Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges (2021) / Chapter Skim
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5 CHAPTER 2: LITERATURE REVIEW This chapter describes the findings from a review of the literature related to freeway and interchange safety. The objective of this review was to identify the highway infrastructurerelated factors that influence the safety of freeways and interchanges.
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6 TABLE 1. Freeway mileage and number of interchanges in the United States Attribute Functional System Rural Urban Total Freeway length, mi 1 Interstate 30,313 16,396 46,709 Non-interstate 2 4,575 8,809 13,384 Total: 34,888 25,205 60,093 Number of interchanges 3 Interstate 6,900 10,900 17,800 Non-interstate 1,000 5,900 6,900 Total: 7,900 16,800 24,700 Number of interchanges 4 Freeway-to-freeway (system)
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7 Other 3% SPU 1% Trumpet 4% Directional 6% Full Cloverleaf 8% Partial Cloverleaf 16% Diamond 62% Figure 1. Typical interchange types.
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8 Typical variations of the diamond and parclo interchange types are shown in Figure 3. It is noted that the SPUI is generally considered to be a diamond-type interchange, but was separately categorized in the distribution shown in Figure 2.
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9 Figure 4. Typical ramp configurations.
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10 Figure 5. Freeway and interchange design components.
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11 with a weaving section (with length Lwev)
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12 Curve 1* Tangent 2 Tangent 1 Curve 2 Tangent 3 Curve 1 Tangent 2 Tangent 1 Curve 2*
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13 TABLE 2. Crash distribution among interchange ramp components Interchange Ramp Component Rural Urban Entrance Exit Ramp Diamond Diamond Diamond Parclo FreeFlow Loop Outer Connection Direct & SemiDirect Total Crashes (All Severities)
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14 Interchange Ramp Proper This subsection describes the findings from a review of the safety literature related to the interchange ramp proper. The ramp proper is defined to be the portion of the ramp between the freeway speed-change lane and the crossroad ramp terminal.
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15 More recently, Khorashadi (1998) examined crash data for nine ramp configurations found in California.
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16 outer connection ramps with a left-hand entrance or exit. The reported data were re-analyzed for this report.
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17 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3,000 5,000 7,000 9,000 11,000 13,000 15,000 Average Daily Traffic Demand, veh/day C ra sh F re qu en cy , c ra sh es /y r Parclo Loop Outer Free-Flow Loop Diamond Urban Exit Ramp Direct Conn. 0.0 0.1 0.2 0.3 0.4 0.5 0 500 1,000 1,500 2,000 2,500 3,000 Average Daily Traffic Demand, veh/day C ra sh F re qu en cy , c ra sh es /y r Parclo Loop Outer Connection Diamond and Direct Conn.
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18 The trends shown in Figure 7 suggest that ramp configuration alone may not provide a sufficient basis upon which to develop SPFs. For example, ramp configuration does not distinguish between ramps that do, and do not, have a ramp-to-ramp merge or diverge point along their length.
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19 1.0 1.2 1.4 1.6 1.8 2.0 0 500 1,000 1,500 2,000 2,500 3,000 Ramp Radius, ft C ra sh M od ifi ca tio n Fa ct or . Two-Lane Highway Ramp Figure 8.
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20 0.6 0.8 1.0 1.2 1.4 1.6 10 12 14 16 18 20 22 Lane Width, ft C ra sh M od ifi ca tio n Fa ct or . Two-Lane Highway AADT > 2,000 veh/day Ramp Two-Lane Highway AADT < 400 veh/day Base Width = 15 ft Figure 9.
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21 0.0 0.4 0.8 1.2 1.6 2.0 0 5,000 10,000 15,000 Average Daily Traffic Demand, veh/day C ra sh F re qu en cy , c ra sh es /y r Parclo Loop Diamond Exit Ramp Single-Vehicle Multiple-Vehicle Figure 10. Relationship between ramp crash frequency and crash type based on total crashes.
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22 AASHTO Speed-Change Lane Length AASHTO Acceleration Length AASHTO Deceleration Length AASHTO Speed-Change Lane Length 12 ft Exit Ramp with Taper Design Entrance Ramp with Parallel Design Ramp Exit Length Ramp Entrance Length *
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23 Sarhan et al.
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24 and crash frequency. The findings reported in these documents are reviewed in the next subsection.
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25 0.8 1.0 1.2 1.4 1.6 1.8 300 500 700 900 Acceleration Length, ft C ra sh M od ifi ca tio n Fa ct or . HSM (Highway, 2010)
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26 0.8 1.0 1.2 1.4 1.6 1.8 200 400 600 800 1,000 Deceleration Length, ft C ra sh M od ifi ca tio n Fa ct or . HSM (Highway, 2010)
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27 reduction in crashes, provided that the lane does not exceed 690 ft. The guidance in the manual is not clear whether this factor applies to the speed-change lane, speed-change lane and ramp proper, or speed-change lane and freeway segment.
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28 This subsection describes the findings from a review of the safety literature related to freeway segments. Topics of discussion include freeway segments that are distant from interchange ramps as well as segments in the vicinity of ramp entrances or exits.
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29 0 10 20 30 40 50 0 20 40 60 80 100 120 Average Daily Traffic Demand (1000s) , veh/day C ra sh R at e, c ra sh es /m i/y r Freeway 1.0-mile segment length 6 Lanes 4 Lanes Safety Improvement a.
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30 0 5 10 15 20 0 20 40 60 80 100 120 140 160 Average Daily Traffic Demand (1000s)
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31 During the development of the SPFs shown in Figures 15e and 15f, Bonneson and Pratt (2008) noted that an examination of crash rates for freeway segments revealed that crash rate tended to increase as the number of lanes increased, similar to the finding by Milton and Mannering (1998)
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32 1.0 1.1 1.2 1.3 1.4 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 Curve Radius, ft C ra sh M od ifi ca tio n Fa ct or . Two-Lane Highway Harwood et al.
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33 0.8 1.0 1.2 1.4 1.6 1.8 10 10.5 11 11.5 12 Lane Width, ft C ra sh M od ifi ca tio n Fa ct or Urban, 4-Lane, Freeway Urban, 6-Lane, Freeway (Hadi et al., 1995) Rural Multilane Divided Highway HSM (Highway, 2010)
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34 0.8 1.0 1.2 1.4 1.6 1.8 9 10 11 12 13 Lane Width, ft C ra sh M od ifi ca tio n Fa ct or Combined AMF (1.5 ft shoulder width) Combined AMF (8 ft shoulder width)
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35 0.9 1.0 1.1 1.2 1.3 4 5 6 7 8 9 10 Inside Shoulder Width, ft C ra sh M od ifi ca tio n Fa ct or . Principal Arterial Highways Milton and Mannering (1998)
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36 The trend lines in Figure 21a apply to total crashes. They show general agreement that narrower medians are associated with more frequent crashes.
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37 1.00 1.02 1.04 1.06 1.08 1.10 10 15 20 25 30 Horizontal Clearance, ft C ra sh M od ifi ca tio n Fa ct or . Barrier or Bridge Rail for 5% of Segment (barrier offset 2 ft from shoulder)
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38 In 2000, Minnesota DOT disabled ramp meters on its freeways at the direction of the State Legislature. A subsequent analysis of total crash data indicated that freeway crash frequency increased 26 percent as a result of the cessation of the ramp meter operation (Cambridge 2001)
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39 0 1 2 3 4 5 6 1,200 1,400 1,600 1,800 2,000 2,200 2,400 Distance from Gore, ft Pe rc en t L an e C ha ng es Upstream of Exit Ramp 1,400 ft Speed-Change Lane Length Downstream of Entrance Ramp Research by Goswami and Bham (2006) was examined to determine the extent of lanechanging activity in the vicinity of an interchange ramp terminal.
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40 0 20 40 60 80 100 120 140 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Distance from Gore, mi C ra sh R at e, c r/1 00 m vm Upstream of Exit Ramp Downstream of Entrance Ramp Urban Freeway Figure 24. Total crash rate as a function of distance from ramp gore.
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41 Figure 25. Relationship between CMF value and distance from gore.
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42 1.0 1.2 1.4 1.6 400 500 600 700 800 900 1,000 Length of Weaving Section, ft C ra sh M od ifi ca tio n Fa ct or Cirillo (1970) Exit Ramp ADT = 1,000 veh/day Entrance Ramp ADT = 1,000 veh/day Bonneson and Pratt (2008)
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43 Spacing Freeway Crossroad 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Spacing, mi C ra sh M od ifi ca tio n Fa ct or . Bared et al.
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44 substantially reduce lane changes, which they speculate may provide "significant gains in the area of safety and driver comfort." Kobelo et al.
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45 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.00 0.05 0.10 0.15 Volume-to-Capacity Ratio C ra sh R at e, c r/m vm Night Day 0 50 100 150 200 250 300 350 0.0 0.2 0.4 0.6 0.8 1.0 Volume-to-Capacity Ratio C ra sh R at e, c r/1 00 m vm Multiple-Vehicle Single-Vehicle Night Day driver alertness may be the reason for the higher rates during late-night hours (as opposed to a low volume-to-capacity ratio)
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46 0.0 0.1 0.2 0.3 0.4 0.5 0 1,000 2,000 3,000 4,000 Traffic Flow Rate, veh/h C ra sh F re qu en cy , c r/m i/y ea r . Single-Vehicle Multiple-Vehicle Rural Freeway 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.2 0.4 0.6 0.8 1.0 Volume-to-Capacity Ratio C ra sh F re qu en cy , c r/m i/y ea r .
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47 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.2 0.4 0.6 0.8 1.0 Volume-to-Capacity Ratio C ra sh M od ifi ca tio n Fa ct or Single-Vehicle, b2 = -5.0 Multiple-Vehicle, b2 = 1.0 Multiple-Vehicle, b2 = 0.4 Single-Vehicle, b2 = -0.5 with, c Vb cv h eCMF 2/ = (8) where, N = estimate of expected crash frequency, cr; bi = regression coefficients, i = 0, 1, 2; L = segment length, mi; Y = time period of crash estimate, yr; Vh = traffic volume, veh/h; CMFv/c = crash modification factor for volume-to-capacity ratio; and c = capacity, veh/h.
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48 HOV Facilities This subsection describes the findings from a review of the safety literature related to HOV facilities on freeways. The focus of the discussion is on HOV facilities that are integral to the freeway cross section.
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49 a. Concurrent flow operation.
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50 The relative safety of the two HOV access types was examined by Newman et al.
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51 0.0 0.2 0.4 0.6 0.8 1.0 0 5 10 15 20 25 Speed Differential, mi/h Fa ta l-a nd -In ju ry C ra sh R at e, cr as he s/ m vm Figure 33. Relationship between FI crash rate and speed differential.
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52 HOV Access Length. The length of an HOV lane entrance or exit has also been thought to have some influence on crash frequency.
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53 different SPFs for each configuration; however, no research has been identified that confirms this speculation. Bared et al.
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54 other two trend lines and is evidence of a likely difference in the safety of ramp terminals, relative to conventional intersections. Number of Legs.
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55 TABLE 6. Elements that may influence the safety of crossroad ramp terminals Category Element Safety Relationship Geometric design Left-turn lane or bay presence Addition of bay correlated with a reduction in crash frequency.
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56 Use of sharp corner radii to discourage wrong-way turns. parclo B (2-quad)
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