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12 Dynamic Operation versus Static Operation â D-PTSU is characterized by variable hours of operation. By definition, D-PTSU facilities must have dynamic signs. S-PTSU has fixed hours of operation. Some S- PTSU facilities have dynamic signs, and others have static signs. Two PTSU facilities were converted from S-PTSU to D-PTSU during the study period. An empirical Bayes before/after study was conducted to quantify the safety effect of the conversion. The study found a 7.3 percent decrease in crashes following conversion from S-PTSU to D-PTSU. This finding is applicable to âtotalâ crashes, which include crashes of all types and severity levels combined. Some recently built D-PTSU facilities include gantries that span all lanes, thus providing lane control signals for all lanes and, in some cases, additional active traffic management (ATM) features. Future research should investigate if there are differences in safety performance associated with different D-PTSU and ATM designs. Project 17-89 Safety Finding Summary Key safety findings from NCHRP Project 17-89, described in the preceding sections, are summarized in Table 4. Table 4. Key PTSU safety performance findings. Topic Finding Overall effect on crash frequency and severity PTSU sites are associated with higher FI and PDO crash frequency than sites without PTSU in most cases. However, the proportion of FI crashes that are severe (K or A severities) decreases in most cases. Shoulder open versus shoulder closed An hourly analysis found that a PTSU site with the shoulder open is associated with 138% more crashes than the same site with the shoulder closed. Shoulder closed versus no PTSU No difference in safety performance between a PTSU site with the shoulder closed and a site without PTSU. Left-side versus right-side PTSU No difference in safety performance of left-side and right-side PTSU facilities, but further research is needed. Dynamic signs versus static signs No difference in safety performance of sites with dynamic signs and sites with static signs. Dynamic operation versus static operation Converting S-PTSU to D-PTSU results in a 7.3% decrease in crash frequency. PTSU Predictive Safety Analysis Introduction NCHRP Project 17-89 models enable the prediction of crash frequency and severity and are documented in draft text prepared for a future edition of the HSM. Readers are referred to the PTSU Safety Evaluation Guidelines for this draft HSM text. A summary of the process to conduct predictive safety analysis is presented in this section. Like other HSM predictive models, the CPMs developed in NCHRP Project 17-89 provide information about the relationship between roadway geometric design features and safety. They are based on research that quantified the relationship between various design elements (e.g., lane width) or design components
13 (e.g., turnouts) and expected average crash frequency. The CPMs are intended to help designers make informed judgments about the safety performance of design alternatives, particularly those involving PTSU. The CPMs developed in this project are the first models developed for the HSM that are capable of analyzing PTSU facilities. Evaluation Process Analysis Steps The HSM Part C predictive method has 18 steps (AASHTO 2014). The steps include the identification of project limits and site limits, gathering data, applying CPMs for each site to computed predicted crashes, andâif desiredâapplying the Empirical Bayes (EB) method to compute expected crashes. The steps are detailed in the draft HSM text in the PTSU Safety Evaluation Guidelines. Data Requirements A list of data needed to apply the NCHRP Project 17-89 CPMs to compute the predicted crash frequency is provided below. Users are referred to Section 2.5.2 of the draft HSM text in the PTSU Safety Evaluation Guidelines for complete definitions of these data elements and instructions for measuring them. Unless noted, data are needed only for the direction of the freeway being analyzed. ï· Number of through lanes ï· Length of freeway segment (i.e., the site being analyzed) ï· AADT of freeway segment ï· Length of speed-change lane, if present ï· AADT of entrance ramp, if present ï· Radius of curve ï· Width of lanes, PTSU lane, outside shoulders, inside shoulders, and medians ï· Length of rumble strips on inside and outside shoulders ï· Length and offset to barrier in median and barrier on roadway ï· Distance to nearest upstream entrance ramp and nearest downstream exit ramp ï· AADT of nearest upstream entrance ramp and nearest downstream exit ramp ï· Clear zone width ï· Proportion of freeway AADT volume that occurs during high volume hours; this is only needed to compute crash severity, not crash frequency. ï· Length of PTSU transition zone present ï· Length of turnout in segment ï· Proportion of time during average day that PTSU operates Segmentation Process HSM analysis of a facility is conducted by subdividing the facility into sites that are homogeneous with respect to key features that impact safety performance. When segmenting a freeway for analysis with the NCHRP 17-89 CPMs, a new segment should be creating if a change if any of the following occurs: ï· Site type (basic freeway segment, ramp entrance speed-change lane, ramp exit speed-change lane) ï· Number of through lanes ï· Through lane width ï· Outside shoulder width