Design Guidelines for Mitigating Collisions with Trees and Utility Poles (2022)

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4 As more freeways and Interstate highways were built in the 1960s, the nature of rural highway crashes began to change. The predominant crash type went from multi-vehicle, head-on crashes, or crashes involving trees immediately adjacent to the roadway, to single-vehicle run-off-road crashes with vehicles colliding with man-made objects (e.g., bridge piers, sign supports, culverts, and ditches). In 1967, the American Association of State Highway Officials (AASHO) Traffic Safety Committee (currently the AASHTO Committee on Safety) issued a report entitled Highway Design and Operational Practices Related to Highway Safety (AASHO 1967). This document became known as the Yellow Book, and its principles were widely used in the design of highway projects, particularly high-speed, controlled-access facilities. The Yellow Book encouraged the use of 30-ft roadside clear zones, since a clear roadside width of 30 ft or more permits about 80% of errant vehicles leaving the road to recover. Most highway agencies began to try to provide a 30-ft clear zone, especially on high-volume, high-speed, rural roadways. The RDG defines a clear zone as “the unobstructed, traversable area provided beyond the edge of the through-traveled way for the recovery of errant vehicles” (AASHTO 2011). The clear zone includes shoulders, bike lanes, and auxiliary lanes, except those auxiliary lanes (such as passing lanes) that function like through lanes. Obstacles located within this clear-zone distance are typically removed, relocated, redesigned, or shielded by traffic barriers or crash cushions. The RDG states that it soon became evident that in some situations, in which the embank- ment sloped significantly downward, a vehicle could encroach farther than 30 ft from the through-traveled way and a 30-ft clear zone might not be adequate. In contrast, on most low- volume, urban, or low-speed facilities, a 30-ft clear-zone distance was considered excessive and was difficult to justify for engineering, environmental, or economic reasons. As a result, the 1977 AASHTO Guide for Selecting, Locating, and Designing Traffic Barriers introduced variable clear-zone distances based on traffic volumes, speeds, and roadside geometry (AASHTO 1977). 2.1 Design Criteria for Clear Zone Distance Table 1 (based on RDG Table 3-1) presents suggested clear-zone distances for selected traffic volumes and speeds. The clear-zone distances provided in Table 1 are general approximations of what is needed for each traffic volume and speed combination; they are based on limited empir- ical data that were extrapolated to provide information for a wide range of speeds. Site-specific conditions (e.g., design speed, area type, practicality) should be considered when selecting an appropriate clear-zone distance. It may not be practical to apply even the minimum values, shown in Table 1, for roadways with low traffic volumes. RDG Chapter 12 presents additional C H A P T E R   2 Design Criteria for Roadside Clear Zones

Design Criteria for Roadside Clear Zones 5   considerations for low-volume roadways, and RDG Chapter 10 provides additional guidance for urban applications. 2.2 Adjustment Factors for Clear Zones on Horizontal Curves Clear-zone distances from Table 1 may be modified, as appropriate, with adjustment factors to account for horizontal curvature, as shown in Table 2 (based on RDG Table 3-2). Generally, such modifications are considered only when crash histories indicate such modifications are necessary, when a site investigation shows a crash potential that could be significantly reduced by increasing the clear zone width, and when such increases are cost-effective. Horizontal curves, particularly those on high-speed facilities, are usually superelevated for greater safety and com- fort. Increasing the banking of curves with inadequate superelevation is an alternate method of Design Speedd (mph) Design ADT Vehicles Per Day (vpd) Foreslopes Backslopes 1V:6H or Flatter 1V:5H to 1V:4H 1V:3H 1V:3H 1V:5H to 1V:4H 1V:6H or Flatter ≤ 40 Under 750c 7–10 7–10 b 7–10 7–10 7–10 750–1500 10–12 12–14 b 10–12 10–12 10–12 1500–6000 12–14 14–16 b 12–14 12–14 12–14 Over 6000 14–16 16–18 b 14–16 14–16 14–16 45–50 Under 750c 10–12 12–14 b 8–10 8–10 10–12 750–1500 14–16 16–20 b 10–12 12–14 14–16 1500–6000 16–18 20–26 b 12–14 14–16 16–18 Over 6000 20–22 24–28 b 14–16 18–20 20–22 55 Under 750c 12–14 14–18 b 8–10 10–12 10–12 750–1500 16–18 20–24 b 10–12 14–16 16–18 1500–6000 20–22 24–30 b 14–16 16–18 20–22 Over 6000 22–24 26–32a b 16–18 20–22 22–24 60 Under 750c 16–18 20–24 b 10–12 12–14 14–16 750–1500 20–24 26–32a b 12–14 16–18 20–22 1500–6000 26–30 32–40a b 14–18 18–22 24–26 Over 6000 30–32a 36–44a b 20–22 24–26 26–28 65–70 Under 750c 18–20 20–26 b 10–12 14–16 14–16 750–1500 24–26 28–36a b 12–16 18–20 20–22 1500–6000 28–32a 34–42a b 16–20 22–24 26–28 Over 6000 30–34a 38–46a b 22–24 26–30 28–30 aWhen a site-specific investigation indicates a high probability of continuing crashes or when such occurrences are indicated by crash history, the designer may provide clear-zone distances greater than the clear zone shown in this table. Clear zones may be limited to 30 ft for practicality and to provide a consistent roadway template if previous experience with similar projects or designs indicates satisfactory performance. bBecause recovery is less likely on the unshielded, traversable 1V:3H fill slope, fixed objects should not be present in the vicinity of the toe of these slopes. Recovery of high-speed vehicles that encroach beyond the edge of the shoulder may be expected to occur beyond the toe of slope. Determination of the width of the recovery area at the toe of slope should consider right-of-way availability, environmental concerns, economic factors, safety needs, and crash histories. Also, the distance between the edge of the through-traveled lane and the 1V:3H fill slope should influence the recovery area provided at the toe of slope. While the application may be limited by several factors, the foreslope parameters that may enter into determining a maximum desirable recovery area are illustrated in RDG Figure 3-2. A 10-ft recovery area at the toe of slope should be provided for all traversable, nonrecoverable fill slopes. cFor roadways with low volumes it may not be practical to apply even the minimum values found in this table. Refer to RDG Chapter 12 for additional considerations for low-volume roadways and to RDG Chapter 10 for additional guidance for urban applications. dWhen design speeds are greater than the values provided, the designer may provide clear zone distances greater than those shown in this table. Table 1. Suggested clear zone distances in feet from the edge of through-traveled lane (adapted from AASHTO 2011).

6 Design Guidelines for Mitigating Collisions with Trees and Utility Poles reducing roadside crashes within a horizontal curve, except where snow and ice conditions limit the use of increased superelevation (AASHTO 2011). For roadsides that are relatively flat and level, the clear-zone concept is simple to apply. How- ever, when the roadway is in a fill or cut section where roadside slopes may be positive, negative, or variable, or where a drainage channel exists near the through-traveled way, applying the clear-zone concept becomes more of a challenge. Radius (ft) Design Speed (mph) 40 45 50 55 65 70 2,950 1.1 1.1 1.1 1.2 1.2 1.2 2,300 1.1 1.1 1.2 1.2 1.2 1.3 1,970 1.1 1.2 1.2 1.2 1.3 1.4 1,640 1.1 1.2 1.2 1.3 1.3 1.4 1,475 1.2 1.2 1.3 1.3 1.4 1.5 1,315 1.2 1.2 1.3 1.3 1.4 -- 1,150 1.2 1.2 1.3 1.4 1.5 -- 985 1.2 1.3 1.4 1.5 1.5 -- 820 1.3 1.3 1.4 1.5 -- -- 660 1.3 1.4 1.5 -- -- -- 495 1.4 1.5 -- -- -- -- 330 1.5 -- -- -- -- -- NOTE: The adjustment factor in this table is multiplied by the applicable clear zone distance in Table 1. This clear zone adjustment factor is applied to the outside of the curve only. Adjustments are typically made only to curves with less than a 2,950-ft radius. Dashes indicate “not applicable”. Table 2. Horizontal curve adjustment factor for clear zone width (adapted from AASHTO 2011).