Consideration of Roadside Features in the Highway Safety Manual (2022) / Chapter Skim
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From page 2... ...
2 BACKGROUND The concept of providing a forgiving roadside first emerged in the 1960s and started to become regularly incorporated in highway designs in the 1970s resulting in the 1974 AASHTO document Highway Design and Operational Practices Related to Highway Safety and the 1977 AAHSTO Guide for Selecting, Locating and Designing Traffic Barriers (i.e., the "Barrier Guide")
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3 embodiment of the encroachment probability model and cost-benefit technique is further discussed below. There are both similarities and differences between the approaches used by Green Book for geometric design and the RDG for roadside design.
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4 Crashes are predicted for different facility types, including road segments and intersections; special facilities and geometric situations; and road networks. Road Segments and Intersections Crash types common to segments differs dramatically from the crash type common to intersections; therefore, the two basic units of analysis in the HSM are road segments and intersections.
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5 quantified countermeasures, but are represented by suggestive trends. Work zone countermeasures have been quantified for freeways only.
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6 CMFs developed following the publication of the HSM in 2010 are available in the literature. The main source for locating CMFs is the CMF clearinghouse, an online database maintained by the Federal Highway Administration (see http://www.cmfclearinghouse.org)
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7 The SPF for rural two-lane roads is applicable across traffic volumes ranging from 0 through 17,800 vpd. The base conditions for this SPF are: • Lane width of 12 feet • Paved, six foot wide shoulders • Roadside Hazard Rating (RHR)
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8 sideswipe, and same-direction sideswipe crashes," however, the horizontal curve and vertical grade CMFs are applicable to the total road segment.[AASHTO10] In other words, the lane width and shoulder width CMFs modify a specific group of crash types, while the horizontal and vertical geometric CMFs modify all crash types equally.
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9 impressions based on comparison of the study segment to standardized reference photographs. The RHR has a scale of 1 to 7 with 1 representing very good and 7 representing very poor roadside conditions, respectively.
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10 Using the text or the photographs, a single RHR is chosen to represent both sides of the road for an entire segment. Variations in the roadside are not captured over the segment using this method.
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11 Figure 3. Photographic Representation of Roadside Hazard Ratings (RHR)
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12 Figure 4. Rural Two-Lane Road Example 1.
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13 Recall the Rural, Two-lane road baseline model presented above. When the model requires modification because the base conditions of the SPF do not match the field conditions, CMFs are used as shown: Nfs= Nspf rs Cr(CMF1r, CMF2r,…, CMFnr)
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14 Figure 6. Example 1 All Crash Types Modified for Roadside Design Over Possible RHR.
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15 for divided roadways. A new segment, therefore, begins at the center of an intersection when there is a change in any of the following characteristics: • AADT; • Presence of median; • Median width; • Sideslope (for undivided roadway segments)
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16 Table 5. Recommended Lane Width Rounding for Segmenting.
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17 Table 7. Rural Multilane Road Base Conditions.
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18 Table 8. Rural Multilane Road CMFs and the Applicable Crash Types.
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19 𝑁𝑁𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = (𝑁𝑁𝑠𝑠𝑝𝑝𝑠𝑠 𝑥𝑥 × �𝐶𝐶𝐶𝐶𝐶𝐶1𝑥𝑥 × 𝐶𝐶𝐶𝐶𝐶𝐶2𝑥𝑥 × … × 𝐶𝐶𝐶𝐶𝐶𝐶𝑦𝑦𝑥𝑥� + 𝑁𝑁𝑝𝑝𝑝𝑝𝑝𝑝𝑥𝑥 + 𝑁𝑁𝑏𝑏𝑝𝑝𝑏𝑏𝑝𝑝𝑥𝑥) × 𝐶𝐶𝑥𝑥 Where: Npredicted = predicted average crash frequency for a specific year on site type x; Nspf x = predicted average crash frequency determined for base conditions of the SPF developed for site type x; Npedx = predicted average number of vehicle-pedestrian collisions per year for site type x; Nbikex = predicted average number of vehicle-bicycle collisions per year for site type x; CMFyx = crash modification factors specific to site type x and specific geometric design and traffic control features y; and Cx = calibration factor to adjust SPF for local conditions for site type x.
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20 There are several segment SPFs for this highway classification and these SPFs are valid at various traffic volumes, as described here: • Two-lane undivided arterials (2U) : 0 to 32,600 vpd • Three-lane arterials including a center two-way left-turn lane (3T)
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21 Table 9. Coefficients for Urban and Suburban Single Vehicle SPF.[AASHTO10]
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22 Table 11. Urban and Suburban Arterial CMFs and the Applicable Crash Types.
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23 For example, if there is 140 feet of guardrail along a roadside located in front of two utility poles located 70 feet apart, the guardrail would be counted as two point objects within the segment and the utility poles would not be considered. On the other hand, if there is 140 feet of guardrail located behind two utility poles, the guardrail would not be considered, only the poles would be considered.
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24 CMF is greater for tighter-spaced objects that are closer to the road. For example, when the objects are spaced 70 feet apart and offset two feet from the travel way, the CMF is 2.00.
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25 Figure 8. Fixed-Object CMF for Two Foot Offset for Various Urban and Suburban Arterial Road Types.
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