Guidelines for Cost-Effective Safety Treatments of Roadside Ditches (2021) / Chapter Skim
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3 CHAPTER 1. INTRODUCTION AND BACKGROUND INTRODUCTION Run-off-road (ROR)
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4 1970s. Additionally, variation exists in the practices across the states for designing and maintaining ditches, and for many miles of roads, the ditches are a remnant of much older design standards.
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5 Figure 1.2. Percentage of fatal crashes involving vehicles striking ditches as FHE, from 1994 to 2008.
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6 Table 1.1. Distributions of crashes by MHE: for fatal crashes involving vehicles striking ditches as FHE in 2008 (6)
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7 lane roads. Vehicle trajectories in slope rollovers were both different from and more complex than fixed-object crashes.
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8 the speed and angle at which the vehicle leaves the roadway; the single and/or combined effects of the side- and backslope steepness; the shape of ditch contour forming the transition from side to backslope; and other related geometric factors. Dynamic response is further affected by vehicle properties such as body dimensions, weight distribution through the suspension system, and attitude of the vehicle prior to leaving the roadway.
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9 in a fill section intersects level ground)
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10 Figure 1.3. Recommendations for design of roadside slope combinations.
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11 Figure 1.4. Recommendations for design of roadside slope combinations to permit vehicle traversal at speeds up to 60 mph (96 km/h)
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12 The AASHTO Roadside Design Guide (RDG) recommends that in addition to providing drainage functions, ditch channels should be proportioned so that they are traversable (4)
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13 Abrupt Slope Changes Chart is applicable to all V-ditches, rounded channels with a bottom width less than 8 ft (2.4 m) , and trapezoidal channels with bottom widths less than 4 ft (1.2 m)
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14 In 2002, Thomson et al.
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15 databases. Foreslope ratios ranging from 3:1 up to 4:1 are considered nonrecoverable but traversable slopes in the RDG.
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16 should be implemented. These site-specific guidelines were then generalized into route-specific guidelines for guardrail performance levels for five different highway functional classes as a function of traffic volume.
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17 To investigate the influence of vehicle type on encroachment stability, the data were segregated into two broad vehicle classifications: passenger cars and light trucks. While a significant increase in rollover percentage occurs for passenger cars between sideslopes of 4:1 and 3:1, the corresponding increase in rollover percentage for light trucks is dramatic.
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18 was observed in the vehicle as it traversed a ditch. The steering input applied while the vehicle was airborne greatly affected vehicle response once the vehicle landed.
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19 The driveway configurations studied included various combinations of foreslopes and driveway slopes of 10:1, 8:1, and 6:1. Simulations of vehicular encroachments across these roadside features were made using the HVOSM program, which was first calibrated using the results of full-scale crash tests with small vehicles.
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20 In 1993, Ross and Bligh conducted a research study for the Minnesota Department of Transportation to investigate the benefits of slope rounding the hinges at the intersections of shoulders and sideslopes (20)
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21 Simulation results indicated that the F-shape concrete barrier had a reasonable probability of acceptable impact performance when placed on slopes as steep as 6H:1V. However, since the finite element pickup truck model used in the simulation analyses had not been thoroughly validated for encroachments across median slopes and ditches, it was recommended that one or more full-scale crash tests be conducted to verify simulation results.
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22 Figure 1.6. Trajectories for two points -- front, right bumper corner and hood corner -- for a pickup truck traversing a 4H:1V slope (22)
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23 DC1 = annualized direct cost of Alternative 1, SC2 = annualized societal cost of Alternative 2, and DC2 = annualized direct cost of Alternative 2. Note that Alternative 2 is normally considered an improvement (i.e., provides a greater level of safety, relative to Alternative 1)
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24 For roadways with a low roadside hazard rating, it is not cost beneficial to provide additional clear-zone width beyond the existing baseline clear-zone width. For purposes of establishing a general clear-zone policy, the researchers recommended clear-zone distances for different AADT ranges, as shown in Table 1.3.
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25 compliance with current safety standards should be replaced by a system that meets safety standards, and (b) selecting a cost-effective sign support for new installations.
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26 The baseline variables are control variables from which a basic set of lateral extent of encroachment relationships can be obtained. The adjustment variables are additional variables that are evaluated independently, with additional simulation runs to provide information regarding the effects of specific roadway or roadside factors on lateral extent of encroachment or vehicle stability.
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27 each encroachment parameter. The probability that a vehicle encroaching onto the roadside will have a lateral extent of movement within a specified range is simply the sum of the probabilities of the simulated encroachments that have a maximum extent of lateral movement within that range.
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28 Roadside channels capture sheet flow from the highway pavement and backslope and convey that runoff to larger channels or culverts within the drainage system. This initial concentration of runoff may create hydraulic conditions that are erosive to the soil that forms the channel boundary.
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29 and has deeper, slower velocity flow. In a small channel, subcritical flow can be observed when a shallow wave moves in both the upstream and downstream direction.
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30 Figure 1.7. Typical open-channel ditch configurations.
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31 2 CASES No. 1 If d ≤ 1/Z, then: No.
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32 in a drowning hazard for the driver or passengers. Embankment slopes and culvert ends that are well maintained minimize the potential to trip a vehicle that runs off the road and passes over the culvert end.
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33 Figure 1.8. Rigid concrete channel lining.
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34 Figure 1.10. Cobble channel lining.
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35 Figure 1.12. Erosion control blanket.
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