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3 CHAPTER 1. INTRODUCTION AND BACKGROUND INTRODUCTION Run-off-road (ROR) traffic crashes account for almost one-third of the deaths and serious injuries each year on U.S. highways. There were 41,259 people killed in motor vehicle crashes in 2007, out of which 15,506 people (>37%) were killed in single-vehicle, off-the-roadway crashes (1). In addition, collisions with fixed objects and non-collisions (e.g., rollovers, which mainly occurred off-road) accounted for about 19% of all crashes, but they were responsible for 46% of fatal crashes. ROR crashes occurred for a variety of reasons, including: Driver inattention. Excessive speed. Driving under the influence of alcohol or drugs. Collision avoidance. Roadway condition (ice, snow, rain). Vehicle component failure. Poor visibility. Inattentive driving, including distracted driving, drowsy driving, or fatigued driving, has been identified as a significant causal factor in crashes of all types (2). While inattentive driving is not always identifiable during crash investigations, such behavior is considered by many to be prevalent among a large number of drivers involved in crashes. Because of the recent trend in talking on a cell phone while driving and texting while driving, it has been suggested that inattentive driving is as serious a problem as impaired driving under the influence of alcohol and drugs (3). Inattentive driving and impaired driving have been and will continue to be responsible for a significant number of inadvertent roadside encroachments and thus ROR crashes. The Fatality Analysis Reporting System (FARS) indicates that over the past 15 years more than 1,000 fatalities annually are attributed to ditch-related crashes. Roadside ditches, built as part of the drainage system, are open-flow areas generally paralleling the highway embankment within the right-of-way (ROW). They are integral parts of the highways and are critical to collecting storm water runoff that drains from the highway and conveying it to the outlets. In addition to channels, elements of the system include curbs, cross- drainage (transverse) structures (pipes and culverts), parallel drainage structures, and drop inlets. The drainage system should be designed, constructed, and maintained based on consideration of both hydraulic function and roadside safety. The following three design options, listed in order of preference, are applicable when considering roadside safety (4): Eliminate nonessential drainage structures. Design or modify drainage structures so they are traversable or present minimal hazard to errant vehicles. If relocation or redesign of drainage structures is impractical, shield with a traffic barrier if in a vulnerable location. The American Association of State Highway Transportation Officials (AASHTO) Roadside Design Guide provides some guidance on preferred configurations for ditches (4). However, this guidance is based on the results of limited testing and simulations conducted in the
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. Limited ROW often dictates the configuration of ditches, and in many cases, the preferred configurations are not practical. Enclosed drainage systems are expensive and may result in additional requirements for treatment and discharge of the runoff. Other drainage elements such as culvert ends, inlets, headwalls, and holding basins may themselves become roadside obstacles. Installing a barrier to shield a ditch reduces the available clear zone, may not always be cost effective, and presents maintenance and operational issues. The need exists to reduce the number and severity of crashes involving roadside ditches. By identifying factors involved in crash events and evaluating the dynamics of vehicles interacting with ditch elements, countermeasures can be developed and implemented to mitigate these crashes. Roadside ditches can be hazardous to errant motorists who leave the roadway. Figure 1.1 shows the number of fatal crashes and fatalities involving vehicles entering ditches as the first harmful event (FHE) for the period from 1994 to 2008. Note that FHE is defined as the first property-damaging or injury-producing event in the crash. These numbers are taken from FARS of the National Highway Traffic Safety Administration (NHTSA). The numbers generally trended up from 1994 to 2006, with slight decreases in 2007 and 2008. There were 761 fatal crashes involving 1,324 fatalities in 1994 and 1,198 fatal crashes involving 1,850 fatalities in 2008. As a fraction of the total number of fatal crashes in the United States, Figure 1.2 indicates that crashes involving vehicles striking ditches as the FHE increased in general during the same period, from about 2.1% in 1994 and 3.5% in 2008. Figure 1.1. Number of fatal crashes and fatalities involving vehicles striking ditches as FHE, from 1994 to 2008. FARS Database: Striking a Ditch as the First Harmful Event Number of Fatal Crashes and Fatalities 0 500 1,000 1,500 2,000 2,500 1993 1995 1997 1999 2001 2003 2005 2007 2009 Calendar Year N um be r of C ra sh es o r Fa ta lit ie s Crashes Fatalities
5 Figure 1.2. Percentage of fatal crashes involving vehicles striking ditches as FHE, from 1994 to 2008. The most harmful event (MHE) for occupants involved in these fatal crashes after striking a ditch as the FHE is presented in Table 1.1 for 2008. By definition, MHE is the single impact that causes the greatest trauma and damage in each crash. Out of the 1,198 fatal crashes that involved ditches as the FHE, 607, 215, 193, and 56 crashes involved vehicle rollover, ditch itself, striking standing trees, and striking utility poles, respectively, as the MHE in the crash sequence. Therefore, vehicle rollover was the MHE of 51% of the crashes, followed by the ditch itself as the MHE for about 18%, and then followed by striking standing trees and utility poles (16% and 4.6%, respectively). Table 1.1 illustrates that on average rollover crashes experience the highest fatalities per crash (1.65 fatalities/crash). Thus, vehicle rollover is by far the deadliest event when an errant vehicle strikes a ditch as the FHE. Note that crashes hitting culverts constitute a small percentage of the MHEs (1.75%), which is consistent with the finding of an earlier study by Viner (5). When striking ditches is the FHE, the most consequent events are rollover and ditch itself, totaling about 69%, which provides a clear indication as to what this projectâs focus is. Vehicles striking standing trees and utility poles as the MHE within or adjacent to ditches constitute another 20.6% of the crashes. These fixed objects should be removed or relocated as practical. FARS Database: Percentage of Fatal Crashes Involving Vehicles Striking Ditches as the First Harmful Event 0.0 1.0 2.0 3.0 4.0 1993 1995 1997 1999 2001 2003 2005 2007 2009 Calendar Year Pe rc en t
6 Table 1.1. Distributions of crashes by MHE: for fatal crashes involving vehicles striking ditches as FHE in 2008 (6). MHE Number of Crashes Number of Fatalities Fatalities per Crash Percentage of Crashes Rollover 607 999 1.65 50.67 Ditch 215 274 1.27 17.95 Tree (standing tree only) 193 299 1.55 16.11 Utility Pole 56 81 1.45 4.67 Culvert 21 28 1.33 1.75 Fire/Explosion 20 32 1.60 1.67 EmbankmentâMaterial Type Unknown 10 13 1.30 0.83 Fell from Vehicle 8 8 1.00 0.67 Other Fixed Object 7 11 1.57 0.58 Other Post/Pole/Support 7 8 1.14 0.58 Immersion 7 9 1.29 0.58 EmbankmentâEarth 5 7 1.40 0.42 Fence 5 7 1.40 0.42 EmbankmentâStone/Rock/Concrete 5 5 1.00 0.42 All Others Combined 32 69 2.16 2.67 Total 1,198 1,850 1.54 100.00 LITERATURE REVIEW Texas A&M Transportation Institute (TTI) researchers conducted a literature search of domestic and international literature related to the safety performance of roadside slopes and ditch designs. The literature was searched for in the National Technical Information Service, Transportation Research Information Service, and Transport databases. The searches were made using the following keywords: ditch, slope, sideslope, embankment, roadside medians, foreslope, backslope, V-ditch, trapezoidal ditch, slope rounding, and roadside. Search results yielded approximately 580 results, some of which were repeated in different databases. These references were then purged based on titles and abstracts to find the ones related to this research. The purged list was reviewed in detail and the most relevant literature was used to prepare the following review. Slope Rollovers and Ditch Crashes In 1995, Viner analyzed national data to define the nature and importance of the problem of ROR vehicles that rolled over on slopes and ditches (5). Data from two specialized databases were also used to obtain insight on vehicle orientation and driver maneuvers in such crashes. This crash type was the leading cause of ROR driver fatalities, accounting for about one-fourth of the total. Highway factors associated with this crash type were identified to assist in the effort to define specific roadway locations where highway design countermeasures may be appropriate. It was determined that attention should be given to the outside of horizontal curves on rural two-
7 lane roads. Vehicle trajectories in slope rollovers were both different from and more complex than fixed-object crashes. Driver fatality counts were developed from 1991 FARS data. Estimates of involved vehicles and A-injuries were developed from 1991 General Estimating System (GES) data. A-injuries are âincapacitatingâ injuries on the KABCO police injury scale. FARS and GES are national crash data bases maintained by NHTSA. FARS is a census of all fatal crashes in the United States. In the FARS data, rollovers on sideslopes and ditches were identified by rollover cases in which the FHE took place outside the shoulder, and both the FHE and MHE were either overturn or ditch or embankment impacts. Since culvert and ditch impacts are combined in one code in GES, overturn data in GES represent rollovers on sideslopes, ditches, and culverts. This inclusion does not appear important. Illinois data indicate that culvert headwall rollovers account for only 0.2% of the total rollovers on sideslopes, ditches, and culverts. Nationwide, rollovers on sideslopes and ditches were identified as the leading cause of ROR fatalities. Fatal rural ROR crashes took place mostly on two-lane roads, representing 82% of slope rollovers and 87% of fixed-object impacts. Pickups were overrepresented in slope- rollover crashes compared to their involvement in fixed-object crashes. There was some indication that utility vehicles and medium/heavy trucks might also be overrepresented. Horizontal curves were seen as a special problem. On two-lane rural roads, about one-third of all crashes and one-half of all fatalities took place on curves for both slope rollovers and fixed- object crash types. In 2004, Turner et al. studied the impact of roadside hazards on rural single-vehicle crashes and investigated the causes for this type of crash in New Zealand (8). The objective of the study was to understand the role of roadside hazards in the occurrence and severity of rural single-vehicle accidents. The researchers collected and examined data from roadside hazards found alongside rural roads in New Zealand to ultimately develop crash prediction models. An analysis of single-vehicle crashes for the 1998â2002 period was performed using the Crash Analysis System. Only crashes where a roadside object had been considered a contributing factor were taken into account. Data were collected for over 850 km (528 mi) of rural roads, including 414 km (257 mi) of state highways. The researchers found that crashes involving roadside objects are often more severe than crashes that do not involve objects. Upright cliffs/banks and ditches were two of the top five objects struck in roadside crashes. Ditches accounted for 2% of fatal, 11% of serious, and 27% of minor injuries. An object-severity score was calculated and defined for each roadside hazard; ditches resulted in an object-severity score of 2.25 (where a maximum score of 3.16 was assigned to poles and a minimum score of 1.43 was assigned to fences). Turner et al. categorized sideslopes (down) as continuous hazards and coded them as follows: D1: Slopes of 6H:1V & 5H:1V just recoverable sideslope (15â20%) D2: Slopes of 4H:1V & 3H:1V unrecoverable sideslope (20â35%) D3: Slopes > 3H:1V vehicle would overturn (>35%) D4: Slopes > 3H:1V with >164-ft (50-m) fall or into water (>35%) Ditch Geometry and Vehicle Dynamics The dynamic response of a vehicle when traversing a roadside terrain feature is dependent on many vehicle and roadway parameters. Of primary importance are such factors as
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. Passenger response is greatly affected by the degree of body restraint existing throughout the maneuver. It is readily apparent that a large number of parameters exert individual influences on vehicle response, and the complexity of the problem is grossly compounded by their interaction. In 1972, Weaver et al. of TTI conducted research to develop a relationship between the sideslope design to highway safety (9). Because of the many variables involved in a study of this scope, the researchers proposed the use of computer simulation to study the problem. Use of a computer simulation model would facilitate investigation of a variety of terrain features within the constraints of time and budget. TTI further proposed that full-scale tests be conducted to validate the model and provide confidence in its ability to analytically investigate the many variables. The panel directed that to reduce project costs of the study, full-scale tests conducted on the earth berm be used to validate the model for the sideslopes as well. Under those guidelines, a TTI modification of the Highway-Vehicle-Object-Simulation Model (HVOSM) (10) was used to determine the dynamic forces on a vehicle as it traversed various roadside configurations. Certain criteria for tolerable forces were selected based on a literature review. Lateral, longitudinal, and vertical accelerations were determined for traversal (60 mph [96 km/h]) at a 25-degree encroachment angle) of each of four ditch contours and 12 combinations of side- and backslope (48 situations). The four ditch configurations were evaluated with respect to suggested limits of human tolerance to accelerations. Combinations of side- and backslope and ditch contours through which traversal is considered tolerable were recommended. Vehicle accelerations predicted by the model were compared to those obtained in five full-scale tests conducted on the earth berm. The tests were conducted on an extremely steep backslope (l.2:l), and at only one encroachment angle (15 degrees). Unfortunately, the limited number of tests did not adequately verify the HVOSM as desired. Although reasonably close agreement was obtained between peak vertical accelerations predicted by the model and those actually experienced, other factors such as vehicle stability and redirection did not compare adequately. The results of the study are summarized: It was recommended that 3:1 sideslopes be used only when flatter slopes are not feasible. The 4:1 sideslope represented an apparent division point in terms of performance level, which was considered one of the most significant findings of the study. In view of the safety benefits, it was recommended that sideslopes of 4:1 or flatter be used whenever possible. With the exception of the V-ditch, flattening the sideslope from 4:1 to 6:1 did not result in an appreciable reduction of vertical Gs for a particular configuration. Although slope combinations having 6:1 sideslopes generally produced somewhat smaller vertical accelerations, the degree of improvement appeared minor. When used with the trapezoidal ditch configuration, backslopes of 4:1 or flatter steepness were not much, if any, more hazardous than flat backslopes (such as where the sideslope
9 in a fill section intersects level ground). For unequal side and backslopes, higher vertical accelerations were generally observed when the sideslope was steeper than the backslope. It was recommended that, when conditions prohibit the use of equal slopes, the steeper slope should be located on the backslope. The trapezoidal ditch with sideslopes of 4:1 or flatter represented the most favorable choice of the four alternatives. Figure 1.3 and 1.4 present recommendations for the design of roadside slope combinations to permit vehicle traversal at speeds up to 60 mph (96 km/h) and encroachment angles up to 25 degrees. The curves are based on evaluation of full-scale tests on the V-ditches, berm tests , and engineering judgment and experience.
10 Figure 1.3. Recommendations for design of roadside slope combinations.
11 Figure 1.4. Recommendations for design of roadside slope combinations to permit vehicle traversal at speeds up to 60 mph (96 km/h) and encroachment angles up to 25 degrees.
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). The shaded areas in Figure 1.5 show RDG preferred cross-sections for parallel ditches with abrupt and gradual slope changes. As indicated in the RDG, where practical, channel sections outside the shaded areas may be reshaped, converted to a closed system (culvert), or shielded by a barrier. For all channels, roadside hardware (for example, sign supports) should not be located in or near channel bottoms or slopes because vehicles leaving the roadway may be funneled along the channel and impact the obstacle. Breakaway hardware may not function properly if impacted by airborne or sideways-sliding vehicles. Foreslopes parallel to the traffic flow may be categorized as recoverable, nonrecoverable, or critical. Recoverable slopes are 1:4 (vertical to horizontal) or flatter. Fixed obstacles, such as culvert headwalls, should not extend above the embankment in this area. Nonrecoverable slopes, generally between 1:4 and 1:3, are traversable, but most motorists will reach the bottom of the slope and not be able to stop or return to the roadway easily. Fixed obstacles should not be constructed along such slopes, and a clear runout area at the bottom of the slope is desirable. Critical slopes, generally steeper than 1:3, are those on which a vehicle is likely to overturn. A barrier might be warranted in such cases. The RDG discusses alternatives that might be considered on critical parallel foreslopes. Many highway agencies construct so-called barn roof sections in embankment conditions. A relatively flat slope is provided adjacent to the roadway, followed by a steeper slope and a clear runout area at the bottom. This procedure is more economical than a continuous flat slope. In applying the clear-zone concept, the RDG says the following âIf the suggested clear-zone distance (as determined from Table 3-1) exists on the flatter slope, the steeper slope then may be critical or non-traversable. Clear-zone distances for embankments with variable foreslopes ranging from essentially flat to 1V:4H may be averaged to produce a composite clear- zone distance. Slopes that change from a foreslope to a backslope cannot be averaged and should be treated as a drainage channel sections and analyzed for traversability as shown in Figures 3-6 and 3-7 (Figure 1.5 herein). Although a weighted average of the foreslope may be used, it is preferable to use the values in Table 3-1 that are associated with the steeper slope. If one slope is significantly wider, the clear-zone computation based on the slope alone may be used.â Changes in slope and toes of slopes should generally be rounded to keep vehicles in contact with the ground and enhance traversability. On backslopes, traversability depends on relative smoothness and the presence of fixed obstacles. If traversable (1:3 slope or flatter) and obstacle-free, it may be acceptable. Conversely, a steep rough-sided rock cut (one that will cause excessive vehicle snagging) should be shielded unless it is outside the clear zone. Transverse slopes may be created by median crossovers, intersecting side roads, or driveways. These slopes generally create a more serious condition than parallel slopes because they can be struck head on by errant vehicles. To minimize the effect, slopes of 1:10 or flatter are desirable where practical. Steeper slopes may be suitable for low-speed facilities. Drainage pipes should be located as far from the roadway as practical. Also, in places where a vehicle could be led into the culvert inlet or outlet by a drainage channel, consideration should be given to special inlet or outlet treatment, as is subsequently discussed.
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) Gradual Slope Changes Chart is applicable to rounded channels with a bottom width of 8 ft (2.4 m) or more and trapezoidal channels with bottom widths equal to or greater than 4 ft (1.2 m). Figure 1.5. Preferred cross-sections for parallel ditches with abrupt and gradual slope changes in RDG (4).
14 In 2002, Thomson et al. (11) investigated the dynamics of errant vehicles entering a V- shaped ditch in a joint Swedish-Finnish test program. Due to natural landscape characteristics of Sweden and Finland, V-shaped ditches represent a significant part of the roadside environment in these countries. The main objective of the study was to investigate the dynamics of errant vehicles entering a V-shaped ditch. In particular, researchers focused on understanding the influence of the impact angle and speed vehicle on crash outcomes. They also tried to evaluate the effects of driver steering on vehicle motion in the ditch. A total of 16 tests with vehicles entering V-ditches were performed. The vehicles used were 1,984-lb (900-kg) passenger test vehicles (Peugeot, Fiat, Ford, and Talbot). Researchers used a 16-ft (5-m) wide ditch with a 1:3 foreslope and a 1:2 backslope. In one of the tests, a U-shaped ditch bottom was used, and in another test, a low, rectangular concrete barrier (20 inches (508 mm) high) was placed on the backslope about 4.3 ft (1.3 m) from the ditch bottom. Encroachment angles and vehicle speeds ranged from 5 to 20 degrees and 50 to 68 mph (80 to 110 km/h), respectively. As the vehicles entered the ditch, a steering input was applied using mechanical actuation of the steering wheel. Crash test results indicated that many vehicles passed over the backslope of the ditch. While rollovers were observed in four tests, they were not strongly related to the impact speed. Rollovers seemed to be related to the first contact mechanism of the errant vehicle with the backslope of the ditch. Researchers noticed that the vehicle motion on the foreslope depended on the approaching angle: an angle of 20 degrees caused the vehicle to lose contact with the foreslope, even when the speed was lowered to 50 mph (80 km/h). No correlation was found between climb of the vehicle up the backslope and its encroachment speed. Also, no conclusion could be made about the effect of the vehicleâs weight and its tendency to climb up the backslope. Guardrail Need Rollovers are the leading cause of fatalities in single-vehicle run-off-road (SVROR) crashes. The rollover crash problem is a source of common concern among state departments of transportation (DOTs), the Federal Highway Administration (FHWA), and NHTSA. Analysis of 6 years of data from the National Automotive Sampling System (NASS) Crashworthiness Data System (CDS), which is administered by NHTSA, indicates that 31% of SVROR crashes result in a rollover. Approximately 75% of these rollover crashes are initiated by vehicles digging into the ground on embankments or in ditches after encroaching onto the roadside. Higher centers of gravity (CGs) make light trucks (e.g., pickup trucks, sport utility vehicles (SUVs), vans) inherently less stable than passenger cars. Numerous crash data studies have documented that light trucks are overrepresented in rollover crashes. For example, the above-referenced analysis of the NASS CDS indicates the risk of a utility vehicle rolling over in a SVROR crash on a high-speed roadway is 2.2 times that of a passenger car. Sales of light trucks continue to increase each year. As a group, light truck sales currently outpace sales of passenger cars, accounting for over 50% of all new passenger vehicles sold. Thus, it is important to update roadside safety guidelines and practices to accommodate the characteristics of light trucks. Few data are available regarding the percentage of overturns versus total vehicle encroachments for different sideslope ratios, due in part to a lack of roadside data in most crash
15 databases. Foreslope ratios ranging from 3:1 up to 4:1 are considered nonrecoverable but traversable slopes in the RDG. Slopes steeper than 3:1 have historically been considered critical foreslopes. The RDG states that such slopes âwill cause most vehicles to overturn and should be treated [i.e., flattened or shielded with a barrier] if they begin within the clear-zone distance of a particular highway.â This guidance is based largely on studies of the relative severity of encroachments on embankments versus impacts with roadside barriers. These studies were conducted in the late 1960s and early 1970s and included a very limited number of full-scale embankment tests and computer simulations with passenger cars. For example, Ross et al. utilized the HVOSM to investigate the dynamic response of a passenger sedan traversing different embankment slopes at various encroachment speeds and encroachment angles (10). The encroachment conditions were selected to match the design impact conditions for the guardrail. The accelerations obtained from the simulated slope traversals were used to compute a severity index that was compared to the severity of a vehicle impacting a strong-post W-beam guardrail system. An equal-severity curve was developed as the basis for recommended guardrail need criterion. The curve established the less severe of the two alternativesâstriking a guardrail or traversing an unprotected slope. The roadside cross-section investigated consisted of a 10-ft (3-m) wide shoulder with a 20H:1V slope, an adjoining sideslope of varying steepness and height, and a flat-bottomed ditch at the bottom of the slope. Slopes of 2H:1V, 3H:1V, and 6H:1V, with heights of 10, 20, 30, and 50 ft (3, 6, 9, and 15 m), were included in the simulation matrix. In addition, 3.25H:1V and 4H:1V slope were evaluated at a height of 20 ft (6.1 m), for a total of 14 roadside embankment configurations. The researchers presented criteria to help engineers make objective decisions on the need for a guardrail to shield embankments. It was concluded that for encroachment angles less than 17.5 degrees and speeds up to 70 mph (113 km/h), impacting a strong-post guardrail is higher in severity than traversing a 3H:1V embankment with a 20-ft (6.1-m) fill height. However, as the encroachment angles increases, the severity of traversing the embankment approaches the severity of striking a guardrail. It is interesting to note that the researchers observed a considerable reduction in the severity index (SI) when the embankment slope was flattened from 3H:1V to 3.25H:1V and to 4H:1V. They noted, âA sharp transition was therefore found to exist in the SI at a slope of about 3:1 for the 20-ft (6.1 m) embankment heightâ (10). More recently, under NCHRP Project 22-12(2) (12), a BCA procedure was utilized to develop general guidelines for guardrail implementation. The primary goal of this research was to identify the most appropriate guardrail test level based on highway and traffic characteristics. The test levels for longitudinal barriers systems are contained in NCHRP Report 350 and consist of a crash test matrix defined by vehicle type, impact speed, and impact angle. Specific roadway, roadside, and hazard conditions to be analyzed were assigned to a set of hazard scenarios for use in the BCA. Both point objects and long hazards, such as steep slopes and roadside ditches, were considered in the study. As part of the process, the layout, construction costs, and crash severities were defined for the various safety treatment options. The Roadside Safety Analysis Program (RSAP), discussed later, was used to analyze each hazard scenario under a wide variety of roadway and traffic characteristics. These RSAP runs were first used to identify site-specific locations where various guardrail performance levels
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. Typically, when a guardrail is used to protect long hazards, such as steep roadside embankments, the crash frequency is not substantially changed, and the ratio of guardrail impacts to hazard crashes prevented may approach 1.0. The primary benefit of using a guardrail to shield a slope is an overall reduction in impact severity, more specifically, a decrease in the number of vehicles that roll over during slope traversal. In this situation, there is a point at which the reduced severity of guardrail crashes relative to crashes involving the slope hazard makes barrier implementation cost beneficial. Two hazard size classifications and three different hazard severities were selected for inclusion in the study, as shown in Table 1.2. In the initial analysis, guardrail treatment of even the most severe point hazard was found never to be cost beneficial. Further investigation led the researchers to believe that the average SI for guardrail impacts in RSAP was too high. Excessive crash severity estimates for guardrails depicted the use of guardrails as much less cost beneficial by overestimating the number of injuries and fatalities associated with barrier crashes. The average crash severity predictions from RSAP were then calibrated by comparing predicted injury distributions to available guardrail crash data. Table 1.2. Hazard classifications and severity levels. Category Severity Level Severe Moderately Severe Moderate Point Hazard 3-ft (0.9-m) diameter bridge pier 10-inch (254-mm) diameter utility pole 6-inch (152-mm) diameter tree Slope Hazard 1.5:1 slope, 26 ft (7.9 m) deep 2:1 slope, 20 ft (6.1 m) deep 2.5:1 slope, 13 ft (4.0 m) deep In subsequent analyses with the modified severity indices, the roadway and roadside conditions under which guardrail treatment of the selected roadside hazards became cost beneficial were identified. Although that research made a significant contribution to the state-of-the-knowledge in regard to guardrail need, further study appears warranted. A concern among members of the research team was that the RSAP severity indices for slope traversal may require updating just like the severity indices for guardrail impacts required updating. However, in the case of ditch severity, it is likely that the current severity indices for some ditch sections are too low. A recent study suggests that some roadside slope conditions that have for many years been considered traversable for passenger cars may not be traversable for light trucks (13). Under NCHRP Project 17-11, results from a comprehensive computer simulation study were weighted with real-world data derived from reconstructed SVROR crashes and used to compute rollover probability as a function of sideslope ratio. Generally speaking, the percentage of vehicle encroachments resulting in overturn increases as the steepness of the sideslope increases. However, a dramatic increase in overturns and decrease in stable vehicle encroachments was observed between sideslope ratios of 4:1 and 3:1. Whereas the probability of rollover increased an average of 12% between sideslope ratios of 6:1 and 4:1, the average change in rollover probability between sideslope ratios of 4:1 and 3:1 was 92% across all the roadway functional classes studied.
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. Across all functional classes, the increase in rollovers for passenger cars between sideslopes of 4:1 and 3:1 averaged 17%. For light trucks, the increase in encroachments resulting in rollover between sideslopes of 4:1 and 3:1 averaged 112%, in contrast to an average increase in light truck rollovers of only 16% between sideslopes of 10:1 and 4:1. For many years, a 3:1 sideslope has been considered a marginally traversable slope and constitutes the break point for when longitudinal barriers become warranted. Project 17-11 suggests that with the steadily increasing percentage of light trucks in the vehicle fleet, the break point for what is considered a traversable versus critical sideslope ratio may need to be reassessed for todayâs vehicle fleet. Proper assessment of slope traversability and guardrail need will help reduce the number of rollover crashes and associated fatalities. A need existed to develop new guidelines for slope traversability that considered the characteristics of the current vehicle fleet, including light trucks. NCHRP Report 911 âGuidelines for Traversability of Roadside Slopesâ developed these guidelines for determining the traversability of roadside slopes considering the characteristics of the current passenger vehicle fleet. Encroachment Simulation Studies Over a period of many years, researchers have used the HVOSM (10) in a large number of studies sponsored by FHWA, NCHRP, and various state agencies. These studies have involved the safety evaluation of a variety of roadway and roadside conditions, including embankments, ditches, driveways, and culverts. These studies have helped define the state-of- the-knowledge about vehicular behavior (lateral distance and stability) when traversing various roadside features. In 1973, TTI conducted research to investigate vehicle behavior as it traversed a roadside under NCHRP Project 20-7 project (9). The objective of the research was to provide guidance in selecting and designing safe roadside configurations. A custom-made multi-rigid body dynamics code HVOSM was used to evaluate vehicle dynamics across the roadside. Simulations were conducted for speeds from 40 to 80 mph (65 to 130 km/h) and encroachment angles of 7, 15, and 25 degrees. Freewheeling and return-to-the-road type steering inputs were applied to the vehicle. Terrain slopes were varied from 3H:1V to 10H:1V. To account for the driverâs reaction time, steering input was applied 1.5 seconds after the vehicle left the pavement, which was followed by a steering input that fully developed in 2 seconds. The vehicle was then driven for 4.5 seconds or until it returned to a position 10 ft (3 m) outside the edge of the pavement. Thus, the vehicle was allowed to travel for a maximum of 8 seconds after leaving the pavement. Twenty-four full-scale vehicle tests were conducted to validate the results obtained from the HVOSM. Even though the HVOSM predicted slightly higher accelerations than the full-scale tests, it showed a good correlation to test data. In the tests performed with a 25-degree encroachment angle, bumper-to-terrain contact and rear overhang drag were observed in all tests above 40 mph (65 km/h). In the absence of a steering and braking input, very little redirection
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. Sharply turned front wheels induced high side forces on landing and increased wheel digging in the terrain. This increase in the side forces increased the potential of the vehicle to rollover. It was also observed that as a freewheeling vehicle crosses a slope hinge point, it is in a state of instability for a short while. However, once all wheels have landed, the vehicle regains stability. In the absence of humps and depressions, a tire-terrain coefficient of friction of 0.6 was reasonable for soil embankments. The path of the vehicle during the return maneuver depended greatly on the tire-terrain coefficient of friction. For a 0.2 friction coefficient, no return maneuvers could be performed successfully. It was noted that front slopes of less than 4H:1V are desired to reduce bumper penetration in the terrain as the vehicle traverses the ditch. In 1975, Ross et al. performed a study to determine the dynamic behavior of a vehicle as it traversed sloped medians and different curb configurations (14). TTIâs version of the HVOSM was used as a tool to perform parametric simulations with a 4,000-lb (1814-kg) vehicle model. This version of the HVOSM was modified to incorporate vehicle body-to-terrain contact representation. The objective of these simulations was to determine the potential of a vehicle vaulting over a barrier that was placed behind a curb or in a sloped median. Simulations were performed with 6-inch (152-mm) and 8-inch (203-mm) curbs and with medians that had slopes of 11H:1V, 8H:1V, and 2.5H:1V. Due to limitations in the computer code, the contact between the vehicle and the barrier was not modeled. Simulations were thus terminated when the vehicle reached the barrier. The performance of the vehicle while traversing existing curb and median configurations was compared to its performance on modified configurations. The comparison was based on vehicular accelerations and bumper height data collected from the HVOSM simulations. Bumper heights were calculated with respect to the local terrain of the vehicle. Simulations were performed assuming a freewheeling mode in which no steering, braking, or throttle inputs were applied to the vehicle as it encroached the roadway. It was concluded that in general barriers should not be placed near curbs. Simulation results indicated that curbs destabilize the vehicle, causing it to vault the barrier, or in some cases impact the barrier at a lower than design impact position. It was suggested that a flat approach area to the barriers should be considered whenever possible. It should be noted, today a few longitudinal barriers have been successfully designed and crash tested for use with a curb placed in front of or directly aligned with the face of the barrier. Under NCHRP Project 22-6, the performance of selected highway safety appurtenances and roadside features for passenger vehicles weighing less than 1,800 lb (820 kg) was assessed, and the limits of vehicle characteristics that can be safely accommodated through improvements in current hardware and roadside features were defined. This work is described in NCHRP Report 318 entitled Roadside Safety Design for Small Vehicles by Ross et al. (15). The roadside geometric features that were evaluated as part of this study included various fill and cut slopes, ditch sections, and driveways. Typical slopes, fill heights, ditch depths, and cross-section combinations were selected for analysis. Embankment parameters investigated included fill slopes of 6:1, 4:1, and 3:1 in combination with fill heights of 5, 10, and 20 ft (1.5, 3.1 and 6.1 m), and cut slopes of 3:1, 2:1, and 1:1. The matrix of ditch parameters examined included foreslope and backslope combinations of 6:1, 4:1, and 3:1 and ditch depths of 5 and 10 ft (1.5 and 3.1 m).
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. In most of the computer runs, the vehicle was directed off the travelway in a tracking condition at an encroachment angle of 15 degrees and an encroachment speed of 60 mph (97 km/h). A limited number of non-tracking encroachment simulations were also conducted, during which the vehicle was given a 15 deg/sec yaw rate as it left the travelway. At a time approximating the perception-reaction time for an average driver, the vehicle was given a steer input to simulate the response of a driver in a panic situation trying to return to the road. For each simulation run, vehicular response in terms of lateral movement, roll and pitch angles, and resultant accelerations were summarized. In addition, the behavior of the vehicle was quantified as either stable, sideslipping, spinning out, or overturning. In a study for the Texas Department of Transportation (TxDOT), Ross et al. developed supporting data from which clear-zone guidelines could be established as a function of embankment sideslope for rural arterial types of roadways (16). In this study, a set of design encroachment conditions were selected using data from pole- and bridge-related accidents (17, 18, 19). The impact speed and angle data from the accident studies were used to establish 85th percentile design encroachment conditions. Since it is not possible to select a unique percentile value for a combination of variables such as speed and angle, the effects of both variables were taken into account by considering the lateral component of vehicular velocity shown below. Vlat = V sinÎ¸ (1.1) where, V = encroachment speed, and Î¸ = encroachment angle. The lateral component of vehicular velocity not only provides a unique percentile value but is considered a good indicator of the potential for lateral movement of an errant vehicle. In order to take unreported accidents into account, the percentile of reported accidents was adjusted using a conservative 1-to-1 ratio of reported to unreported accidents. Thus, the 85th percentile design encroachment conditions for both reported and unreported accidents were equivalent to the 70th percentile impact conditions for reported accidents. Using this approach, the lateral speeds for 53 accidents on rural arterial highways were calculated, and the resulting data were fitted with a logistic normal distribution. Based on this theoretical distribution, the 70th percentile of lateral speed for reported accidents was 9.7 mph (15.7 km/h), from which design encroachment conditions were determined for speeds of 45, 50, and 55 mph (72, 80, and 89 km/h). For these design encroachment conditions, the HVOSM computer simulation program (10) was used to determine the response of large and small cars for a selected range of roadside slope conditions and a selected set of driver steer inputs. The results were used to help define appropriate clear-zone distances for rural arterials roadways with embankment slopes of 6:1, 4:1, and 3:1.
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). Analysis was performed to investigate vehicle dynamics of a vehicle as it traversed different roadside shoulder and sideslope configurations. The HVOSM was used to perform this simulation analysis. Some initial simulations were performed to identify the sensitivity of tire-terrain friction, vehicle type, and driver response to the dynamics of an encroaching vehicle. It was determined that the tire-terrain friction coefficient of 1.0 was more critical and that it would be an appropriate value for a soft soil condition. Higher vehicular accelerations and roll angles were observed for the small car than for the pickup truck. Initial investigation also revealed that a panic return-to-the-road type steering input was more critical compared to no steer input. Once these critical parameters were identified, a more detailed parametric simulation analysis was performed. Vehicle encroachment speeds of 45 and 65 mph (72 and 105 km/h), at encroachment angles of 5, 15, 25, 35, and 45 degrees were investigated. Sideslopes of 6H:1V, 4H:1V, and 3H:1V were investigated with a shoulder and roadway cross slope of 25H:1V and 50H:1V, respectively. All simulations were performed with a Honda Civic vehicle with a return- to-the-road steering input of 8 degrees at the wheels. It was determined that the body-to-terrain contact was an important factor for this analysis. Therefore, a TTI version of the HVOSM (V-3) that provided this type of contact was used. Curved roadways and V-ditches were not considered in this research and all roadsides ditches were assumed to have flat bottoms. Vehicle overturn was predicted for rounded and unrounded sideslopes for several combinations of encroachment speeds, angles, and ditch configurations. It was determined that high cornering forces due to panic steering, in combination with body-to-terrain contact, resulted in significant vehicular instability. For many cases, this instability led to vehicle overturns. Occupant risk was evaluated for each simulation, and an SI was assigned for the BCA. The BCA was used to develop guidelines for identifying scenarios when slope rounding becomes beneficial. In 2005, Sheikh et al. conducted research to evaluate the performance of concrete median barriers placed on or adjacent to medians with 6H:1V cross slopes using finite element analysis (21). According to the existing guidelines, the maximum slope on which a concrete barrier could be placed was 1V:10H. The researchers performed vehicular impact simulations on an F-shape barrier installed at various offsets in a V-ditch with 6H:1V cross slopes. The simulations in this research were performed using commercially available finite element analysis code LS-DYNA. The objective of the research was to determine if the concrete median barrier could be placed on steeper cross slopes, thus allowing placement of the barrier farther from the travelway. To evaluate barrier performance, lateral offset positions of the barrier most likely to result in vehicle override or instability were determined. Initial simulations were performed to determine the encroachment trajectory of the vehicle as it freely traversed a 6H:1V slope in the absence of a barrier. The results of these simulations were used to trace the path of the vehicleâs bumper with respect to the local ground elevation as a function of the vehicleâs lateral movement down a 6H:1V slope in the absence of a backslope. By using these curves of nominal bumper heights, critical barrier offset locations were identified and evaluated in full-scale vehicular impact simulations.
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. In a subsequent research project, the researchers performed full-scale crash tests to successfully evaluate the performance of the F-shaped barrier on 6H:1V cross slopes. In 2007, Marzougui et al. conducted a study to evaluate the performance of a three-strand cable barrier placed on sloped terrains (22). The researchers utilized LS-DYNA finite element simulations to analyze the performance of the cable system when impacted by a 2000P (C2500 pickup) per NCHRP Report 350 TL-3 criteria and validate the results using existing full-scale crash tests. Later, the researchers used a vehicle dynamic (multi-rigid body) model to study the trajectory of a large sedan (Ford Crown Victoria), a small sedan (Mitsubishi Mirage), and a pickup truck (Chevrolet C2500) when they traversed a median slope. Then, two full-scale crash tests of the large sedan were conducted using NCHRP Report 350 TL-3 conditions. In both tests, the cable barrier was placed on the slope of a symmetric median that was 16-ft (4.9-m) wide, with both a foreslope and backslope of 6H:1V. In the first test, the barrier was placed at a 4-ft (1.2-m) offset from the median centerline, while in the second test it was placed at a 1-ft (0.3-m) offset from the median centerline. The large sedan underrode the cable barrier in the first test and crossed the median with little resistance from the cable. However, in the second test, the vehicle was successfully redirected by the barrier. Both tests were correlated by a series with multi-rigid bodiesâ vehicle dynamics code human-vehicle-environment (HVE). The pickup truck simulation indicated that a cable rail placed at a 4-ft (1.2-m) offset from the ditch would successfully redirect the pickup truck. Figure 1.6 shows a graph of two points on the front right (or left) corners of the pickup truck model that was traced by the HVE program as the vehicle traverses a 4H:1V slope. Based on these results, a recommendation was made not to place a cable guardrail from 1 ft to 4 ft (0.3 m to 1.2 m) from the center of a V-ditch median on a 6H:1V slope. Addition of a fourth cable was also recommended for better performance.
22 Figure 1.6. Trajectories for two pointsâfront, right bumper corner and hood cornerâfor a pickup truck traversing a 4H:1V slope (22). Cost-Effectiveness Methodology Due to limited funds, highway engineers are continually faced with the difficult problem of determining when, where, and to what extent safety improvements should be provided on the roadside. Under the proposed project, the problem to be addressed is what treatments of roadside ditches are cost effective for a given roadway classification, average daily traffic (AADT), alignment, roadside geometry, and more. In recent years, cost-effectiveness, or BCA procedures, has become widely accepted as a reasonable approach for evaluating alternatives for such situations. Basically, the benefit/cost (B/C) methodology compares the benefits derived from a safety improvement, such as extending the recovery-area distance, to the direct highway agency costs incurred as a result of the improvement. Benefits are measured in terms of reductions in accident costs due to a reduction in the frequency or severity of predicted accidents. Direct costs are those associated with the initial cost, maintenance, and accident repair costs of a safety improvement. A ratio between the benefits and costs of an improvement (B/C ratio) is used to determine if the improvement is cost beneficial, as shown in the following equation: BC2-1 = (SC1 â SC2)/(DC2 â DC1) (1.2) where, BC2-1 = benefit-cost ratio of Alternative 1 compared with Alternative 2, SC1 = annualized societal cost of Alternative 1,
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). When the B/C ratio for a safety improvement is less than 1.0, the improvement should not normally be implemented. When the B/C ratio is greater than 1.0, the improvement is cost effective and its implementation should be considered. However, given the uncertainty of future conditions, user agencies often make implementation decisions based on a B/C ratio of 2 to 4. In one study for the Minnesota Department of Transportation (20), the benefits of rounding the hinge at the intersection of the shoulder and sideslope were evaluated. The HVOSM computer simulation program was used to determine occupant risk parameters for encroachments on unrounded and rounded sideslopes that, in turn, were used to estimate an SI for input into the BCA program. Occupant risk was measured in terms of vehicular accelerations and vehicular stability as determined by roll angle. The BCA evaluated unrounded and three degrees of rounded treatments for sideslopes of 6:1, 4:1, and 3:1. Results of the BCA were used to develop recommended slope rounding guidelines for freeways and rural arterial roadways as a function of sideslope and AADT. In a study sponsored by TxDOT, TTI researchers used the B/C approach to determine an appropriate and cost-beneficial clear-zone width requirement for suburban, high-speed, arterial highways with curb-and-gutter cross-sections for upgrading and/or reconstruction situations (16). The study focused on situations in which growth in traffic volume and frequency of turning movements necessitates the widening of an existing two-lane highway to four or more lanes. For purposes of the study, a baseline clear-zone width was defined as the clear-zone width that would be available after a roadway is widened, assuming no additional ROW is acquired. The question that was addressed was âunder what conditions is it cost beneficial to provide additional clear zone through the purchase of additional ROW?â Typical site conditions for this class of roadway were defined based on field data obtained from a selected sample of highway sections. Three levels of roadside hazard rating were defined in terms of utility poles and stress to represent varying roadside conditions, from a relatively clear roadside (low rating) to a roadside cluttered with hazards (high rating). The direct costs associated with increasing the clear-zone width included ROW purchase cost, clearing cost, and the cost to relocate existing utility poles. Benefits were measured in terms of reductions in accident or societal costs from decreases in the frequency or severity of accidents. An incremental BCA was used to determine incremental B/C ratios for various combinations of baseline clear-zone width, traffic volume (AADT), roadside hazard rating, and unit ROW acquisition cost. The results were tabulated in a series of tables to identify AADT ranges for which different clear-zone widths become cost beneficial. Based on these results, the following general observations were made: It is not cost beneficial to purchase 5 ft (1.5 m) or less of additional ROW. For unit ROW acquisition costs greater than $4/ft2 ($43.1/m2), it is not cost beneficial to provide additional clear-zone width by the purchase of additional ROW.
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. Table 1.3. Clear-zone distances for different AADT ranges. AADT Recommended Clear-Zone Distance,a ft (m) Minimum Desirable <8,000 10 (3.0) 10 (3.0) 8,000â12,000 10 (3.0) 20 (6.1) 12,000â16,000 10 (3.0) 25 (7.6) >16,000 20 (6.1) 30 (9.1) a Purchase of 5 ft (1.5 m) or less of additional ROW strictly for satisfying clear-zone provisions is not cost beneficial and thus not required. In another study conducted by TTI for FHWA, the B/C procedure was applied to develop general guidelines for positive work zone barrier use for four common work zone activities (23). These included bridge widening, roadway widening, major structural work (such as construction of grade separation structures) near the travelway, and two-lane, two-way operations on what is normally a divided highway. Up to 16 typical work zones were established for each of these activities. Variations included the number of construction personnel, number of heavy equipment hazards, work zone offset, and depth/severity of drop-off. The B/C algorithm was used to determine combinations of highway operating speeds, average daily traffic, and project duration for which the use of a portable concrete safety-shaped barrier becomes cost effective (i.e., B/C ratio equal 1.0) for each of these situations. Severities associated with barrier impacts were extrapolated from available crash test data. Under NCHRP Project 17-8, NCHRP Report 358: Traffic Barriers and Control Treatments for Restricted Work Zones (24), a BCA procedure was used to develop end treatment selection guidelines for concrete barriers placed in restricted work zones. Factors that were examined in selecting the most cost-beneficial treatments included (a) rate or angle at which the barrier is flared away from the travelway, (b) offset or lateral distance from end of barrier to travelway, and (c) how the ends of the barrier are treated. Five basic options were evaluated for treating the end of the concrete safety shape barrier: (a) a baseline option of leaving the end untreated, (b) a conventional sloped-end treatment, (c) inertial crash cushion designs, (d) the GREAT, and (e) the ADIEM. Estimates of the SI for the various barrier treatment alternatives were derived from crash test and computer simulation data. Selection guidelines were developed for various roadway classes and traffic conditions, defined in terms of volume and operating speeds. Given that the use of a barrier is warranted, the guidelines are intended to assist an engineer in the selection of the most appropriate end treatment and its placement. In a study for the Arizona Department of Transportation (ADOT) (25), Ross et al. developed guidelines to assist ADOT in (a) determining if an existing sign installation not in
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. The BCA procedure was used to establish these guidelines based on traffic volume and offset from the travelway. NCHRP Project 17-11 It has been recognized that updated guidelines are needed to aid designers in determining safe and cost-effective recovery areas while still recognizing the constraints associated with building or improving the highway system. As a first step in addressing this need, Bligh et al. conducted research under NCHRP Project 17-11 (13). The objective of this research was to develop relationships between recovery-area distance and roadway and roadside features, vehicle factors, encroachment parameters, and traffic conditions for the full range of highway functional classes and design speeds that can subsequently be used to establish clear-zone guidelines. The use of crash data for determining the statistics on the extent of lateral movement of vehicles encroaching onto the roadside is often limited by a vehicle striking a fixed object or rolling over. Therefore, any lateral extent of encroachment distribution derived from crash data will be a truncated distribution, and the full effect of sideslopes and other variables on lateral extent of encroachments is only partially observed. A research approach that combined crash data analyses with computer simulation results was developed to overcome this limitation. Use of computer simulation permitted a detailed analysis of vehicle trajectory and resulting vehicle kinematics for a wide range of variables for which data were not otherwise available. When combined with real-world crash data, the results can be used to develop relationships between various encroachment parameters. Clinical reconstruction and analyses of NASS CDS data were conducted to investigate key encroachment parameters for ROR crashes, including encroachment speed, encroachment angle, vehicle orientation at encroachment (e.g., tracking, non-tracking), and driver control input (e.g., steering, braking, or both). Three years of NASS CDS crash cases from 1997 to 1999 were considered in this study. A total of 559 cases were selected for the study based on an adopted set of criteria. A supplemental data collection effort was planned and executed to obtain desired roadway and roadside characteristics associated with the crash sites. The weighted crash data were used to develop probability distributions for the key encroachment parameters for nine highway functional classes. The functional classes were defined in terms of roadway type, land use, and posted speed limit. The distributions were then segregated into prescribed value categories for purposes of developing probability input matrices for application to the simulation data. The computer simulation study was performed using the HVOSM computer simulation program (10). The variables in the computer simulation study included vehicle type, encroachment speed and angle, vehicle orientation, driver input, HC, vertical grade, shoulder width, foreslope ratio, foreslope width, ditch width, backslope ratio, backslope width, and tire- terrain friction. A simulation matrix of all possible combinations of these variables would consist of literally millions of simulation runs. Since this was obviously not practical, it was necessary to prepare a selective simulation matrix. The approach utilized was to categorize the variables into two groups: baseline variables and adjustment variables.
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. For example, the effect of HC can be quantified by comparing the lateral extent of encroachment obtained for a curved section of roadway with that of a tangent section of roadway having similar roadside characteristics. The effects of these adjustment variables can be presented in the form of factors that can be applied to a basic set of relationships rather than requiring the development of numerous sets of relationships. The baseline variables and their selected values included: Vehicle type (1800-lb (820-kg) passenger car, 3000-lb (1500-kg) passenger sedan, 4400-lb (2000-kg) pickup truck, and small SUV). Encroachment speed (30, 45, 55, and 68 mph (50, 70, 90, and 110 km/hr). Encroachment angle (5, 15, and 25 degrees). Driver control response (steering and combined steering and braking). Foreslope ratio (flat, 10:1, 6:1, 4:1 and 3:1). Roadside coefficient of friction (longitudinal/lateralâ0.5/1.2). Vehicle orientation (tracking and non-tracking with yaw rate of 15 deg/sec). Note that under the baseline conditions, the roadway was assumed to be straight and level, and the foreslope was assumed to extend indefinitely (i.e., no ditch or backslope). Each adjustment variable was analyzed individually, not in combination with other adjustment variables. Selected values for each adjustment variable were evaluated with the same set of encroachment parameters used for the baseline simulations, which permits a more direct comparison of the baseline and adjustment variable simulation results as well as the use of the same probability distributions. The adjustment variables and their values included: HC (3, 6, and 9 degrees). Vertical grade (3 and 6% downgrade and 3 and 6% upgrade). Shoulder width (2 ft, 6 ft, and 12 ft [0.6 m, 1.8 m, and 3.6 m]). Ditch configuration: o Foreslope width (13, 26, 40 ft [4, 8, and 12 m]). o Ditch width (3 ft and 10 ft [1.0 m and 3.0 m]). o Backslope ratio (6:1, 4:1, and 2.5:1). o Backslope width (20 ft and 40 ft [6 m and 12 m]). The ranges used for these variables were generally intended to comprise current design practice for the classes of roadways being investigated. The magnitude of the simulation effort conducted under this project was unprecedented (>45,000 runs). In order to establish functional relationships from the discrete simulation data points, the probability distributions develop from the weighted NASS CDS data were applied to each encroachment parameter to obtain a probability for each value category of that parameter used in the simulation matrix. The combined probability for a given simulation with a unique set of encroachment conditions was determined by multiplying the individual probabilities assigned to the value of
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. In this manner, exceedance curves were developed and used to create lateral extent of movement relationships that combine simulation and real-world crash data such that they are a function of multiple encroachment parameters. Following this procedure, relationships for lateral extent of movement were developed for each functional class in terms of foreslope ratio. For a given functional class and foreslope ratio, the exceedance curves can be used to determine the percentage of encroachments that will exceed a certain lateral distance. The simulation runs conducted for the adjustment variables were analyzed in the same manner as the baseline runs. The exceedance curves developed for each value of the adjustment variables were compared to those of the baseline simulations to develop adjustment factors. The adjustment factors can be applied to the baseline exceedance curves to account for the effect of adjustment variable on lateral extent of encroachment. The simulation results were also analyzed for purposes of developing encroachment severity relationships for the different roadway and roadside variables of interest. Such relationships can be used for determining accident costs associated with roadside encroachments. Vehicular resultant accelerations and angular displacements were captured as output from the simulations and used to help assess encroachment severity. The simulation results were used to compute rollover probability as a function of sideslope ratio. The injury severity for the rollover encroachments was estimated using 6 years of NASS CDS data. Non-rollover encroachments were treated separately as a function of maximum resultant vehicle accelerations obtained from the computer simulations. The relationships developed under this study provided data and relationships from which recovery-area guidelines can be developed for selected highway functional classes. The researchers recommended that the future guideline development process involve some form of cost-effectiveness procedure and/or utilize a BCA program such as RSAP. The lateral extent of encroachment relationships developed under this study can be used to update accident frequency or rate as a function of recovery distance and sideslope. The severity relationships can be used to update accident severity in the RSAP model expressed in terms of injury probability and vehicle stability (i.e., rollover probability). Hydraulic Design Considerations The Design of Roadside Channels with Flexible Linings, Hydraulic Engineering Circular Number 15, Third Edition, Publication No. FHWA-NHI-05-114, September 2005 (26), addressed the design of small open channels called roadside channels that are constructed as part of a highway drainage system. Roadside channels play an important role in the highway drainage system as the initial conveyance for highway runoff. Roadside channels are often included as part of the typical roadway section. Therefore, the geometry of roadside channels depends on available ROW, flow capacity requirements, and the alignment and profile of the highway. The procedures for hydraulic design of roadside channels contained in the aforementioned manual may also be used for ancillary roadside drainage features, such as rundowns.
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. To perform reliably, the roadside channel is often stabilized against erosion by placing a protective lining over the soil. The manual presents various classes of flexible channel linings that are well suited for construction of small roadside channels. Changes or modifications in the foreslope, backslope, ditch width, and ditch material lining all have an effect on the hydraulic performance of the drainage system. When evaluating the safety performance of ditches and channels, the designer must also evaluate the effect changes in geometry and lining will have on the mechanical performance of the drainage system. A brief discussion of the theory of open-channel flow is presented to identify the variables affected when modifying open drainage systems. The discussions that follow are from excerpts of Chapter 2 of Design of Roadside Channels with Flexible Linings. The design method presented in the circular is based on the concept of maximum permissible tractive force. The method has two parts, computation of the flow conditions for a given design discharge and determination of the degree of erosion protection required. The flow conditions are a function of the channel geometry, design discharge, channel roughness, channel alignment and channel slope. The erosion protection required can be determined by computing the shear stress on the channel lining (and underlying soil, if applicable) at the design discharge and comparing that stress to the permissible value for the type of lining/soil that makes up the channel boundary. 2.1 OPEN-CHANNEL FLOW 2.1.1 Type of Flow For design purposes in roadside channels, hydraulic conditions are usually assumed to be uniform and steady. This means that the energy slope is approximately equal to average ditch slope, and that the flow rate changes gradually over time. This allows the flow conditions to be estimated using a flow resistance equation to determine the so-called normal flow depth. Flow conditions can be either mild (subcritical) or steep (supercritical). Supercritical flow may create surface waves whose height approaches the depth of flow. For very steep channel gradients, the flow may splash and surge in a violent manner and special considerations for freeboard are required. More technically, open-channel flow can be classified according to three general conditions: uniform or non-uniform flow steady or unsteady flow subcritical or supercritical flow. In uniform flow, the depth and discharge remain constant along the channel. In steady flow, no change in discharge occurs over time. Most natural flows are unsteady and are described by runoff hydrographs. It can be assumed in most cases that the flow will vary gradually and can be described as steady, uniform flow for short periods of time. Subcritical flow is distinguished from supercritical flow by a dimensionless number called the Froude number (Fr), which is defined as the ratio of inertial forces to gravitational forces in the system. Subcritical flow (Fr < 1.0) is characterized as tranquil
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. Supercritical flow (Fr > 1.0) is characterized as rapid and has shallow, high velocity flow. At critical and supercritical flow, a shallow wave only moves in the downstream direction. 2.1.2 Normal Flow Depth The condition of uniform flow in a channel at a known discharge is computed using the Manning's equation combined with the continuity equation: (2.1) where, Q = discharge, ft3/s (m3/s) n = Manning's roughness coefficient, dimensionless A = cross-sectional area, ft2 (m2) R = hydraulic radius, ft (m) Sf = friction gradient, which for uniform flow conditions equals the channel bed gradient, So, ft/ft (m/m) Î± = unit conversion constant, 1.49 (CU), 1.0 (SI) The depth of uniform flow is solved by rearranging Equation 2.1 to the form given in Equation 2.2. This equation is solved by trial and error by varying the depth of flow until the left side of the equation is zero. (2.2) Figure 1.7 and the two cases presented directly below it, taken from Appendix B of Design of Roadside Channels with Flexible Linings, illustrate typical open-channel ditch configurations and geometry equations used in highway roadside design. Application guidance is provided in the AASHTO RDG.
30 Figure 1.7. Typical open-channel ditch configurations.
31 2 CASES No. 1 If d â¤ 1/Z, then: No. 2 If d > 1/Z, then: Note: The equations for V-shape with rounded bottom only apply in customary units for a channel with a 4-ft (1.2-m) wide rounded bottom. Maintenance and Vegetation The AASHTO Maintenance Manual: The Maintenance and Management of Roadways and Bridges, 1999 (27), provides the following recommendations for maintaining drainage systems: Drainage systems for highways are designed to limit water damage to the roadway by controlling or directing the free flow of water over, under, or adjacent to the highway and to control the movement of water through the pavementâs structural support where necessary. . . . Roadway drainage maintenance focuses on retaining the intended design efficiency of the drainage system and on adapting the existing drainage system to accommodate environmental changes to the degree possible with minor changes. . . . â¢ Cleaning roadside ditches to provide a uniform flow line and consistent channel shape. â¢ Removing trees and other debris from natural water courses or ditches. . . . Good roadway drainage maintenance has a significant positive effect on roadway safety. Well-maintained foreslopes allow vehicles leaving the traveled lanes to be brought under control with minimum damage. Roadside ditches that are cleaned to prevent standing water minimize the possibility that a vehicle falling into the ditch after a storm will result
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. Good maintenance patrol inspection of roadway drainage will identify culvert end sections or inlets that are potential hazards to vehicles and report these features to the engineering sections responsible for a redesign, mitigating the hazard. . . . [To ensure good inspection procedures] check for silt deposits or erosion of the ditch cross section and profile. . . . [Roadside ditches] will be open unless they cross under a side road, a driveway, or a walkway. Roadside ditches should be maintained as near as practical to the alignment, grade profile, depth, and cross section to which they were originally designed and constructed or as subsequently reconstructed. At periodic intervals, the roadside ditches should be inspected for and cleaned of fallen rock, heavy vegetation, sediment creating ponding, and other debris that may restrict design drainage flows. . . . Channel Lining Many materials have been used to line or surface roadway drainage channels to aid in the prevention of erosion of the roadside and channel cross-section. Commonly used materials for channels are (a) grass; (b) gravel, rip rap, and cobble; and (c) flexible and rigid pavements, such as asphaltic and Portland cements. Additionally, the material used to line the ditch and surrounding roadside can have a profound effect on the performance of an errant motoristâs ability to recover when they depart the roadway. Differential surface friction between the roadway and roadside can affect the motoristâs ability to steer and recover. Additionally, both lower and higher surface friction along the roadside can contribute to a vehicle furrowing or tripping, thus resulting in the vehicle rolling over. Figures 1.8 through 1.12 illustrates examples of channels lined with these materials (26). Grass-lined channels have been widely used in roadway drainage systems for many years. They are easily constructed and maintained and work well in a variety of climates and soil conditions. Grass linings provide good erosion protection and can trap sediment and related contaminants in the channel section. Routine maintenance of grass-lined channels consists of mowing, control of weedy plants and woody vegetation, repair of damaged areas, and removal of sediment deposits. Riprap, cobble, and gravel linings are considered permanent flexible linings. They may be described as a noncohesive layer of stone or rock with a characteristic size ranging from 0.6- inch (15-mm) to 22-inches (550-mm). The boundary between gravel, cobble, and riprap sizes are defined by the following ranges: Gravel: 0.6â2.5 inches (15â64 mm). Cobble: 2.5â5.0 inches (64â130 mm). Riprap: 5.0â22.0 inches (130â550 mm). Other differences between gravels, cobbles, and riprap may include gradation and angularity. Gravel mulch, although considered permanent, is generally used as a supplement to aid in the establishment of vegetation. It may be considered for areas where vegetation establishment is difficultâfor example, in arid-region climates.
33 Figure 1.8. Rigid concrete channel lining. Figure 1.9. Vegetative lining.
34 Figure 1.10. Cobble channel lining. Figure 1.11. Rip rap channel lining.
35 Figure 1.12. Erosion control blanket. CONCLUSIONS The researchers performed a literature search and review of the domestic and international research related to safety performance of roadside slopes and ditch designs. A summary of the reviewed references was presented in this chapter. While the literature search and review provided useful insights regarding different aspects of roadside ditch design features and related past research, the existing literature did not expose any innovative methods for mitigating hazards associated with motorists entering the ditches. Some of the existing but underutilized mitigation methods, such as slope rounding and surface treatments, have been briefly evaluated in the past, which can be further investigated in more detail.