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Development of Clear Recovery Area Guidelines (2024)

Chapter: Chapter 8 - Recovery Area Guideline Development

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Page 118
Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Page 122
Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Page 125
Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
×
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Page 128
Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
×
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Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Page 130
Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
×
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Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
×
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Page 132
Suggested Citation:"Chapter 8 - Recovery Area Guideline Development." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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118 C H A P T E R   8 The CZ-GAP risk analysis tool was used to estimate P(K+A) for encroachments occurring on various roadway and roadside design configurations for various clear zone distances and obstacle spacing at the edge of the clear zone. The results obtained from the risk analysis were developed into guidelines for recommended clear recovery distances using the relative risk approach. The basic premise of the relative risk approach is that the risk associated with an unshielded roadside should be less than the risk of shielding that roadway segment with guardrail. The guidelines provide recommended clear recovery distances based on a given set of road- way and roadside characteristics while also considering the obstacle spacing that exists at the clear zone edge. As the spacing of the obstacles at the clear zone edge increases, the risk of impact with a hazard and, hence, the probability of a severe or fatal injury crash, P(K+A), decreases. Thus, the recommended clear zone distance is determined as the distance at which the injury risk associated with impacting a hazard at the clear zone edge drops below the injury risk associated with impacting a guardrail. For wide enough obstacle spacing, the risk associ- ated with guardrail is greater than the risk associated with impacting an obstacle at a given lateral offset. Note that rollover risk was not explicitly included in the guideline development process. Although the CZ-GAP analysis tool was developed with the capability of estimating rollover risk for different design configurations, rollover risk is related to the traversability of a given terrain configuration rather than the clear zone distance. While slope or ditch traversability is another important aspect of roadside design, this issue has recently been addressed under previ- ous research (18, 19). Including it in the clear zone guidelines would be, in some ways, supersed- ing the previous research and would be a fundamental shift in clear zone philosophy that has historically been based on reducing crashes with fixed objects on the roadside. The guidelines were developed in two forms to present different options for a designer. The first form consists of equations derived using statistical regression analysis. The equations allow the user to calculate the minimum recommended clear zone distance for a given set of variables. The second form consists of charts in which multiple variables of interest are displayed. The charts provide the user with the ability to find the minimum recommended clear zone distance as well as understand the magnitude of the relative risk. Guideline Equations Given that the risk of injury is highly sensitive to obstacle spacing, a greater range of spacings from 50 ft to 500 ft was incorporated into the statistical models used to develop the equations for the recommended clear zone distance. Recovery Area Guideline Development

Recovery Area Guideline Development 119   Fixed Effects Estimate StandardError z-Value P-Value Vertical Grade 0.006 131.19 <0.001 Horizontal Curve 0.494 464.77 <0.001 Shoulder Width 0.016 140.63 <0.001 Foreslope 0.011 64.81 <0.001 Foreslope Width 0.023 195.71 <0.001 Bottom Ditch Width 0.023 205.08 <0.001 Backslope 0.103 328.12 <0.001 Backslope Width −0.009 −79.13 <0.001 Lateral Distance −0.026 −1,117.95 <0.001 Hazard Density −0.005 −1,546.74 <0.001 Facility Type 0.127 136.97 <0.001 Intercept −3.977 4.46E-05 1.06E-03 1.13E-04 1.73E-04 1.16E-04 1.13E-04 3.13E-04 1.16E-04 2.32E-05 3.11E -06 9.26E-04 3.09E-03 −1,285.64 <0.001 Model Performance Metrics Multiple R-Squared Adjusted R-Squared F-Statistic Degree of Freedom P-Value 0.9043 0.9043 3.739e+05 435,444 < 0.001 Table 58. Linear regression results for P(K1A) for roadways with high posted speed limit. Fixed Effects Estimate StandardError z-Value P-Value Vertical Grade 0.007 4.48E-05 164.65 <0.001 Horizontal Curve 0.465 1.07E-03 434.72 <0.001 Shoulder Width 0.017 1.13E-04 153.66 <0.001 Foreslope 0.012 1.74E-04 67.47 <0.001 Foreslope Width 0.023 1.16E-04 198.58 <0.001 Bottom Ditch Width 0.024 1.13E-04 208.42 <0.001 Backslope 0.105 3.15E-04 332.87 <0.001 Backslope Width −0.009 1.16E-04 −79.86 <0.001 Lateral Distance −0.038 2.33E-05 −1,635.44 <0.001 Hazard Density −0.005 3.12E-06 −1,538.80 <0.001 Facility Type 0.129 9.31E-04 138.15 <0.001 Intercept −4.542 3.11E-03 −1,460.73 <0.001 Model Performance Metrics Multiple R-Squared 0.9267 Adjusted R-Squared 0.9267 F-Statistic 5.005e+05 Degree of Freedom 435,444 P-Value < 0.001 Table 59. Linear regression results for P(K1A) for roadways with low posted speed limit. Table 58 and Table 59 present the linear regression results for the probability of fatal and injury crashes [P(K+A)] for the high and low categories of the posted speed limit, respectively. The high-speed range includes posted speed limits of 60 mph and above, and the low-speed range represents posted speed limits from 45 mph to 55 mph. Along with the coefficients/estimates for each of the design variables, the tables also present the model performance metrics. It can be observed that the R-squared values for the two models are relatively high. The implication is that the design variables are able to explain the variability in P(K+A) by about 90% and 92% for high- and low-posted-speed-limit ranges, respectively. The research team utilized these statistical models to develop equations that can calculate a recommended clear zone distance for a given roadway and roadside design configuration. The

120 Development of Clear Recovery Area Guidelines clear zone distance is defined as the distance at which the P(K+A) associated with the obstacles at the clear zone edge is equal to the P(K+A) associated with a guardrail at the roadway edge. The team adopted the guardrail P(K+A) values from NCHRP Research Report 972: Development of Safety Performance-Based Guidelines for the Roadside Design Guide (28). Figure 112 shows values of P(K+A) at 65 mph for different longitudinal barriers. The research team selected the P(K+A) for the strong-post W-beam barrier to represent the guardrail risk for guideline development. For roadways within the higher-posted-speed-limit category (i.e., ≥ 60 mph), the team chose to use the guardrail P(K+A) for a 65-mph posted speed limit, which is given as 0.0094. For roadways within the lower-posted-speed-limit category (i.e., 45 mph ≤ PSL ≤ 55 mph, where PSL = posted speed limit), a guardrail P(K+A) associated with a 50-mph posted speed limit was selected. To calculate the guardrail P(K+A) at 50 mph, Eq. 9 from NCHRP Research Report 972 was used (28). The guardrail P(K+A) at 50 mph was calculated as follows: .P K AP K A 65 50 0 0043 65 3 3 50 )+ = + = J L KK` ` N P OOj j (9) The clear zone distance equations were determined by setting the regression equations for P(K+A) equal to the guardrail risk value for the appropriate posted-speed-limit category and solving for the lateral distance. Furthermore, the research team used default values for the vari- ables that had little significance on the prediction of the P(K+A) estimate as determined by the sensitivity and importance analyses presented in Chapter 7. The default values adopted in the clear zone equation development process for these variables are: • Backslope width = 12 ft, • Foreslope ratio = 1V:6H, and • Vertical grade = 0 percent. The team developed two equations for estimating clear zone distance. Each equation applies to a range of posted speed limits to be used. For the higher-posted-speed-limit category (i.e., ≥60 mph), Eq. 10 should be used to deter- mine a recommended clear zone distance (CZD): . . . . . . . . CZ HCurveR ShldW FSW DBtW BS ObS FType 19013 4 1 0 610 0 872 0 889 0 1 23 950 85 4 881 4 894 D ) ) ) ) ) ) ) = + + + + - + + (10) NOTE: subscript 65 = 65mph. Figure 112. Severity probabilities for longitudinal barriers from NCHRP Research Report 972 (28).

Recovery Area Guideline Development 121   For the lower-posted-speed-limit category (i.e., 45 mph ≤ PSL ≤ 55 mph), Eq. 11 should be used to determine the recommended clear zone distance: . . . . . . . . CZ HCurveR ShldW FSW DBtW BS ObS FType 12232 9 1 0 458 0 608 0 621 2 758 0 126 3 384 22 786 D ) ) ) ) ) ) ) = + + + + - + + (11) where: HCurveR = Radius of horizontal curve (ft), ShldW = Shoulder width (ft), FSW = Foreslope width (ft), DBtW = Ditch bottom width (ft) (= 0 for V-ditch), BS = Backslope ratio = horizontal component of slope (e.g., BS = 6 for 1V:6H slope), ObS = Average obstacle spacing at clear zone edge (ft), and FType = Facility type = ;for U1 2 .for D0 4 ( To determine a clear zone distance, the user inputs values for the indicated roadway and roadside design variables using the equation associated with the appropriate speed limit category for the facility. The backslope ratio (BS) is input as the horizontal component of the slope. For example, for a 1V:6H backslope, BS = 6 in the equations. The obstacle spacing (ObS) at the clear zone edge is input as an average obstacle spacing (e.g., tree spacing). The facility type (FType) is a binary variable depending on whether the facility is 2U or 4D. Guideline Charts Several different formats were evaluated for chart-based clear zone guidelines. Despite the challenge of having a large number of variables involved, the researchers managed to develop guidelines in the form of meaningful and legible charts. In addition to providing the minimum required clear zone distance, some of the charts also display the relative risk values for different clear zone distances for each design configuration. Use of these numbers can assist a designer with understanding the change in relative risk if different clear zone distances are contemplated based on engineering, environmental, or cost factors. It was important to generate charts that were practical to use, reasonable in number, and leg- ible within acceptable page margins. To address these challenges, the researchers introduced an innovative chart style that permitted the integration of multiple variables onto a single chart. Sev- eral of the design variables that were shown to have little importance in determining clear zone distance in Chapter 7 were excluded from the charts to assist with meeting the chart objectives. For the excluded variables, default values were used to compute the associated P(K+A) and rela- tive risk for a given design configuration. The variables that were excluded and their associated default values used in the clear zone guideline chart development are • Backslope width = 8 ft, • Foreslope ratio = 1V:6H, • Vertical grade = 0 percent, • Shoulder width = 6 ft, and • Facility type = 2U. The charts are populated with values of relative risk defined as a ratio of P(K+A) associated with impacting obstacles at the clear zone edge over the P(K+A) of a guardrail impact. The P(K+A) values associated with impacting obstacles at the clear zone edge were generated from

122 Development of Clear Recovery Area Guidelines CZ-GAP for each design configuration and respective posted-speed-limit category. As discussed in the previous section, the P(K+A) values for a guardrail impact were adopted from NCHRP Research Report 972 for each posted speed limit category (28). A risk ratio less than or equal to 1 indicates a recommended clear zone distance. A risk ratio greater than 1 indicates a larger clear zone value, or shielding the roadway segment with a guard- rail should be considered. This relationship is presented in Eq. 12. ,Clear Zo e Recommendedn1# Relative Risk Ratio P K A P K A Guadrail Tree= + + = , Shielding Recommended1>` ` j j ( (12) One chart-based guideline format contains three charts with each being associated with a dif- ferent obstacle spacing at the clear zone edge. Figure 113, Figure 114, and Figure 115 show clear zone selection guideline charts for obstacle spacings of less than 150 ft, 150 ft to 300 ft, and greater than 300 ft, respectively. The most significant design variables are located along the left and right sides of these charts. The primary outcome of interest is the recommended clear zone distance, which is located at the bottom of the charts. When a risk ratio is greater than 1, the cell containing the value is shaded, indicating that the clear zone distance is not recommended for that design configuration. The recommended clear zone distance is the leftmost unshaded cell among the group of cells associ- ated with a particular roadway and roadside configuration. Another chart-based guideline that combines all three obstacle spacing ranges into a single chart is presented in Figure 116 in a landscape layout. A similar, alternative chart format without the risk ratio values is shown in Figure 117, also in a landscape layout. Presenting the clear zone guidelines in a single chart provides for a more compact presentation. An advantage of having the risk ratios included in Figure 116 is that the chart can be updated for different guardrail risk values that might become available from future research. This can be accomplished by multiplying the relative risk in the chart by the original guardrail P(K+A) value to return to the original P(K+A) associated with impacting obstacles at the clear zone edge and dividing by the new guardrail P(K+A). The equations can be similarly updated in the future to account for a new guardrail P(K+A), but this requires the use of the regression models presented in this chapter to formulate the new equations. Guideline Examples A set of examples is provided to demonstrate the use of both the clear zone guideline chart and clear zone guideline equations. Example 1 Given: Facility type: 2U Posted speed limit: 50 mph Horizontal curvature: 0 degrees (none) Shoulder width: 2 ft Foreslope width: 10 ft Ditch Bottom width: 0 ft (V-ditch) Backslope ratio: 1V:4H Average obstacle spacing at clear zone edge: 200 ft

Recovery Area Guideline Development 123   Clear Zone (ft) ≤4 2.31 1.63 1.04 0.61 0.34 0.19 0.12 2.03 1.62 1.18 0.79 0.50 0.33 0.23 >4 2.66 1.96 1.32 0.83 0.50 0.29 0.18 2.33 1.94 1.49 1.06 0.72 0.49 0.34 ≤4 2.90 2.16 1.47 0.91 0.53 0.31 0.19 2.55 2.16 1.67 1.18 0.79 0.53 0.37 >4 3.22 2.51 1.80 1.19 0.74 0.45 0.28 2.83 2.50 2.04 1.54 1.10 0.77 0.54 ≤4 3.16 2.43 1.69 1.08 0.64 0.38 0.23 2.78 2.42 1.93 1.41 0.97 0.66 0.47 >4 3.46 2.77 2.04 1.39 0.89 0.55 0.34 3.04 2.76 2.31 1.81 1.33 0.95 0.68 ≤4 2.63 1.92 1.27 0.78 0.45 0.26 0.16 2.31 1.90 1.43 0.99 0.66 0.43 0.30 >4 2.97 2.26 1.58 1.03 0.63 0.38 0.24 2.60 2.24 1.77 1.31 0.92 0.63 0.44 ≤4 3.19 2.46 1.73 1.11 0.67 0.40 0.25 2.80 2.45 1.96 1.44 1.00 0.69 0.49 >4 3.49 2.81 2.08 1.43 0.92 0.57 0.36 3.06 2.79 2.34 1.84 1.36 0.98 0.70 ≤4 3.42 2.72 1.96 1.30 0.81 0.49 0.31 3.01 2.71 2.23 1.69 1.21 0.85 0.61 >4 3.69 3.05 2.32 1.64 1.09 0.69 0.44 3.25 3.03 2.62 2.12 1.61 1.19 0.87 ≤4 2.61 1.92 1.30 0.81 0.48 0.29 0.18 2.29 1.91 1.46 1.04 0.70 0.48 0.33 >4 2.95 2.25 1.59 1.04 0.65 0.40 0.25 2.59 2.23 1.78 1.33 0.95 0.66 0.47 ≤4 3.16 2.46 1.76 1.16 0.72 0.44 0.27 2.77 2.45 1.99 1.50 1.07 0.75 0.54 >4 3.45 2.79 2.08 1.45 0.95 0.60 0.38 3.03 2.77 2.35 1.86 1.40 1.02 0.74 ≤4 3.39 2.71 1.99 1.35 0.87 0.54 0.34 2.98 2.70 2.26 1.76 1.29 0.93 0.67 >4 3.65 3.02 2.32 1.66 1.12 0.73 0.47 3.21 3.01 2.63 2.15 1.66 1.24 0.91 ≤4 2.92 2.22 1.55 1.00 0.62 0.37 0.23 2.56 2.20 1.74 1.28 0.90 0.62 0.44 >4 3.24 2.56 1.86 1.27 0.82 0.51 0.33 2.84 2.53 2.09 1.61 1.18 0.84 0.60 ≤4 3.42 2.75 2.03 1.39 0.90 0.56 0.36 3.00 2.73 2.29 1.79 1.33 0.96 0.69 >4 3.69 3.06 2.36 1.70 1.15 0.75 0.49 3.24 3.04 2.66 2.18 1.69 1.27 0.93 ≤4 3.62 2.98 2.26 1.60 1.06 0.68 0.44 3.18 2.97 2.56 2.06 1.57 1.16 0.86 >4 3.86 3.28 2.60 1.92 1.34 0.90 0.59 3.39 3.25 2.92 2.46 1.97 1.52 1.14 ≤4 2.90 2.21 1.56 1.03 0.64 0.40 0.25 2.54 2.19 1.75 1.31 0.93 0.66 0.47 >4 3.21 2.54 1.87 1.28 0.84 0.53 0.34 2.81 2.52 2.09 1.63 1.21 0.87 0.63 ≤4 3.38 2.73 2.04 1.42 0.93 0.60 0.38 2.97 2.72 2.30 1.83 1.38 1.01 0.74 >4 3.65 3.04 2.36 1.72 1.18 0.78 0.51 3.20 3.02 2.66 2.20 1.73 1.31 0.97 ≤4 3.58 2.96 2.27 1.63 1.10 0.72 0.47 3.15 2.95 2.57 2.10 1.63 1.23 0.91 >4 3.81 3.25 2.59 1.93 1.37 0.93 0.62 3.35 3.22 2.91 2.48 2.01 1.57 1.19 ≤4 3.18 2.51 1.83 1.25 0.81 0.51 0.33 2.79 2.49 2.05 1.59 1.17 0.84 0.61 >4 3.48 2.84 2.15 1.53 1.03 0.67 0.44 3.05 2.80 2.40 1.93 1.48 1.09 0.80 ≤4 3.62 3.00 2.31 1.67 1.14 0.75 0.49 3.18 2.98 2.61 2.14 1.67 1.26 0.94 >4 3.85 3.29 2.64 1.98 1.41 0.96 0.64 3.38 3.26 2.95 2.52 2.04 1.59 1.21 ≤4 3.79 3.21 2.54 1.88 1.32 0.89 0.59 3.32 3.19 2.86 2.42 1.94 1.50 1.14 >4 3.99 3.48 2.85 2.20 1.60 1.12 0.76 3.51 3.44 3.19 2.80 2.33 1.87 1.46 10 20 30 40 50 60 70 10 20 30 40 50 60 70 6:1 <2 ≤122-5 >5 <2 >122-5 >5 >5 4:1 <2 ≤122-5 >5 <2 >122-5 >5 2:1 <2 ≤122-5 >5 <2 >122-5 oitaR epolskcaB (H :V ) mottoB hcti D W id th (ft ) <150 ft Obstacle Spacing at Clear Zone Edge H or iz on ta l Cu rv at ur e (d eg ) Fo re sl op e W id th (ft )Posted Speed Limit Low (45-55 mph) High (≥ 60 mph) Figure 113. Guidelines for less than 150 ft of obstacle spacing at clear zone edge. Clear zone distance, in feet, is the row across the bottom of each chart.

124 Development of Clear Recovery Area Guidelines ≤4 1.16 0.82 0.52 0.31 0.17 0.10 0.06 1.02 0.81 0.59 0.39 0.25 0.16 0.11 >4 1.33 0.98 0.66 0.42 0.25 0.15 0.09 1.17 0.97 0.74 0.53 0.36 0.24 0.17 ≤4 1.45 1.08 0.73 0.45 0.26 0.15 0.09 1.27 1.08 0.83 0.59 0.39 0.26 0.18 >4 1.61 1.26 0.90 0.60 0.37 0.23 0.14 1.42 1.25 1.02 0.77 0.55 0.39 0.27 ≤4 1.58 1.22 0.85 0.54 0.32 0.19 0.12 1.39 1.21 0.96 0.70 0.48 0.33 0.23 >4 1.73 1.39 1.02 0.70 0.45 0.28 0.17 1.52 1.38 1.16 0.90 0.67 0.48 0.34 ≤4 1.32 0.96 0.64 0.39 0.22 0.13 0.08 1.15 0.95 0.72 0.50 0.33 0.22 0.15 >4 1.49 1.13 0.79 0.51 0.32 0.19 0.12 1.30 1.12 0.89 0.65 0.46 0.32 0.22 ≤4 1.59 1.23 0.87 0.56 0.34 0.20 0.12 1.40 1.23 0.98 0.72 0.50 0.34 0.24 >4 1.75 1.40 1.04 0.71 0.46 0.29 0.18 1.53 1.39 1.17 0.92 0.68 0.49 0.35 ≤4 1.71 1.36 0.98 0.65 0.40 0.25 0.15 1.51 1.36 1.12 0.85 0.60 0.42 0.30 >4 1.85 1.52 1.16 0.82 0.54 0.35 0.22 1.62 1.51 1.31 1.06 0.81 0.59 0.43 ≤4 1.31 0.96 0.65 0.40 0.24 0.14 0.09 1.15 0.95 0.73 0.52 0.35 0.24 0.17 >4 1.48 1.13 0.80 0.52 0.33 0.20 0.13 1.29 1.12 0.89 0.67 0.47 0.33 0.23 ≤4 1.58 1.23 0.88 0.58 0.36 0.22 0.14 1.39 1.22 1.00 0.75 0.54 0.38 0.27 >4 1.73 1.40 1.04 0.72 0.48 0.30 0.19 1.52 1.39 1.17 0.93 0.70 0.51 0.37 ≤4 1.70 1.36 0.99 0.68 0.43 0.27 0.17 1.49 1.35 1.13 0.88 0.65 0.46 0.34 >4 1.83 1.51 1.16 0.83 0.56 0.36 0.23 1.61 1.50 1.31 1.07 0.83 0.62 0.46 ≤4 1.46 1.11 0.78 0.50 0.31 0.19 0.12 1.28 1.10 0.87 0.64 0.45 0.31 0.22 >4 1.62 1.28 0.93 0.63 0.41 0.26 0.16 1.42 1.27 1.04 0.80 0.59 0.42 0.30 ≤4 1.71 1.37 1.01 0.70 0.45 0.28 0.18 1.50 1.37 1.15 0.90 0.66 0.48 0.35 >4 1.85 1.53 1.18 0.85 0.58 0.38 0.24 1.62 1.52 1.33 1.09 0.84 0.63 0.47 ≤4 1.81 1.49 1.13 0.80 0.53 0.34 0.22 1.59 1.48 1.28 1.03 0.79 0.58 0.43 >4 1.93 1.64 1.30 0.96 0.67 0.45 0.29 1.70 1.63 1.46 1.23 0.99 0.76 0.57 ≤4 1.45 1.11 0.78 0.51 0.32 0.20 0.13 1.27 1.10 0.88 0.65 0.47 0.33 0.23 >4 1.61 1.27 0.93 0.64 0.42 0.27 0.17 1.41 1.26 1.05 0.81 0.60 0.44 0.31 ≤4 1.69 1.37 1.02 0.71 0.47 0.30 0.19 1.49 1.36 1.15 0.92 0.69 0.51 0.37 >4 1.83 1.52 1.18 0.86 0.59 0.39 0.25 1.60 1.51 1.33 1.10 0.86 0.65 0.49 ≤4 1.79 1.48 1.14 0.81 0.55 0.36 0.23 1.57 1.47 1.29 1.05 0.82 0.61 0.46 >4 1.91 1.63 1.29 0.97 0.68 0.46 0.31 1.68 1.61 1.46 1.24 1.01 0.78 0.59 ≤4 1.59 1.26 0.92 0.62 0.40 0.26 0.16 1.39 1.24 1.03 0.79 0.58 0.42 0.30 >4 1.74 1.42 1.08 0.76 0.52 0.34 0.22 1.52 1.40 1.20 0.97 0.74 0.55 0.40 ≤4 1.81 1.50 1.16 0.83 0.57 0.37 0.24 1.59 1.49 1.30 1.07 0.83 0.63 0.47 >4 1.93 1.65 1.32 0.99 0.70 0.48 0.32 1.69 1.63 1.48 1.26 1.02 0.80 0.61 ≤4 1.89 1.61 1.27 0.94 0.66 0.44 0.29 1.66 1.60 1.43 1.21 0.97 0.75 0.57 >4 2.00 1.74 1.42 1.10 0.80 0.56 0.38 1.75 1.72 1.60 1.40 1.17 0.94 0.73 10 20 30 40 50 60 70 10 20 30 40 50 60 70 Clear Zone (ft) 6:1 <2 ≤122-5 >5 <2 >122-5 >5 >5 4:1 <2 ≤122-5 >5 <2 >122-5 >5 2:1 <2 ≤122-5 >5 <2 >122-5 oitaR epolskcaB (H :V ) mottoB hcti D W id th (ft ) 150 - 300 ft Obstacle Spacing at Clear Zone Edge H or iz on ta l Cu rv at ur e (d eg ) Fo re sl op e W id th (ft )Posted Speed Limit Low (45-55 mph) High (≥ 60 mph) Figure 114. Guidelines for 150 ft to 300 ft of obstacle spacing at clear zone edge. Clear zone distance, in feet, is the row across the bottom of each chart.

Recovery Area Guideline Development 125   ≤4 0.81 0.55 0.36 0.21 0.11 0.06 0.04 0.71 0.55 0.40 0.27 0.17 0.11 0.08 >4 0.98 0.65 0.43 0.27 0.16 0.10 0.06 0.86 0.64 0.49 0.35 0.24 0.16 0.11 ≤4 1.11 0.73 0.50 0.31 0.18 0.10 0.06 0.97 0.72 0.56 0.40 0.27 0.18 0.12 >4 1.19 0.88 0.59 0.39 0.24 0.15 0.09 1.05 0.87 0.67 0.50 0.36 0.25 0.18 ≤4 1.19 0.84 0.56 0.35 0.21 0.13 0.08 1.05 0.84 0.63 0.46 0.32 0.22 0.15 >4 1.21 1.00 0.69 0.47 0.30 0.18 0.11 1.06 1.00 0.78 0.61 0.45 0.32 0.22 ≤4 0.96 0.63 0.42 0.25 0.15 0.09 0.05 0.84 0.63 0.47 0.32 0.22 0.15 0.10 >4 1.13 0.76 0.54 0.35 0.21 0.13 0.08 0.99 0.75 0.60 0.44 0.31 0.21 0.14 ≤4 1.19 0.85 0.57 0.36 0.22 0.13 0.08 1.04 0.85 0.65 0.47 0.33 0.22 0.16 >4 1.21 1.01 0.70 0.48 0.31 0.20 0.12 1.07 1.01 0.79 0.62 0.46 0.33 0.24 ≤4 1.21 0.98 0.66 0.44 0.28 0.17 0.10 1.07 0.97 0.75 0.57 0.41 0.29 0.20 >4 1.30 1.11 0.80 0.54 0.36 0.23 0.15 1.14 1.10 0.90 0.70 0.53 0.40 0.29 ≤4 0.93 0.63 0.42 0.26 0.16 0.10 0.06 0.82 0.62 0.47 0.34 0.23 0.16 0.11 >4 1.10 0.76 0.54 0.35 0.22 0.13 0.08 0.96 0.75 0.60 0.45 0.32 0.22 0.16 ≤4 1.20 0.84 0.58 0.38 0.24 0.15 0.09 1.05 0.84 0.66 0.49 0.35 0.25 0.18 >4 1.23 1.00 0.70 0.49 0.32 0.20 0.13 1.08 0.99 0.79 0.63 0.48 0.34 0.24 ≤4 1.23 0.96 0.66 0.46 0.29 0.18 0.11 1.08 0.96 0.76 0.59 0.44 0.31 0.22 >4 1.28 1.11 0.80 0.55 0.37 0.25 0.16 1.12 1.10 0.90 0.71 0.55 0.42 0.31 ≤4 1.08 0.74 0.52 0.34 0.21 0.12 0.08 0.95 0.74 0.58 0.43 0.30 0.21 0.15 >4 1.19 0.88 0.61 0.41 0.27 0.17 0.11 1.04 0.87 0.69 0.53 0.39 0.28 0.20 ≤4 1.23 0.97 0.68 0.47 0.31 0.19 0.12 1.08 0.97 0.77 0.60 0.45 0.32 0.23 >4 1.29 1.11 0.81 0.56 0.38 0.25 0.16 1.13 1.10 0.91 0.72 0.56 0.42 0.31 ≤4 1.26 1.10 0.77 0.52 0.35 0.23 0.15 1.11 1.09 0.87 0.68 0.52 0.39 0.29 >4 1.37 1.14 0.92 0.65 0.45 0.29 0.19 1.20 1.13 1.04 0.83 0.66 0.50 0.38 ≤4 1.04 0.74 0.52 0.35 0.22 0.13 0.08 0.92 0.73 0.59 0.44 0.32 0.22 0.16 >4 1.20 0.87 0.62 0.42 0.28 0.18 0.11 1.05 0.86 0.69 0.53 0.40 0.29 0.21 ≤4 1.24 0.96 0.68 0.48 0.32 0.20 0.13 1.09 0.95 0.77 0.62 0.47 0.33 0.24 >4 1.26 1.11 0.81 0.56 0.39 0.26 0.17 1.11 1.10 0.91 0.72 0.57 0.44 0.33 ≤4 1.25 1.07 0.77 0.53 0.37 0.24 0.16 1.10 1.07 0.87 0.69 0.54 0.42 0.31 >4 1.34 1.14 0.92 0.65 0.45 0.31 0.21 1.18 1.13 1.03 0.84 0.67 0.52 0.40 ≤4 1.19 0.85 0.61 0.41 0.26 0.17 0.11 1.04 0.84 0.69 0.52 0.38 0.27 0.20 >4 1.23 1.00 0.72 0.51 0.35 0.23 0.15 1.08 0.99 0.80 0.65 0.50 0.37 0.27 ≤4 1.25 1.09 0.79 0.55 0.38 0.25 0.17 1.10 1.08 0.89 0.70 0.55 0.42 0.32 >4 1.36 1.14 0.94 0.67 0.47 0.31 0.21 1.19 1.13 1.04 0.85 0.68 0.52 0.40 ≤4 1.33 1.13 0.90 0.63 0.44 0.29 0.20 1.16 1.13 1.01 0.81 0.65 0.49 0.38 >4 1.43 1.20 0.99 0.77 0.54 0.38 0.25 1.26 1.19 1.11 0.98 0.78 0.63 0.48 10 20 30 40 50 60 70 10 20 30 40 50 60 70 oitaR epolskcaB (H :V ) mottoB hcti D W id th (ft ) >300 ft Obstacle Spacing at Clear Zone Edge H or iz on ta l Cu rv at ur e (d eg ) Fo re sl op e W id th (ft )Posted Speed Limit Low (45-55 mph) High (≥ 60 mph) 2:1 <2 ≤122-5 >5 <2 >122-5 >5 4:1 <2 ≤122-5 >5 <2 >122-5 >5 Clear Zone (ft) 6:1 <2 ≤122-5 >5 <2 >122-5 >5 Figure 115. Guidelines for greater than 300 ft of obstacle spacing at clear zone edge. Clear zone distance, in feet, is the row across the bottom of each chart.

≤4 2.31 1.63 1.04 0.61 0.34 0.19 0.12 1.16 0.82 0.52 0.31 0.17 0.10 0.06 0.81 0.55 0.36 0.21 0.11 0.06 0.04 2.03 1.62 1.18 0.79 0.50 0.33 0.23 1.02 0.81 0.59 0.39 0.25 0.16 0.11 0.71 0.55 0.40 0.27 0.17 0.11 0.08 >4 2.66 1.96 1.32 0.83 0.50 0.29 0.18 1.33 0.98 0.66 0.42 0.25 0.15 0.09 0.98 0.65 0.43 0.27 0.16 0.10 0.06 2.33 1.94 1.49 1.06 0.72 0.49 0.34 1.17 0.97 0.74 0.53 0.36 0.24 0.17 0.86 0.64 0.49 0.35 0.24 0.16 0.11 ≤4 2.90 2.16 1.47 0.91 0.53 0.31 0.19 1.45 1.08 0.73 0.45 0.26 0.15 0.09 1.11 0.73 0.50 0.31 0.18 0.10 0.06 2.55 2.16 1.67 1.18 0.79 0.53 0.37 1.27 1.08 0.83 0.59 0.39 0.26 0.18 0.97 0.72 0.56 0.40 0.27 0.18 0.12 >4 3.22 2.51 1.80 1.19 0.74 0.45 0.28 1.61 1.26 0.90 0.60 0.37 0.23 0.14 1.19 0.88 0.59 0.39 0.24 0.15 0.09 2.83 2.50 2.04 1.54 1.10 0.77 0.54 1.42 1.25 1.02 0.77 0.55 0.39 0.27 1.05 0.87 0.67 0.50 0.36 0.25 0.18 ≤4 3.16 2.43 1.69 1.08 0.64 0.38 0.23 1.58 1.22 0.85 0.54 0.32 0.19 0.12 1.19 0.84 0.56 0.35 0.21 0.13 0.08 2.78 2.42 1.93 1.41 0.97 0.66 0.47 1.39 1.21 0.96 0.70 0.48 0.33 0.23 1.05 0.84 0.63 0.46 0.32 0.22 0.15 >4 3.46 2.77 2.04 1.39 0.89 0.55 0.34 1.73 1.39 1.02 0.70 0.45 0.28 0.17 1.21 1.00 0.69 0.47 0.30 0.18 0.11 3.04 2.76 2.31 1.81 1.33 0.95 0.68 1.52 1.38 1.16 0.90 0.67 0.48 0.34 1.06 1.00 0.78 0.61 0.45 0.32 0.22 ≤4 2.63 1.92 1.27 0.78 0.45 0.26 0.16 1.32 0.96 0.64 0.39 0.22 0.13 0.08 0.96 0.63 0.42 0.25 0.15 0.09 0.05 2.31 1.90 1.43 0.99 0.66 0.43 0.30 1.15 0.95 0.72 0.50 0.33 0.22 0.15 0.84 0.63 0.47 0.32 0.22 0.15 0.10 >4 2.97 2.26 1.58 1.03 0.63 0.38 0.24 1.49 1.13 0.79 0.51 0.32 0.19 0.12 1.13 0.76 0.54 0.35 0.21 0.13 0.08 2.60 2.24 1.77 1.31 0.92 0.63 0.44 1.30 1.12 0.89 0.65 0.46 0.32 0.22 0.99 0.75 0.60 0.44 0.31 0.21 0.14 ≤4 3.19 2.46 1.73 1.11 0.67 0.40 0.25 1.59 1.23 0.87 0.56 0.34 0.20 0.12 1.19 0.85 0.57 0.36 0.22 0.13 0.08 2.80 2.45 1.96 1.44 1.00 0.69 0.49 1.40 1.23 0.98 0.72 0.50 0.34 0.24 1.04 0.85 0.65 0.47 0.33 0.22 0.16 >4 3.49 2.81 2.08 1.43 0.92 0.57 0.36 1.75 1.40 1.04 0.71 0.46 0.29 0.18 1.21 1.01 0.70 0.48 0.31 0.20 0.12 3.06 2.79 2.34 1.84 1.36 0.98 0.70 1.53 1.39 1.17 0.92 0.68 0.49 0.35 1.07 1.01 0.79 0.62 0.46 0.33 0.24 ≤4 3.42 2.72 1.96 1.30 0.81 0.49 0.31 1.71 1.36 0.98 0.65 0.40 0.25 0.15 1.21 0.98 0.66 0.44 0.28 0.17 0.10 3.01 2.71 2.23 1.69 1.21 0.85 0.61 1.51 1.36 1.12 0.85 0.60 0.42 0.30 1.07 0.97 0.75 0.57 0.41 0.29 0.20 >4 3.69 3.05 2.32 1.64 1.09 0.69 0.44 1.85 1.52 1.16 0.82 0.54 0.35 0.22 1.30 1.11 0.80 0.54 0.36 0.23 0.15 3.25 3.03 2.62 2.12 1.61 1.19 0.87 1.62 1.51 1.31 1.06 0.81 0.59 0.43 1.14 1.10 0.90 0.70 0.53 0.40 0.29 ≤4 2.61 1.92 1.30 0.81 0.48 0.29 0.18 1.31 0.96 0.65 0.40 0.24 0.14 0.09 0.93 0.63 0.42 0.26 0.16 0.10 0.06 2.29 1.91 1.46 1.04 0.70 0.48 0.33 1.15 0.95 0.73 0.52 0.35 0.24 0.17 0.82 0.62 0.47 0.34 0.23 0.16 0.11 >4 2.95 2.25 1.59 1.04 0.65 0.40 0.25 1.48 1.13 0.80 0.52 0.33 0.20 0.13 1.10 0.76 0.54 0.35 0.22 0.13 0.08 2.59 2.23 1.78 1.33 0.95 0.66 0.47 1.29 1.12 0.89 0.67 0.47 0.33 0.23 0.96 0.75 0.60 0.45 0.32 0.22 0.16 ≤4 3.16 2.46 1.76 1.16 0.72 0.44 0.27 1.58 1.23 0.88 0.58 0.36 0.22 0.14 1.20 0.84 0.58 0.38 0.24 0.15 0.09 2.77 2.45 1.99 1.50 1.07 0.75 0.54 1.39 1.22 1.00 0.75 0.54 0.38 0.27 1.05 0.84 0.66 0.49 0.35 0.25 0.18 >4 3.45 2.79 2.08 1.45 0.95 0.60 0.38 1.73 1.40 1.04 0.72 0.48 0.30 0.19 1.23 1.00 0.70 0.49 0.32 0.20 0.13 3.03 2.77 2.35 1.86 1.40 1.02 0.74 1.52 1.39 1.17 0.93 0.70 0.51 0.37 1.08 0.99 0.79 0.63 0.48 0.34 0.24 ≤4 3.39 2.71 1.99 1.35 0.87 0.54 0.34 1.70 1.36 0.99 0.68 0.43 0.27 0.17 1.23 0.96 0.66 0.46 0.29 0.18 0.11 2.98 2.70 2.26 1.76 1.29 0.93 0.67 1.49 1.35 1.13 0.88 0.65 0.46 0.34 1.08 0.96 0.76 0.59 0.44 0.31 0.22 >4 3.65 3.02 2.32 1.66 1.12 0.73 0.47 1.83 1.51 1.16 0.83 0.56 0.36 0.23 1.28 1.11 0.80 0.55 0.37 0.25 0.16 3.21 3.01 2.63 2.15 1.66 1.24 0.91 1.61 1.50 1.31 1.07 0.83 0.62 0.46 1.12 1.10 0.90 0.71 0.55 0.42 0.31 ≤4 2.92 2.22 1.55 1.00 0.62 0.37 0.23 1.46 1.11 0.78 0.50 0.31 0.19 0.12 1.08 0.74 0.52 0.34 0.21 0.12 0.08 2.56 2.20 1.74 1.28 0.90 0.62 0.44 1.28 1.10 0.87 0.64 0.45 0.31 0.22 0.95 0.74 0.58 0.43 0.30 0.21 0.15 >4 3.24 2.56 1.86 1.27 0.82 0.51 0.33 1.62 1.28 0.93 0.63 0.41 0.26 0.16 1.19 0.88 0.61 0.41 0.27 0.17 0.11 2.84 2.53 2.09 1.61 1.18 0.84 0.60 1.42 1.27 1.04 0.80 0.59 0.42 0.30 1.04 0.87 0.69 0.53 0.39 0.28 0.20 ≤4 3.42 2.75 2.03 1.39 0.90 0.56 0.36 1.71 1.37 1.01 0.70 0.45 0.28 0.18 1.23 0.97 0.68 0.47 0.31 0.19 0.12 3.00 2.73 2.29 1.79 1.33 0.96 0.69 1.50 1.37 1.15 0.90 0.66 0.48 0.35 1.08 0.97 0.77 0.60 0.45 0.32 0.23 >4 3.69 3.06 2.36 1.70 1.15 0.75 0.49 1.85 1.53 1.18 0.85 0.58 0.38 0.24 1.29 1.11 0.81 0.56 0.38 0.25 0.16 3.24 3.04 2.66 2.18 1.69 1.27 0.93 1.62 1.52 1.33 1.09 0.84 0.63 0.47 1.13 1.10 0.91 0.72 0.56 0.42 0.31 ≤4 3.62 2.98 2.26 1.60 1.06 0.68 0.44 1.81 1.49 1.13 0.80 0.53 0.34 0.22 1.26 1.10 0.77 0.52 0.35 0.23 0.15 3.18 2.97 2.56 2.06 1.57 1.16 0.86 1.59 1.48 1.28 1.03 0.79 0.58 0.43 1.11 1.09 0.87 0.68 0.52 0.39 0.29 >4 3.86 3.28 2.60 1.92 1.34 0.90 0.59 1.93 1.64 1.30 0.96 0.67 0.45 0.29 1.37 1.14 0.92 0.65 0.45 0.29 0.19 3.39 3.25 2.92 2.46 1.97 1.52 1.14 1.70 1.63 1.46 1.23 0.99 0.76 0.57 1.20 1.13 1.04 0.83 0.66 0.50 0.38 ≤4 2.90 2.21 1.56 1.03 0.64 0.40 0.25 1.45 1.11 0.78 0.51 0.32 0.20 0.13 1.04 0.74 0.52 0.35 0.22 0.13 0.08 2.54 2.19 1.75 1.31 0.93 0.66 0.47 1.27 1.10 0.88 0.65 0.47 0.33 0.23 0.92 0.73 0.59 0.44 0.32 0.22 0.16 >4 3.21 2.54 1.87 1.28 0.84 0.53 0.34 1.61 1.27 0.93 0.64 0.42 0.27 0.17 1.20 0.87 0.62 0.42 0.28 0.18 0.11 2.81 2.52 2.09 1.63 1.21 0.87 0.63 1.41 1.26 1.05 0.81 0.60 0.44 0.31 1.05 0.86 0.69 0.53 0.40 0.29 0.21 ≤4 3.38 2.73 2.04 1.42 0.93 0.60 0.38 1.69 1.37 1.02 0.71 0.47 0.30 0.19 1.24 0.96 0.68 0.48 0.32 0.20 0.13 2.97 2.72 2.30 1.83 1.38 1.01 0.74 1.49 1.36 1.15 0.92 0.69 0.51 0.37 1.09 0.95 0.77 0.62 0.47 0.33 0.24 >4 3.65 3.04 2.36 1.72 1.18 0.78 0.51 1.83 1.52 1.18 0.86 0.59 0.39 0.25 1.26 1.11 0.81 0.56 0.39 0.26 0.17 3.20 3.02 2.66 2.20 1.73 1.31 0.97 1.60 1.51 1.33 1.10 0.86 0.65 0.49 1.11 1.10 0.91 0.72 0.57 0.44 0.33 ≤4 3.58 2.96 2.27 1.63 1.10 0.72 0.47 1.79 1.48 1.14 0.81 0.55 0.36 0.23 1.25 1.07 0.77 0.53 0.37 0.24 0.16 3.15 2.95 2.57 2.10 1.63 1.23 0.91 1.57 1.47 1.29 1.05 0.82 0.61 0.46 1.10 1.07 0.87 0.69 0.54 0.42 0.31 >4 3.81 3.25 2.59 1.93 1.37 0.93 0.62 1.91 1.63 1.29 0.97 0.68 0.46 0.31 1.34 1.14 0.92 0.65 0.45 0.31 0.21 3.35 3.22 2.91 2.48 2.01 1.57 1.19 1.68 1.61 1.46 1.24 1.01 0.78 0.59 1.18 1.13 1.03 0.84 0.67 0.52 0.40 ≤4 3.18 2.51 1.83 1.25 0.81 0.51 0.33 1.59 1.26 0.92 0.62 0.40 0.26 0.16 1.19 0.85 0.61 0.41 0.26 0.17 0.11 2.79 2.49 2.05 1.59 1.17 0.84 0.61 1.39 1.24 1.03 0.79 0.58 0.42 0.30 1.04 0.84 0.69 0.52 0.38 0.27 0.20 >4 3.48 2.84 2.15 1.53 1.03 0.67 0.44 1.74 1.42 1.08 0.76 0.52 0.34 0.22 1.23 1.00 0.72 0.51 0.35 0.23 0.15 3.05 2.80 2.40 1.93 1.48 1.09 0.80 1.52 1.40 1.20 0.97 0.74 0.55 0.40 1.08 0.99 0.80 0.65 0.50 0.37 0.27 ≤4 3.62 3.00 2.31 1.67 1.14 0.75 0.49 1.81 1.50 1.16 0.83 0.57 0.37 0.24 1.25 1.09 0.79 0.55 0.38 0.25 0.17 3.18 2.98 2.61 2.14 1.67 1.26 0.94 1.59 1.49 1.30 1.07 0.83 0.63 0.47 1.10 1.08 0.89 0.70 0.55 0.42 0.32 >4 3.85 3.29 2.64 1.98 1.41 0.96 0.64 1.93 1.65 1.32 0.99 0.70 0.48 0.32 1.36 1.14 0.94 0.67 0.47 0.31 0.21 3.38 3.26 2.95 2.52 2.04 1.59 1.21 1.69 1.63 1.48 1.26 1.02 0.80 0.61 1.19 1.13 1.04 0.85 0.68 0.52 0.40 ≤4 3.79 3.21 2.54 1.88 1.32 0.89 0.59 1.89 1.61 1.27 0.94 0.66 0.44 0.29 1.33 1.13 0.90 0.63 0.44 0.29 0.20 3.32 3.19 2.86 2.42 1.94 1.50 1.14 1.66 1.60 1.43 1.21 0.97 0.75 0.57 1.16 1.13 1.01 0.81 0.65 0.49 0.38 >4 3.99 3.48 2.85 2.20 1.60 1.12 0.76 2.00 1.74 1.42 1.10 0.80 0.56 0.38 1.43 1.20 0.99 0.77 0.54 0.38 0.25 3.51 3.44 3.19 2.80 2.33 1.87 1.46 1.75 1.72 1.60 1.40 1.17 0.94 0.73 1.26 1.19 1.11 0.98 0.78 0.63 0.48 10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 Clear Zone (ft) 6:1 <2 ≤122-5 >5 <2 >122-5 >5 >5 4:1 <2 ≤122-5 >5 <2 >122-5 >5 2:1 <2 ≤122-5 >5 <2 >122-5 H or iz on ta l Cu rv at ur e (d eg ) Fo re sl op e W id th (ft ) Obstacle Spacing at Clear Zone Edge (ft) <150 150-300 >300 <150 150-300 >300 Posted Speed Limit epolskcaB ) V: H( oitaR mottoB hcti D W id th (ft ) Low (45-55 mph) High (≥ 60 mph) Figure 116. Combined chart for ranges of obstacle spacing at clear zone edge.

≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 ≤4 >4 10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 Clear Zone (ft) 6:1 <2 ≤122-5 >5 <2 >122-5 >5 >5 4:1 <2 ≤122-5 >5 <2 >122-5 >5 2:1 <2 ≤122-5 >5 <2 >122-5 H or iz on ta l Cu rv at ur e (d eg ) Fo re sl op e W id th (ft ) Obstacle Spacing at Clear Zone Edge (ft) <150 150-300 >300 <150 150-300 >300 Posted Speed Limit epolskcaB ) V: H( oitaR mottoB hcti D W id th (ft ) Low (45-55 mph) High (≥ 60 mph) Figure 117. Alternative combined chart for ranges of obstacle spacing at clear zone edge without risk ratios.

128 Development of Clear Recovery Area Guidelines Use of Clear Zone Guideline Chart The chart presented in Figure 116 is used for purposes of this example. An illustrated, step- by-step procedure for finding the applicable clear zone distance for the prescribed roadway and roadside configuration is as follows. Step 1 – For the given PSL = 50 mph, observe only the three major columns on the left side of the chart corresponding to Low PSL (45–55 mph) (see Figure 118). Step 2 – For the given average Obstacle Spacing at Clear Zone Edge = 200 ft, select the middle major column in this group corresponding to 150–300 ft (see Figure 119). Step 3 – For the given Backslope Ratio = 1V:4H, select the second (or middle) major row. This now defines a prescribed box or group of numbers (see Figure 119). Step 4 – For the given Foreslope Width = 10 ft, reduce to the top six rows of the identified box of numbers corresponding to ≤ 12 ft (see Figure 120). Step 5 – For the given Horizontal Curvature = 0 degrees, reduce to only the top two rows cor- responding to < 2 degrees (see Figure 120). Step 6 – For the given Ditch Bottom Width = 0 ft, reduce to the top row corresponding to ≤ 4 ft. This row provides the range of relative risk ratios for the given roadway and roadside design configuration, which range from 1.31 on the left to 0.09 on the right (see Figure 121). Step 7 – Select the leftmost unshaded value in the final row, which for this example is 0.96 (see Figure 121). Step 8 – Project down the minor column containing the risk value of 0.96 to obtain the recom- mended Clear Zone distance = 20 ft (see Figure 121). Use of Clear Zone Equations Now consider determining the recommended clear zone distance for the same example using the clear zone guideline equations. For the given lower-posted-speed-limit category (i.e., 45 mph ≤ PSL ≤ 55 mph), Eq. 11 should be used to determine the recommended clear zone distance. The design values to use in the equation are as follows. HCurveR = Radius of horizontal curve = 0 (i.e., no curvature). ShldW = Shoulder width = 2 ft. FSW = Foreslope width = 10 ft. DBtW = Ditch bottom width = 0 (i.e., V-ditch). BS = Backslope ratio = 4 (horizontal component of slope ratio). ObS = Average obstacle spacing at clear zone edge = 200 ft. FType = Facility type = 1 (2U). Entering these values into Eq. 11 yields Eq. 13 as follows: . . . . . . . . CZ 12232 9 0 0 458 2 0 608 10 0 621 0 2 758 4 0 126 200 3 384 1 22 786 19 ft D ) ) ) ) ) ) ) = + + + + - + + = (13) The recommended clear zone distance using the clear zone guideline equation for the example roadway segment is 19 ft, which is similar to the result from the chart. Note that the equations may not always yield the same result as the chart, because the chart is based on ranges of the design variable values and increments of clear zone distance, whereas the equation computes the recommended clear zone value for specific values of the design variables.

Figure 118. Example of Step 1.

130 Development of Clear Recovery Area Guidelines Figure 119. Example of Step 2 and Step 3.

Recovery Area Guideline Development 131   Figure 120. Example of Step 4 and Step 5.

132 Development of Clear Recovery Area Guidelines Figure 121. Example of Step 6, Step 7, and Step 8.

Next: Chapter 9 - Conclusions, Recommendations, and Suggested Research »
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The clear zone concept for roadside design emerged in the mid-1960s as a single distance for lateral clearance that reduced the likelihood of an errant vehicle striking a roadside obstacle. Subsequent recovery area guidance that evolved over the next two decades provided a variable distance expressed in terms of traffic volume, design speed, sideslope, and other roadway and roadside factors.

NCHRP Research Report 1097: Development of Clear Recovery Area Guidelines, from TRB's National Cooperative Highway Research Program, develops updated guidelines for roadside clear zones expressed in terms of key roadway and roadside design parameters. These updated guidelines can aid designers in better understanding the risk associated with roadside encroachments while recognizing and working within the associated design constraints.

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