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4 ï· Shorter implementation timeframe than a conventional widening project. Some PTSU projects have been interim in nature and eventually were removed when a subsequent widening project occurred (Jenior 2016). Before/after studies of PTSU operation have consistently identified operational benefits of PTSU operation. Several studies have also been conducted for the purpose of quantifying the safety benefits of PTSU operation, and the majority have shown increases in crash frequency following PTSU implementation. The findings from several of these studies are summarized below in the PTSU Tradeoffs section of this document. NCHRP Project 17-89 conducted a more rigorous and comprehensive study of PTSU safety performance, and findings are presented in the PTSU Tradeoffs section as well. PTSU Tradeoffs This section discusses documented operational performance and safety performance of PTSU operation. It is intended to give planners and engineers a high-level understanding of these two performance measures on PTSU facilities. Later sections of this document discuss how to perform predictive safety analysis of specific PTSU sites. Operational Performance Before/after studies of facilities on which PTSU operation was implemented have shown operational improvements. Specific findings include the following: ï· Following implementation of PTSU on a two-lane rural section of I-70 eastbound in Colorado with heavy recreational traffic, there was a 14 percent increase in the throughput, a 38 percent improvement in travel time in general purpose lanes, and an 18 percent increase in the average vehicle speeds during high traffic volumes on the weekends (CDOT 2017). ï· Following implementation of PTSU on a two-lane, 1.55-mile bridge on US 2 in Washington State, peak period peak direction delays reduced from 8-10 minutes to 1-2 minutes (Dowling 2011). ï· Following implementation of PTSU on a freeway in Germany with three lanes in each direction, a 20- 25 percent increase in capacity was reported (Geistefeldt 2102). ï· Following implementation of PTSU on a 9.32-mile section of Highway X and a 7.45-mile section of Highway W in Denmark, travel time was reduced 1-3 minutes on Highway Z and 5 minutes on Highway W, despite increases in traffic volume from local road traffic shifting to the freeways (Danish Road Directorate 2016). ï· Studies of multiple freeways in the Netherlands (A20, A27, A13, A7, and A28) showed decreases in peak and off-peak travel times following implementation of PTSU (Veld 2009). Additional information on these studies is found in the NCHRP Project 17-89 Final Report. The capacity added by the shoulderâwhen it is openâis highly dependent on design features, and dimensions meeting or exceeding criteria in the AASHTO Green Book (AASHTO 2018) and AASHTOâs Policy on Design Standards â Interstate System (AASHTO 2011) should be provided when possible. Field- measured capacity ranges from 1,000 to 2,000 vehicles per hour (Dowling 2011). The capacity of right- side PTSU that continues through interchanges increases as ramp-merge distances increase. Right-side PTSU where the width of the shoulder used for travel is 12 feet or more has higher capacity than right-side PTSU with a narrower shoulder. Data on left-side PTSU capacity is limited but is expected to be similar to (and perhaps slightly lower than) that of the adjacent general purpose lanes (Jenior 2016).
5 Safety Performance Before/after studies of facilities on which PTSU operation was implemented have shown operational improvements. Specific findings include the following: ï· Following implementation of PTSU on US 2 in Washington, an empirical Bayes evaluation showed a 15 percent increase in expected crash frequency. This was attributed to the site being âuniqueâ (Margiotta 2014). ï· Two studies of I-66 in Virginia examined differences between time periods when PTSU was operational (i.e., the shoulder was open to traffic) and when it was not. A 2007 study found an 8 percent decrease in crash frequency in the hours PTSU was operational (Suliman et al. 2017). A later study found a 38 percent increase in crash frequency in the hours PTSU was operational (Lee 2012: Kuhn 2013). ï· Following implementation of PTSU on I-35W in Minneapolis, an empirical Bayes evaluation showed a 28 percent increase in crash frequency. This was attributed to changes in traffic volume and traffic patterns (Margiotta 2014; Davis 2017). ï· Following implementation of PTSU on A3 in Hessen and A7 in Schleswig-Holstein, Germany, there was an increase in crash frequency on the PTSU segment and a decrease in crash frequency upstream of PTSU that was attributed to a decrease in congestion. PTSU implementation also resulted in increased volume on these freeways (Geistefeldt 2012; Jones 2011). Additional information on these studies is found in the NCHRP Project 17-89 Final Report. NCHRP Project 17-89 developed crash prediction models (CPMs) for freeways with and without PTSU, and these models provide a means of assessing the overall effect of PTSU on crash frequency and severity. Safety findings presented in this section were primarily obtained through the use of count regression modeling, with crashes assumed to follow a negative binomial distribution. This approach is consistent with the manner in which CPMs in the Highway Safety Manual (HSM) were developed. The dataset used for model development consisted of 728 sites with a total length of 164.8 miles; 177 of the sites had PTSU and 551 did not. Sites were classified as basic freeway segments, entrance speed-change lanes, or exit speed-change lanes. Sites were located in Georgia, Hawaii, Minnesota, Ohio, and Virginia. Sites and CPMs are directional (i.e., they encompass and analyze one direction of the freeway). The sites collectively had 4,807 fatal-and-injury (FI) crashes and 11,937 property-damage-only (PDO) crashes. All sites were in urban areas. Project 17-89 Crash Frequency Prediction Model Findings Models that predict (1) the frequency of FI crashes and (2) the frequency of PDO crashes were developed and applied to assess the overall effect of PTSU on total and severe crash frequency. A third modelâthe severity distribution function (SDF)âwas developed and applied to predict the distribution of FI crashes among different severity levels. Each model includes variables that describe the traffic demand characteristics, geometric elements, and PTSU operational features. The variables related to PTSU design and operation are provided in the following list: ï· Proportion of time that PTSU operates; ï· PTSU lane width; ï· Proportion of segment length with PTSU transition zone present between, upstream of, or downstream of a PTSU lane; ï· Number of through-lanes on segment (including managed lanes but not including auxiliary lanes or PTSU); ï· Proportion of segment length with turnout present; and ï· Turnout spacing.
6 The first four variables in the aforementioned list are incorporated in a PTSU operation adjustment factor (AF). The last three variables in the aforementioned list are incorporated in the turnout presence AF. An AF is analogous in its interpretation to a crash modification factor (CMF) in Part C of the HSM. Transition zones are locations upstream, downstream, or between portions of a freeway with a PTSU typical section. Turnouts are paved areas adjacent to a shoulder used for PTSU that function as refuge areas for disabled vehicles. Table 1 illustrates the application of the PTSU operation and turnout presence AFs for FI crashes for various combinations of PTSU lane width, proportion of time PTSU was operating, and number of through lanes. The values are shown in the top two-thirds of Table 1 are for freeway segments with PTSU, and those values in the bottom one-third of Table 1 are for PTSU transition zone segments. Typical values for the variables identified in Table 1 include 0.5-mile turnout spacing, a proportion time PTSU was operating of 0.2, an 11-foot PTSU lane width, and four lanes. For these values, the AF value is 1.41. This AF value suggests that PTSU operation can increase the annual FI crash frequency by 41 percent for typical conditions. Table 1. Estimated PTSU operation AF for FI crashes. PTSU Type PTSU Lane Width (ft) Proportion Time PTSU Operatinga AF Value by Number of Lanes 2 4 6 PTSU lane (no turnouts) 11 0.0 0.80 0.89 0.93 0.1 1.11 1.19 1.22 0.2 1.42 1.49 1.52 0.3 1.73 1.79 1.82 0.4 2.04 2.09 2.11 12 0.0 0.78 0.88 0.92 0.1 1.08 1.17 1.20 0.2 1.37 1.45 1.48 0.3 1.67 1.74 1.77 0.4 1.96 2.03 2.05 PTSU lane (turn- out every 0.5 mi) 11 0.0 0.72 0.85 0.89 0.1 1.00 1.13 1.18 0.2 1.28 1.41 1.46 0.3 1.56 1.70 1.75 0.4 1.84 1.98 2.04 12 0.0 0.71 0.84 0.89 0.1 0.97 1.11 1.16 0.2 1.24 1.38 1.43 0.3 1.51 1.65 1.70 0.4 1.77 1.92 1.97 PTSU transition zoneb Any 0.0 1.00 1.00 1.00 0.1 1.11 1.11 1.11 0.2 1.22 1.22 1.22 0.3 1.33 1.33 1.33 0.4 1.43 1.43 1.43 a Proportion time PTSU operating = (weekday hours Ã 5/7 + weekend hours Ã 2/7)/24 b Segment length is 0.27 mile.
7 A similar table of AF values was prepared for the PTSU-related AFs included in the PDO crash prediction model. The AF values for the PDO model follow the same trends as those for FI crashes but tend to be about 10 percent larger in value (i.e., AFPDO â 1.1 Ã AFFI). The trends in the AF values in the last three columns of Table 1 are summarized in the following list: ï· Urban freeway segments with PTSU operation are typically associated with a larger FI crash frequency than those segments without PTSU operation. ï· FI crash frequency was found to be higher on segments where the proportion of time that PTSU operates is larger. ï· Segments with a turnout present were associated with a smaller FI crash frequency than those sites without a turnout present. The trends in Table 1 suggest that the provision of turnouts at 0.5-mile spacing can reduce the PTSU operation AF value by 5 to 10 percent ï· Segments with an average lane width of 11 feet were found to have a larger FI crash frequency than those segments with an average lane width of 12 feet. Figure 2. Turnout, SR 3 Massachusetts Project 17-89 Severity Distribution Prediction Model Findings An SDF was developed to be used with the FI crash prediction model to provide the means for estimating the frequency of crashes by severity level K, A, B, or C. Table 2 was developed from the SDF. It lists the predicted severity distribution for freeway segments as a function of proportion time that the PTSU operates, proportion AADT during high volume hours as defined in the HSM Supplement (AASHTO 2014), and proportion of segment adjacent to a barrier.
8 Table 2. Comparison of predicted severity distribution for freeway segments. Proportion Time PTSU Operates Proportion AADT in High Volume Hours Proportion Segment Adjacent to Barrier Crash Proportion by Severity K A B C 0.0 0.05 0.1 0.005 0.056 0.417 0.521 0.5 0.005 0.051 0.378 0.567 0.9 0.004 0.046 0.339 0.611 0.25 0.1 0.005 0.051 0.375 0.570 0.5 0.004 0.045 0.336 0.615 0.9 0.004 0.040 0.299 0.657 0.45 0.1 0.004 0.045 0.333 0.618 0.5 0.004 0.040 0.296 0.660 0.9 0.003 0.035 0.261 0.700 0.5 0.05 0.1 0.001 0.039 0.439 0.521 0.5 0.001 0.036 0.397 0.567 0.9 0.000 0.032 0.357 0.611 0.25 0.1 0.001 0.035 0.394 0.570 0.5 0.000 0.032 0.353 0.615 0.9 0.000 0.028 0.314 0.657 0.45 0.1 0.000 0.031 0.350 0.618 0.5 0.000 0.028 0.311 0.660 0.9 0.000 0.025 0.275 0.700 The proportions shown in the four right columns of Table 2 indicate that the proportion of K, A, and B crashes decrease (while the proportion of C crashes increase), with an increase in the proportion of segment adjacent to barrier. The proportions shown in the table also indicate that an increase in the proportion of AADT during high volume hours corresponds to a decrease in the proportion of K, A, and B crashes. Finally, the proportions shown indicate that an increase in the proportion of time that PTSU operates corresponds to a decrease in K and A crashes (while the proportion of B crashes increases, and the proportion of C crashes stays about the same). Thus, the increase in FI crash frequency with PTSU operation is partially offset by a shift in the severity distribution away from the most severe crashes. Project 17-89 Crash Cost Evaluation The trends identified in the two preceding sections were examined in combination using a crash cost analysis. To facilitate this examination, one direction of a hypothetical freeway section was evaluated. The section has two 12-ft through traffic lanes in the subject travel direction. The freeway experiences an average of one FI crash for every two PDO crashes. The directional AADT volume is 31,500 vehicles per day. The base condition for this examination is a freeway without PTSU operation. The PTSU operation, turnout presence, and outside shoulder width AFs were used to compute the AF values shown in the third and fourth columns of Table 3. The SDF was used to compute the severity distribution shown in the fifth to eighth columns. The relative cost of a crash was computed for each row of the table. This cost is relative to the number of crashes predicted for the base condition (which has a cost of $31,118). The relative crash cost tends to increase with an increase in the âproportion time PTSU operatingâ due to the associated increase in crash frequency relative to the base condition. The relative cost is shown to be smaller than the base condition cost when the âproportion time PTSU operatingâ is small. This trend results because the width of the PTSU lane is effectively serving as additional shoulder width
9 when PTSU operation is not allowed. Additional information on how values in the table were calculated are presented in the NCHRP Project 17-89 Final Report. Table 3. Change in crash frequency, severity, and cost associated with PTSU operation. PTSU Type Proportion Time PTSU Operating AF Value by Severity1 Severity Distribution Relative Crash Cost ($)2 Proportion Change in Cost3 FI PDO K A B C PTSU lane (no turnouts Base cond.4 1.00 1.00 0.004 0.042 0.313 0.641 31,118 0.05 1.10 1.17 0.004 0.042 0.313 0.641 33,255 1.07 0.1 1.27 1.39 0.003 0.039 0.317 0.641 37,594 1.21 0.2 1.62 1.83 0.002 0.037 0.320 0.642 46,078 1.48 0.3 1.97 2.27 0.001 0.034 0.323 0.642 54,486 1.75 0.4 2.31 2.71 0.001 0.032 0.326 0.641 62,925 2.02 PTSU lane (turnout every 0.5 mi) Base cond. 4 1.00 1.00 0.004 0.042 0.313 0.641 31,118 0.05 0.99 1.02 0.004 0.042 0.313 0.641 29,842 0.96 0.1 1.15 1.21 0.003 0.039 0.317 0.641 33,726 1.08 0.2 1.46 1.60 0.002 0.037 0.320 0.642 41,319 1.33 0.3 1.77 1.98 0.001 0.034 0.323 0.642 48,843 1.57 0.4 2.09 2.36 0.001 0.032 0.326 0.641 56,395 1.81 1 â Product of PTSU operation, turnout presence, and outside shoulder width AFs. PTSU design conditions include two through lanes, PTSU lane width of 12 feet, and paved outside shoulder width of 2 feet. AF values for FI crashes are from Table 1. AF values for PDO crashes from Table 5-12 of the NCHRP Project 17-89 Final Report. AF values for outside shoulder width are also from the NCHRP Project 17-89 Final Report. 2 â Societal crash costs from Table 7-1 of the HSM (AASHTO 2010). 3 â Proportion change in crash cost equals the crash cost for the subject row divided by the crash cost for the base condition. 4 â Base condition: no PTSU lane, no PTSU operation, no turnout, and 10 foot outside shoulder width. The proportion change in crash cost is shown in the last column of Table 3. This proportion can be compared with the AF values shown in columns 3 and 4. For a given row (i.e., the value of proportion time of the PTSU operating), the proportion change in cost value is less than or equal to the FI AF and PDO AF values. This trend reflects the reduction in the proportion of very severe crashes associated with PTSU operation. In fact, when a turnout is provided every 0.5 mile and the proportion time PTSU operating is 0.06 or less (e.g., â¤ 2 hours/day during the weekdays and 0 h/day during the weekends), the change in crash cost is less than 1.0 and there is a safety benefit associated with PTSU operation. Project 17-89 Supplemental Safety Findings In addition to the CPMs developed for inclusion in a future edition of the HSM, additional count regression modeling of crash data was conducted to examine targeted research questions. A summary of major findings is presented herein, with additional information available in the NCHRP Project 17-89 Final Report.
10 Shoulder Open versus Shoulder Closed â The safety performance of PTSU sites with the shoulder closed to traffic and the shoulder open to traffic was compared. This analysis was conducted with a dataset consisting of 48 observations per siteâone observation for each weekday hour and one for each weekend hour of the study period. The analysis results indicate that sites with the shoulder open during a given hour have 138 percent more crashes than sites at which the shoulder is closed (all other geometric design elements, traffic control features, and traffic volumes being the same). This finding is applicable to âtotalâ crashes, which include crashes of all types and severity levels combined. a. Shoulder open, I-66 Virginia. b. Shoulder closed, I-264 Virginia Figure 3. PTSU facilities with the shoulder open and closed. Shoulder Closed versus No PTSU â The safety performance of PTSU sites with the shoulder closed to traffic and âcomparisonâ sites without PTSU was compared. This analysis was conducted with a dataset consisting of 48 observations per siteâone observation for each weekday hour and one for each weekend hour of the study period. The analysis results indicate that there is no practical difference between the crash potential of sites with PTSU closed to traffic and the crash potential of sites without PTSU. This is consistent with the data in Table 1, which shows the increase in crashes on PTSU facilities to largely be associated with PTSU being operational rather than simply present but not in operation. a. Shoulder closed, I-70 Colorado b. Freeway without PTSU Figure 4. PTSU facility with shoulder closed and freeway without PTSU.
11 Left-side versus Right-side PTSU â The ability to assess the safety performance of left-side versus right-side PTSU was limited by the relatively small number of left-side PTSU sites in the NCHRP Project 17-89 dataset. Within this limited dataset, the analysis results indicate that there is no practical difference between the crash potential of sites with right-side PTSU and the crash potential of sites with left-side PTSU. This finding is qualified as being applicable to PTSU facilities with ramps providing right-side access to the freeway. Several left-side PTSU facilities have opened in the US in recent years, and future study of the safety performance of left-side versus right-side PTSU is recommended. a. Left-side PTSU, I-495 Virginia b. Right side PTSU, I-66 Virginia Figure 5. Left-side and right-side PTSU facilities. Dynamic Signs versus Static Signs â Dynamic signs electronically change their display when the shoulder is open or closed. Typically, a green arrow is used to indicate the shoulder is open and a red X is used to indicate the shoulder is closed. Static signs use reflective sheeting to list the days and hours in which the shoulder is open. The analysis results indicate that there is no practical difference between the crash potential of sites with dynamic signs and the crash potential of sites with static signs. a. Dynamic sign, I-495 Virginia b. Static sign, SR 3 Massachusetts Figure 6. PTSU facilities with dynamic signs and static signs