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From page 177... ... 179 C H A P T E R 8 -SUPPLEMENTAL SAFETY FINDINGS Supplemental Safety Findings Introduction The literature review and agency interviews identified several research questions that would provide a means of assessing the safety performance of freeways that contain bus-on-shoulder (BOS) and part-time shoulder use (PTSU) Read the entire page →
From page 178... ... 180 The remainder of this chapter is organized into 10 sections. The first section below provides an overview of the statistical methods used to answer the research questions. Read the entire page →
From page 179... ... 181 where Var(λi) is the variance of observed crashes occurring at site i; E(λi) Read the entire page →
From page 180... ... 182 Step 3: Compare Predicted and Observed Safety Performance to Estimate Treatment Effectiveness An unbiased estimate of the safety effect θ of the treatment or countermeasure was obtained using Equation 198, where Equation 199 is used to compute the variance term needed in Equation 198. Equation 198 𝜃 𝑁 𝑁 1 𝑉𝑎𝑟 𝑁𝑁 with Equation 199 𝑉𝑎𝑟 𝑁 𝑟 1 𝑤 𝑁 where θ is the unbiased estimate of safety effect of the countermeasure, and 𝑁 is the observed crashes at the site during the after period. Read the entire page →
From page 181... ... 183 = 1) , is shown in Equation 203, and the probability of crash n experiencing severity outcome i = 0, Pn(i = 0) Read the entire page →
From page 182... ... 184 Table 73. Descriptive statistics of BOS sites. Read the entire page →
From page 183... ... 185 Equation 207 𝑓 𝑃 𝑃 /2 where frs is the factor for rumble strips; Pir and Por are the proportion of the site length with inside and outside rumble strips, respectively. Equation 208 𝑓 𝐼 /𝑛 where fdrop is the factor for lane drop; Idrop is an indicator variable for lane drop presence (= 1.0 if lane drop is present; 0 otherwise) Read the entire page →
From page 184... ... 186 Table 74. Fixed parameters model of total crashes for BOS sites. Read the entire page →
From page 185... ... 187 Table 75. Fixed parameters model of FI crashes for BOS sites. Read the entire page →
From page 186... ... 188 effectively an indicator for the site being located on I-71 in Ohio. There is no means for determining whether the coefficient is reflecting "location on I-71" or "presence of left-side BOS." Based on these findings, it is concluded that BOS presence on the left side of the freeway is unlikely to have a meaningful influence on crash frequency. Read the entire page →
From page 187... ... 189 Table 76. Descriptive statistics of PTSU sites. Read the entire page →
From page 188... ... 190 FI and PDO Crash Frequency Models Two crash frequency prediction models were developed for the purpose of evaluating the effect of a proposed PTSU design on total and severe crash frequency. One model predicts the frequency of FI crashes. Read the entire page →
From page 189... ... 191 Table 78. Estimated PTSU operation AF for FI crashes. Read the entire page →
From page 190... ... 192 Table 79. Comparison of predicted severity distribution for freeway segments. Read the entire page →
From page 191... ... 193 Table 80. Change in crash frequency, severity, and cost associated with PTSU operation. Read the entire page →
From page 192... ... 194 Questions 2a and 2b: Safety Effect of PTSU Presence Question 2a: What is the difference in safety performance when the shoulder is open or closed? Question 2b: When the shoulder is closed, is there a difference in safety performance between a freeway with shoulder use and a freeway without shoulder use? Read the entire page →
From page 193... ... 195 Table 81. Descriptive statistics of PTSU sites and comparison sites. Read the entire page →
From page 194... ... 196 addition, state indicator variables were included in the model to control for differences in crash reporting among the agencies represented in the database. The site length was assumed to be directly proportional to the number of total crashes and was included as an offset variable in the model. Read the entire page →
From page 195... ... 197 Table 82. Fixed parameters model of total crashes per hour for PTSU sites. Read the entire page →
From page 196... ... 198 Question 2b Results The "proportion of time during an hour that the PTSU lane is open" variable was set to zero using the model shown in Table 82. The indicator for "PTSU facility presence" variable was then used to compare the safety performance of site with a PTSU lane that is closed to a site without a PTSU lane. Read the entire page →
From page 197... ... 199 Empirical Setting A database containing PTSU sites was developed to answer Question 4. The database included all PTSU sites and PTSU comparison sites. Read the entire page →
From page 198... ... 200 The site length was assumed to be directly proportional to the number of total crashes and was included as an offset variable in the model. Additionally, since the number of crashes observed at each location was for some predefined time period (e.g., between 3 to 5 years) Read the entire page →
From page 199... ... 201 Table 84. Fixed parameters model of total crashes for PTSU sites. Read the entire page →
From page 200... ... 202 the null hypothesis that this value equals 0.0 cannot be rejected at a 5 percent significance level. There is a probability of 0.120 that a coefficient value this large could be observed due to random variation when the true value equals 0.0. Read the entire page →
From page 201... ... 203  Right-side PTSU with static signing  Right-side PTSU with dynamic signing  Left-side PTSU with static signing  Left-side PTSU with dynamic signing Given the characteristics described in the previous paragraph, the indicators for static signs and dynamic signs in the model in Table 84 cannot be used by themselves to assess the safety of PTSU presence (with static or dynamic signs) versus a comparison facility without PTSU. Read the entire page →
From page 202... ... 204 were not included in the estimation data. There are a total of 123 site observations in the database. Read the entire page →
From page 203... ... 205 PTSU to D-PTSU conversion was undertaken with no infrastructure changes. Both sites had dynamic signs during the S-PTSU operational period. Read the entire page →
From page 204... ... 206 change in the width of the shoulder used for BOS travel has an effect on freeway safety is considered to be of no practical value. Therefore, this section addresses the question of whether a change in the width of the shoulder used for PTSU travel has an effect on freeway safety. Read the entire page →
From page 205... ... 207 rumble strips presence, and other site-specific features were considered as candidate independent variables for inclusion in the model. The site length was assumed to be directly proportional to the number of total crashes and was included as an offset variable in the model. Read the entire page →
From page 206... ... 208 Results The model shown in Table 89 is used to answer the question about the safety effect of changing shoulder width. The right-side shoulder width variable for "sites without PTSU operation on the right side" has a coefficient value of −0.032. Read the entire page →
From page 207... ... 209  Step 3: Compare the ratio of the nighttime indicators results from Steps 1 and 2 to determine if there is difference in nighttime crash severities among BOS or PTSU facilities and comparable non-BOS or non-PTSU facilities. For this analysis, crashes that were reported between 10:00 PM and 4:59 AM were defined as nighttime events, while crashes that were reported between 8:00 AM and 3:59 PM were defined as daytime period crashes. Read the entire page →
From page 208... ... 210 Table 90. Summary of PTSU crash severity data. Read the entire page →
From page 209... ... 211 Table 91. Summary of BOS crash severity data. Read the entire page →
From page 210... ... 212 than 1.0 indicates that the odds of a crash being a fatal or severe crash increases as the value of the independent variable increases (or changes from zero to unity for an indicator variable) Read the entire page →
From page 211... ... 213 Table 94. Fatal-and-severe-injury crash severity model for matched PTSU freeway segments. Read the entire page →
From page 212... ... 214 null hypothesis that this value equals 1.0 can be rejected at a 5 percent significance level. The probability the odds ratio is this large because of random variation (when the true value equals 1.0) Read the entire page →
From page 213... ... 215 The model in Table 96 was estimated using only freeway segments on comparison facilities. Only crashes that occurred during the daytime and nighttime periods were considered in the model. Read the entire page →
From page 214... ... 216 1.084. However, the null hypothesis that this value equals 1.0 cannot be rejected at a 5 percent significance level. Read the entire page →
From page 215... ... 217 Summary of Findings This chapter presented analysis used to answer research questions related to the safety performance of PTSU and BOS. Table 98 summarizes the findings related to each question. Read the entire page →

From page 177...

... 179 C H A P T E R 8 -SUPPLEMENTAL SAFETY FINDINGS Supplemental Safety Findings Introduction The literature review and agency interviews identified several research questions that would provide a means of assessing the safety performance of freeways that contain bus-on-shoulder (BOS) and part-time shoulder use (PTSU)