Safety Performance of Part-Time Shoulder Use on Freeways, Volume 1: Informational Guide and Safety Evaluation Guidelines (2021) / Chapter Skim
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31 Chapter 2 – Draft Highway Safety Manual Text 2.1 Overview This chapter presents the predictive method for urban freeways with part-time shoulder use (PTSU)
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32 The predictive method described in this chapter is used to evaluate a freeway facility with a PTSU lane serving the subject direction of travel. This facility can represent an existing freeway, a design alternative for an existing freeway, or a new freeway.
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33 and possible injury C In this document, a "FI crash" is any crash designated has having a K, A, B, or C severity level.
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34 point (as shown in Figure 8b)
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35 Table 3 identifies the site, crash type, and severity configurations for which SPFs have been developed. Table 3.
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36 type distributions are presented in Section 2.8.4. Guidance for establishing the value of the calibration factor is described in Section 2.10.
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37 Figure 3. The HSM predictive method.
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38 to include a very long corridor for the purposes of network screening (as discussed in HSM Chapter 4)
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39 recorder data or estimated by a sample survey. For a future period, the AADT volume may be a forecast estimate based on appropriate land use planning and traffic volume forecasting models.
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40 Step 4 -- Determine geometric design features, traffic control features, and site characteristics for all sites in the project limits. A range of data is needed to apply a predictive model.
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41 SPF with which it is used (unless indicated otherwise in the AF description)
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42 distributions are presented in Sections 2.7.4 and 2.8.4. The distributions can provide a more reliable indication of local crash characteristics if they are updated based on local data as part of the calibration process.
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43 Ne,en(i) ,at,as,j = expected average crash frequency of ramp entrance speed-change lane site i for year j (includes all crash types at and all severities as)
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44 freeway adjacent to the speed-change lane. The predictive models are limited to freeways with two to seven lanes.
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45 Figure 5. Freeway speed-change lane length.
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46 Figure 6. Measurement of cross section data elements.
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47 distance is equal to 0.0 mi. If the ramp does not exist or is located more than 0.5 mi from the segment, then this distance can be set to a large value (e.g., 999)
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48 Clear zone width -- This width is measured from the edge of traveled way to the typical limits of vertical obstruction (e.g., non-traversable slope, fence line, utility poles) along the roadway.
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49 Entrance ramp AADT volume, Exit Ramp AADT volume -- The annual average daily traffic volume of the ramp. The evaluation of a freeway segment requires the nearest upstream entrance ramp AADT volume (if within 0.5 miles of the segment)
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50 In Figure 10c, the site is shown as a freeway segment between two PTSU lanes. Its length is shown to include two transition zones.
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51 Popen, wkday,j = proportion of hour j that PTSU is operating during typical weekday of year; and Popen, wkend,j = proportion of hour j that PTSU is operating during typical weekend day of year.
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52 The predictive model for speed-change lane sites estimates the average frequency of crashes that are associated with the presence and operation of the speed-change lane. These crashes occur in Region A of Figure 13 (this region is defined by the gore point and the taper point)
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53 new site if the rounded value for the current point changes from that of the previous point (e.g., from 6 to 7 ft)
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54 a. Add PTSU lane.
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55 Guidance regarding the location of the lane, shoulder, and median width measurement points is provided in the text associated with Figure 6. Each width represents an average for the site.
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56 Table 4. SPFs for freeway segments.
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57 which these SPFs are applicable is shown in Table 6. Application of the SPFs to segments with AADT volume substantially outside these ranges may not provide reliable results.
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58 Table 7. SPF coefficients for freeway segments.
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59 SPF for fatal-and-injury crashes (fs, at, fi) SPF for property-damage-only crashes (fs, at, pdo)
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60 AF3,fs,at,z -- Inside Shoulder Width Two AFs are used to describe the relationship between average inside shoulder width and predicted crash frequency. The SPFs to which they apply are identified in the following list: SPF for fatal-and-injury crashes (fs, at, fi)
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61 Pib = proportion of site length with a barrier present in the median (i.e., inside) ; Wicb = distance from edge of inside shoulder to barrier face (ft)
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62 Table 12. Coefficients for median barrier AF -- freeway segments.
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63 AADTe,ext = AADT volume of exit ramp located at distance Xe,ext downstream of the subject segment (veh/day) ; Xb,ent = distance from segment begin milepost to nearest upstream entrance ramp gore point (mi)
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64 The base condition is "no outside shoulder rumble strips present" (i.e., Por = 0.0)
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65 Table 14. Coefficients for outside clearance AF -- freeway segments.
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66 Equation 37 𝐴𝐹 , , , 1.0 𝑃 1.0 𝑃 exp 𝑎/𝑛 with Equation 38 𝑃 𝐿 , /𝐿 , where AF12,fs,at,z = adjustment factor for turnout presence in a freeway segment; for all crash types and severity z; Ls,fs = length of freeway segment (mi) ; Lturnout,fs = length of turnout within segment (i.e., between segment begin and end mileposts)
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67 AF13,fs,at,z = adjustment factor for PTSU operation in a freeway segment; for all crash types and severity z; IptsuLane = indicator variable for PTSU lane presence (= 1.0 if PTSU lane is present [Wptsu,s > 0] , 0.0 otherwise)
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68 The general form for the severity distribution prediction equation for freeway segments is shown in the following equation. Equation 45 𝑁 , , , 𝑁 , , , 𝑃 , , where Np,fs,at,j = predicted average crash frequency of a freeway segment; for all crash types at and severity level j (j = K: fatal, A: incapacitating injury, B: non-incapacitating injury, C: possible injury)
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69 This SDF is applicable to Phv, Pib, and Pob values in the range of 0.0 to 1.0. Guidance for computing the variables Pib and Pob is provided in Section 2.9.
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70 Table 18. Default distribution of crashes by crash type for freeway segments.
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71 Table 19. SPFs for speed-change lanes.
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72 6. The SPFs are applicable to ramp AADT volume up to 30,700 veh/day.
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73 a. Fatal-and-injury crash frequency.
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74 AADTf = one-directional AADT volume of freeway in speed-change lane site (veh/day) ; Ls,ex = length of ramp exit speed-change lane site (≤ length of ramp exit speed-change lane, as measured from gore point to taper point)
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75 2.8.2. SPF Adjustment Factors for Speed-Change Lanes with PTSU The AFs for geometric design and traffic control features of speed-change lane sites are presented in this section.
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76 AF2,w,at,z = adjustment factor for lane width at a speed-change lane site; for site type w (w = en: entrance ramp speed-change lane; ex: exit ramp speed-change lane) , all crash types, and severity z; and Wl = through lane width (ft)
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77 SPF for fatal-and-injury crashes, ramp entrance (en, at, fi) SPF for property-damage-only crashes, ramp entrance (en, at, pdo)
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78 Wicb ranges from 0.75 to 20 ft. The width of the PTSU lane is 16.8 ft or less.
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79 AF6,w,at,fi = adjustment factor for rumble strips on the inside shoulder of a speed-change lane site; for site type w (w = en: ramp entrance speed-change lane; ex: ramp exit speed-change lane) , all crash types, and fatal-and-injury fi crashes; n = number of through lanes within site; and Pir = proportion of site length with a rumble strips present on the inside shoulder.
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80 Table 28. Coefficients for PTSU operation AF -- speed-change lanes.
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81 SPF for fatal-and-injury crashes, ramp exit (ex, at, fi) SPF for property-damage-only crashes, ramp exit (ex, at, pdo)
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82 Pw,at,j = proportion of crashes with severity level j (j = K: fatal, A: incapacitating injury, B: nonincapacitating injury, C: possible injury) for all crash types at on site type w (w = en: ramp entrance speed-change lane; ex: ramp exit speed-change lane)
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83 Table 31. Coefficients for severity distribution function -- speed-change lanes.
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84 Table 32. Default distribution of crashes by crash type for speed-change lanes.
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85 2.9. Supplemental Calculations to Apply SPF Adjustment Factors Some of the AFs in Section 2.7.2 and Section 2.8.2 require the completion of supplemental calculations before they can be applied.
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86 The summation term "∑" in parentheses in Equation 92 applies to short lengths of barrier in the median that are located between the shoulder and continuous barrier. It indicates that the ratio of barrier length Lib,i to clearance distance (= Woff,in,i –Wptsu,s –Wis,s)
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87 Iptsu,s,o = indicator variable for PTSU lane location in the subject travel direction (= 1.0 if outside shoulder is allocated to part-time vehicular traffic use at any time of the day; otherwise 0.0)
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88 Table 33. Crash frequency predictive models with a calibration factor.
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89 2.11. Limitations of Predictive Method This section discusses limitations of the predictive models described in this chapter.
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90 immediate vicinity of a toll plaza, (b) is widened to accommodate vehicle movements through the toll plaza, (c)
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91 freeway segment. Sample Problem 2 illustrates how to calculate the predicted average crash frequency for a ramp entrance speed-change lane.
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92 Downstream exit ramp is 0.30 miles from the segment and has an AADT volume of 7600 veh/day (Xe,ext = 0.30; AADTe,ext = 7600) ; as shown in Figure 19.
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93 Step 10 -- Multiply the result obtained in Step 9 by the appropriate AFs. The AF's in the predictive model equation for freeway segments are described in this step.
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94 The AF for PDO crashes is computed using the same equation but with a different coefficient. This AF is computed as AF4,fs,at,pdo = 1.056.
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95 𝐴𝐹 , , , exp 𝑎𝑛 min 𝑊 , 12 10 The segment has 3 through lanes. The coefficient a is obtained from Table 13.
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96 The 0.10-mi turnout is located wholly within the length of the 0.50-mi segment. Based on the guidance in Figure 11, "length of turnout within the segment" Lturnout,fs is equal to the total turnout length.
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97 1.318 The coefficient d is obtained from Table 17. It equals 1.305 for FI crashes.
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98 Apply the site-specific EB method to a future time period, if appropriate. The site-specific EB method is not applied in this sample problem.
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99 𝑆 , , exp 2.128 𝑓 , , , 𝑓 , , 𝑓 , , 0.119 0.795 0.905 0.958 0.0821 𝑆 , , exp 0.126 𝑓 , , , 𝑓 , , 𝑓 , , 0.882 0.795 0.905 1.006 0.6381 The fifth step is to compute the proportion of crashes with severity level j. The proportions are computed using Equation 46 to Equation 49.
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100 Apply the crash type distribution, if desired. The first step in applying the crash type distribution proportions is to select the desired crash type proportion from Table 18.
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101 AF value of 0.954 suggests that FI crash frequency is 4.6 percent lower on a segment with a turnout present, relative to a segment without a turnout. The outside shoulder width AF value of 1.131 suggests that the 1-ft outside shoulder is associated with a 13.1 percent increase in FI crash frequency, relative to a segment with a 10-ft outside shoulder.
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102 No rumble strips on inside shoulder. Continuous median barrier; 10-ft offset from edge of traveled way to barrier face.
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103 AF3,en,at,z -- Inside Shoulder Width The subject site has 6.0-ft paved inside shoulders, which is the base condition for the inside shoulder width AF. Hence, AF3,en,at,fi and AF3,en,at,pdo are equal to 1.000.
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104 AF13,en,at,z -- PTSU Operation The PTSU operation AF is computed using Equation 70. This equation is repeated below.
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105 The site is located within a 0.25-mi speed-change lane. The coefficient a is obtained from Table 29.
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106 Apply the severity distribution, if desired. The severity distribution is not needed for this evaluation.
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