS-RAL. Through the comparison it has become per week over 4 weeks) is 27.5%. At the 3.9% intervention evident that a combination of crossable gaps, yields, and utiliza- rate for PS-RAL this likelihood increases to 79.6%. tion rates all contribute to making a site more or less accessible.