Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
101 APPENDIX E: ROADSIDE SAFETY OVERVIEW A HISTORY OF ROADSIDE SAFETY Highway design was reaching maturity during the 1940s, and focus shifted to safety in the late 1960s. In 1956, President Dwight Eisenhower signed the Federal-Aid Highway Act of 1956. The bill created a 41,000-mile âNational System of Interstate and Defense Highways.â The interstates were to provide a uniform transcontinental roadway design network that would eliminate unsafe roads, inefficient routes, and traffic jams and provide fast safe transcontinental travel. The 1960s focused on safety for both automobiles and highways because many motorists were losing their lives in automobile accidents. The National Traffic and Motor Vehicle Safety Act in 1966 empowered the federal government to set and administer new safety standards for motor vehicles and road traffic safety. The act was the first mandatory federal safety standards for motor vehicles. During the 1960s, highway engineers also began to focus on the design of the roadside as well as the roadway itself. The forgiving roadside concept recognized that motorists do run off the roadway and that serious accidents and injuries could be lessened if a roadside had a clear and traversable recovery area. Collisions with fixed objects have been significantly reduced thanks to federal and state transportation officialsâ dedication to the forgiving roadside concept. This roadside recovery area came to be known as the clear zone and starts at the edge of the traveled way and extends out to a current unshielded minimum of 3 m (10 feet) for low-speed rural collectors and local roads. The desired clear zone width is a function of traffic volumes, speeds, and roadside geometry. Ideally, the clear zone should be free of obstacles such as unyielding sign and luminaire supports, non-traversable drainage structures, utility poles, steep slopes, and other fixed objects. The AASHTO RDG generally defines fixed objects as the following: ⢠Post/pole ⢠Culvert/ditch ⢠Guardrail, crash cushion, or traffic barrier ⢠Tree ⢠Curb ⢠Embankment ⢠Fence ⢠Other fixed object (bridge, wall, bush, etc.) AASHTO has design options for the treatment of undesirable roadside objects, and those options are generally considered in the following order: ⢠Remove the obstacle or redesign it so it can be safely traversed. ⢠Relocate the obstacle to a point where it is less likely to be struck. ⢠Reduce impact severity by using an appropriate breakaway device. ⢠Redirect a vehicle by shielding the obstacle with a longitudinal traffic barrier and/or crash cushion. ⢠Delineate the obstacle if the above alternatives are not appropriate
102 As transportation facilities continue to expand and most often without the benefit of additional ROW, the most common practice is to shield the obstacle with some type of roadside barrier or attenuator. However, this practice, because of limited ROW, in essence comprises the concept of the clear recovery zone. The placement of re-directive roadside barriers, as an example, that are intended to protect the motorist from an immovable hazard(s) has now created a secondary hazard to the motorists and has increased the exposure area. However, these barriers are installed where the consequence of striking the barrier is still less than the consequence of striking the object(s) the barrier is protecting the vehicle from striking. Many objects are placed along the roadside and within the clear zone or roadway environment, such as sign supports, that provide information that guides, warns, and regulates the motoristâs operation along the highway. Likewise, barriers are used to shield steep roadside slopes and protect infrastructure, trees, and other hazards. All these types of devices placed in roadway environment must be subjected to full-scale vehicular crash testing to verify that they are indeed crashworthy. The procedures for crash testing and evaluating highway safety appurtenances have evolved from the first published guidelines in 1962, Highway Research Correlation Services Circular 482, to the current 2006 MASH. For approximately six decades, the United States has been committed to highway safety. Guidelines for crash testing roadside safety appurtenances originated in 1962 with a one-page documentâHighway Research Circular 482, entitled Proposed Full-Scale Testing Procedures for Guardrails. This document includes four specifications on test article installation, one test vehicle, six test conditions, and three evaluation criteria. Since the release of Highway Research Circular 482, six more documents have followed pertaining to crash testing and evaluating highway safety appurtenances. Those documents are NCHRP Reports 115, 118, 153, 230, and 350 and Transportation Research Circular 191. A brief discussion of each document follows. In 1971, NCHRP Report 115: Guardrail Performance Design and NCHRP Report 118: Location, Selection, and Maintenance of Highway Traffic Barriers were published. NCHRP Report 115 examines and uses a six degrees of freedom mathematical model to describe the dynamic behavior of vehicle impacts into barriers. The study was one of the first of its kind to examine the correlation between computer-predicted behavior (simulation) of vehicle/barrier impacts and full-scale crash testing. In addition, human impact tolerance levels were examined and evaluated for 25 full-scale crash tests. It was first suggested here that occupants would require lap and shoulder belts to avoid serious injury when impacting roadside barriers. NCHRP Report 118 presents a synthesis of existing information on warrants, service requirements, and performance criteria for all traffic barrier systems. Vehicle impact characteristics and dynamic performance and evaluation criteria are presented for all types of highway safety barrier hardware. In 1974, NCHRP Report 153: Recommended Procedures for Vehicle Crash Testing of Highway Appurtenances was published. This 16-page document provides the first complete test matrix for evaluating safety features. Data collection methods, evaluation criteria, and limited guidance on reporting formats are included. These procedures gained wide acceptance following their publication, but it was recognized at that time that periodic updating would be needed. Published in 1978, Transportation Research Circular 191: Recommended Procedures for Vehicle Crash Testing of Highway Appurtenances provides limited interim changes to NCHRP
103 Report 153 to address minor changes requiring modified treatment of problem areas. Extensive revision and update to these procedures were made in 1981 with the publication of the 42-page NCHRP Report 230: Recommended Procedures for the Safety Performance Evaluation of Highway Appurtenances. In 1993, NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features was published. This 132-page document represents a comprehensive update to crash test and evaluation procedures. It incorporates significant changes and additions to procedures for safety performance evaluation, and updates reflecting the changing character of the highway network and the vehicles using it. NCHRP Report 350 contains guidelines for evaluating the safety performance of roadside features, such as longitudinal barriers, terminals, crash cushions, and breakaway structures. FHWA formally adopted this document as the national standard in 1993 with an implementation date of late 1998. In 1998, AASHTO and FHWA agreed that most types of safety features installed along the National Highway System must meet NCHRP Report 350 safety performance evaluation criteria. FHWA formally adopted the new performance evaluation guidelines for highway safety features set forth in NCHRP Report 350 as a âGuide or Referenceâ document in the Federal Register, Volume 58, Number 135, dated July 16, 1993, which added paragraph (a)(13) to 23 CFR, Part 625.5. FHWA subsequently mandated that starting in September 1998, only highway safety appurtenances that have successfully met the performance evaluation guidelines set forth in NCHRP Report 350 may be used on new construction projects on the National Highway System. Since the reportâs adoption, many states, counties, and municipalities have begun to use crashworthy safety appurtenances. NCHRP Report 350 was unique in that it created six test levels. Test levels one through three (TL-1 through TL-3) were based on speed: TL-1 was 50 km/h (30 mi/h), TL-2 was 70 km/h (43 mi/h), and TL-3 was 100 km/h (62 mi/h). Three additional test levels (TL-4 through TL-6) were for heavier commercial-type vehicles at 80 km/h (50 mi/h). The test vehicle weights ranged from 820 kg (1800 lb) to 36,000 kg (80,000 lb). Through various pooled-fund studies and other research projects, FHWA and state DOTs tested the most widely used nonproprietary safety appurtenances. Additionally, manufacturers worked toward recertification of their proprietary products. Ultimately, numerous changes and modifications to existing hardware were required to comply with NCHRP Report 350. Many of these changes were attributed to the change from the 4500-lb passenger vehicle to the 4400-lb (2000P) pickup truck. The pickup truck represented an SUV class of vehicle that had a higher CG and was inherently less stable than the large passenger vehicle used under NCHRP Report 230. In addition, the pickup truck had a shorter front overhang, often resulting in snagging of the front wheel and subsequent displacement of the wheel and tire into the floor/toe pan. As a result of snagging and wheel displacement, excessive intrusion into the occupant compartment was frequently observed. Work zone devices, such as portable sign stands and barricades, were tested, many for the first time. These devices often failed due to intrusion into the small 1800-lb (820C) passenger compartment through the roof and windshield. In 2009, AASHTO published MASH, the update to NCHRP Report 350. MASH provides: ⢠A basis on which researchers and user agencies can compare the impact performance merits of candidate safety features
104 ⢠Guidance for developers of new safety features ⢠A basis on which user agencies can formulate performance specifications for safety features MASH reflected a changing vehicle fleet in that the small car weight increased to 1100 kg (2420 lb), the pickup truck weight increased to 2270 kg (5000 lb), and the TL-4 single-unit truck weight increased from 8000 kg (17,600 lb) to 10,000 kg (22,040 lb). In addition, the impact angle for the small car increased from 20 degrees to 25 degrees. The impact angles for terminals and crash cushions also increased to 25 degrees. An extensive cable barrier test matrix was presented for both level terrain and in ditches. Much like in NCHRP Report 350, an implementation plan also existed in MASH. An example of a typical test matrix in MASH is Table 2-2A, âRecommended Test Matrices for Longitudinal Barriers,â and is presented in Figure E1. This table illustrates the test levels, speed, angle, impact severity, and evaluation criteria used for evaluating a longitudinal barrier. The normal or standard test level for most highway applications is TL-3â100 km/h (62 mi/h). Research (Mak et al. 1986) (NCHRP in- progress) and reconstructions of run-off-the-road passenger vehicle crashes on high-speed roadways indicate that an impact speed of 100 km/h (62 mi/h) and an impact angle of 25 degrees approximate the 85th percentile of the real-world impacts. These conditions are reflected in MASH TL-3.
105 Figure E1. Recommended Test Matrices for Longitudinal Barriers (AASHTO 2016). On November 12, 2015, FHWA issued a memorandum indicating that all modifications to NCHRP 350âtested devices will require testing under MASH to receive a federal-aid eligibility letter from FHWA. In addition, a Federal Register notice was also issued regarding this action. This action provided a significant step forward to the implementation of MASH. Furthermore, another FHWA-issued memorandum dated January 7, 2016, regarding the federal-aid eligibility of highway safety hardware after December 31, 2016, says that: ⢠FHWA will no longer issue eligibility letters for highway safety hardware that has not been successfully crash tested to the 2016 edition of MASH. ⢠Modifications of eligible highway safety hardware must use criteria in the 2016 edition of MASH for reevaluation and/or retesting.
106 ⢠Non-significant modifications of eligible hardware that have a positive or inconsequential effect on safety performance may continue to be evaluated using finite element analysis. Highway safety appurtenances will continue to evolve, and guidance will be updated to reflect changes in the vehicle population and new knowledge about vehicle/safety hardware and vehicle/occupant interactions. HIGHWAY SAFETY APPURTENANCE PERFORMANCE EVALUATION The goal of a highway safety appurtenance/feature, as previously discussed, is to provide a forgiving roadway environment so that when an errant motorist leaves the roadway, the risk of a serious crash is reduced. The goal is to reduce the risk of serious injury or death. When the roadside cannot be made clear of all fixed objects or obstructions and/or cannot be made to be traversable, then a safety appurtenance is used so that the consequences of striking the device are less severe than the fixed object or terrain feature it is protecting. The safety appurtenance used should be crashworthy and evaluated in accordance with the appropriate guidelines at the time the device is installed, such as is currently used (AASHTO 2016). A highway safety appurtenance is evaluated based on the performance of full-scale crash tests, as previously discussed. A full-scale crash test is defined by the impact speed and angle, the weight of the impacting vehicle, and the impact location. These parameters are selected to represent the âworst practical conditionâ and are believed to represent the 85th percentile crash severity. The safety performance of a highway appurtenance cannot be measured directly. However, the crashworthiness performance of an appurtenance can be evaluated in terms of the risk of injury to occupants of the impacting vehicle, the structural adequacy of the safety feature, the exposure to workers and pedestrians that may be behind a barrier or in the path of debris resulting from impact with a safety feature, and the post-impact behavior of the vehicle. This philosophy has been stated in NCHRP Report 230 and reiterated in both NCHRP Report 350 and AASHTO MASH. As previously stated, the crashworthy performance of a highway safety appurtenance is evaluated in terms of structural adequacy, occupant risk, and post-impact vehicle trajectory. A brief discussion of each of these evaluation criteria is presented hereafter. AASHTO MASH provides a thorough discussion of how the crash performance of a highway safety appurtenance is evaluated through crash testing. The criteria for evaluating structural adequacy in MASH are shown in Figure E1, which reproduces MASH Table 5-1A. As noted in MASH, the structural adequacy criteria refer to the structural requirements associated with the impact itself and no other structural aspects of the device. For example, the criteria do not imply that a sign support system that meets the structural adequacy requirements of a test will meet the structural adequacy requirements of wind and ice loads or other environmental considerations when applicable.
107 Table E1. Safety Evaluation Guidelines for Structural Adequacy. Evaluation Factors Evaluation Criteria Applicable Tests Structural Adequacy (see Section 5.2.1) A. Test article should contain and redirect the vehicle or bring the vehicle to a controlled stop; the vehicle should not penetrate, underride, or override the installation although controlled lateral deflection of the test article is acceptable. 10, 11, 12, 20, 21, 22, 30a, 31a, 32a, 33a, 34a, 35, 36, 37a, 38a B. The test article should readily activate in a predictable manner by breaking away, fracturing, or yielding. 60, 61, 62, 70, 71, 72, 80, 81, 82 C. Acceptable test article performance may be by redirection, controlled penetration, or controlled stopping of the vehicle. 30b, 31b, 32b, 33b, 34b, 37b, 38b, 40, 41, 42, 43, 44, 50, 51, 52, 53, 90, 91 a = non-gating terminals and crash cushions, b = gating terminals and crash cushions Occupant risk is evaluated according to measured vehicular accelerations that occur as a function of the highway safety appurtenance performance and its effect on the external structural design of the test vehicle. Whereas the highway engineer is ultimately concerned with the safety of the vehicleâs occupants, the occupant risk criteria of MASH Table 5-IB, shown as Table E2, are considered as guidelines for generally acceptable dynamic performance.
108 Table E2. Safety Evaluation Guidelines for Occupant Risk. Evaluation Factors Evaluation Criteria Applicable Tests Occupant Risk (see Section 5.2.2) D. Detached elements, fragments, or other debris from the test article should not penetrate or show potential for penetrating the occupant compartment, or present undue hazard to other traffic, pedestrians, or personnel in a work zone. Deformations of, or intrusions into, the occupant compartment should not exceed limits set forth in Section 5.2.2 and Appendix E. All E. Detached elements, fragments, or other debris from the test article, or vehicular damage should not block the driver's vision or otherwise cause the driver to lose control of the vehicle. 70, 71, 72 F. The vehicle should remain upright during and after collision. The maximum roll and pitch angles are not to exceed 75 degrees. All except those listed in Criterion G G. It is preferable, although not essential, that the vehicle remain upright during and after collision. 12, 22 H. Occupant impact velocities (see Appendix A, Section A5.2.2 for calculation procedure) should satisfy the following limits: Occupant Impact Velocity Limits, ft/s (m/s) Component Preferred Maximum Longitudinal and Lateral 30 ft/s (9.1 m/s) 40 ft/s (12.2 m/s) 10, 11, 20, 21, 30, 31, 32, 33, 34, 35, 36, 37, 38, 40, 41, 42, 43, 44, 50, 51, 52, 53, 80, 81, 82, 90, 91 Longitudinal 10 ft/s (3.0 m/s) 16 ft/s (4.9 m/s) 60, 61, 62, 70, 71, 72 I. The occupant ridedown acceleration (see Appendix A, Section A5.2.2 for calculation procedure) should satisfy the following limits: Occupant Ridedown Acceleration Limits (G) Component Preferred Maximum Longitudinal and Lateral 15.0 G 20.49 G 10, 11, 20, 21, 30, 31, 32, 33, 34, 35, 36, 37, 38, 40, 41, 42, 43, 45, 50, 51, 52, 53, 54, 60, 61, 62, 70, 71, 72, 80, 81, 90, 91
109 Post-impact vehicular response is a measure of the potential of the vehicle to result in a secondary collision with other vehicles and/or fixed objects, thereby increasing the risk of injury to the occupants of the impacting vehicle and/or other vehicles. Table E3, which reproduces MASH Table 5-1C, provides the guidelines for post-impact vehicular response. Table E3. Safety Evaluation Guidelines for Post-Impact Vehicular Response. Evaluation Factors Evaluation Criteria Applicable Tests Post-Impact Vehicular Response (see Section 5.2.3) J. through M. Reserved. N. Vehicle trajectory behind the test article is acceptable. 30b, 31b, 32b, 33b, 34b, 37b, 38b, 40, 41, 42, 43, 44, 45, 60, 61, 70, 71, 72, 80, 81, 82, 90, 91 A = non-gating terminals and crash cushions, b = gating terminals and crash cushions AASHTO MASH says that in addition to satisfying these impact performance evaluation criteria in a series of crash tests, the user agency should continue to monitor a safety deviceâs field performance through in-service performance evaluations because its performance cannot necessarily be measured by a series of a few tests alone. Crash tests are performed in a controlled environment, and while a safety appurtenance may meet all test and evaluation criteria, the device may encounter field or in-service conditions that may not have been evaluated in testing. The discussion hereafter presents material to aid the designer in applying the understanding and the application of how a safety appurtenance is evaluated with regard to structural adequacy, occupant risk, and vehicle trajectory. Examples are presented, as an introduction, to assist in guiding the designer through the thought process of the selection, design, and crash performance of a terminal and sign support. An end terminal for a concrete safety shape roadside barrier/guardrail and a sign support are unique and in some respects unlike most traditional engineering design. In traditional engineering design, a product is designed and built to sustain or exceed a specific load or structural requirement. In highway safety appurtenance engineering, a design window methodology is used. An end terminal, for example, is designed to withstand the impact of a vehicle and bring that vehicle to a controlled stop at minimal deceleration levels and/or safely redirect the vehicle to the roadway without placing other motorists on the road at risk. Thus begins the dilemma: the highway structure must be structurally adequate to redirect an errant vehicle when struck along its side while yielding or forgiving enough to provide acceptable occupant acceleration ridedown levels when struck on or near its end or terminus, in accordance with values presented in MASH Table 5-1B. Unlike conventional structural design, forgiving devices must not be overdesigned. The upper bound for strength may be more critical than the lower bound. In lay terms, this means that the strength must be just strong enough but not overly strong. The goal of designing any roadside safety appurtenance is to reduce risk to the errant motorist to the lowest level practical. For a longitudinal barrier end terminal, structural adequacy simply means that the terminal must stop the vehicle in a controlled manner or in some cases prevent the device from gating and allowing the vehicle to penetrate or pass to the other side of the installation. If the vehicle is not brought to a controlled stop, then it may require redirecting the vehicle along the barrierâs travel side
110 longitudinal axis. In other words, the vehicle cannot be permitted to reach through or over the barrier and reach what the barrier is protecting. Controlled stop, as noted previously, refers to safely decelerating an errant vehicle without causing undue harm to its occupants. The external forces exerted on the errant vehicle from the terminal are or may be the result of metal or other materials. The terminal is constructed of deforming the support posts, if applicable, anchored into or onto the ground or road surface, deforming and displacing through the soil or detachment from their method of anchoring to the road surface or base. The energy-absorbing elements of a terminal should not be altered in any way from its design because this will likely alter its crashworthy performance. The energy- absorbing elements can literally be any part of the terminalâs design. The performance of a guardrail, guardrail terminal, and guardrail transition is typically very dependent on the displacement of the supporting posts displacing through the soil in a controlled manner. Anything done to alter the characteristics of the way the posts displace through the soil can seriously alter the performance of the safety appurtenance. The absorption of kinetic energy by displacing the posts through the soil is part of the energy- absorbing ability of the guardrail system and should not be altered. A change in energy- absorbing contribution from one system component such as a post, whether it be more or less energy contribution, can affect how the other system component functions. In the example of guardrail, if a post is restrained from rotating properly, it can cause additional energy transfer to the metal beam element of the guardrail and thus result in possibly tearing or rupturing of the rail. This phenomenon can be readily observed if an improperly constructed concrete or asphalt mow strip is added around wood guardrail posts. The restrained posts do not rotate and consume energy, the posts fracture, excessive deflection of the guardrail occurs, excessive energy is transferred to the rail, and this rail ruptures, allowing the impacting vehicle to penetrate the rail. This phenomenon can also be true of surface-mounted posts if the designed post failure mechanism is altered. Additionally, the vehicle and components of the vehicle, such as the tire and suspension, can snag on the post(s) and/or form a pocket in the rail. This leads to snagging and results in rupturing the rail and frequently excessive deformations of the occupant compartment, which may also lead to excessive changes in occupant velocities and ridedown accelerations. For the example of a sign support, the sign support must be designed structurally to support the dead load of the sign panel and be capable of sustaining the wind and ice loads the support(s) and sign panel will be subjected to while also being capable of safely yielding to an impacting vehicle in a controlled manner. In this example, controlled manner refers to: ⢠When the sign is struck, it must break away or yield to the impacting vehicle while not causing excessive change in occupant impact velocity or excessive occupant ridedown accelerations ⢠The trajectory or rotation of the detached sign installation (i.e., sign support, sign panel, and their associated components) over the vehicle should not penetrate into the occupant compartment or cause excessive deformation to the windshield or roof. The occupant risk evaluation criteria of these values are presented in MASH Table 5-1B, as previously presented, for sign support Tests 60, 61, and 62 for this example. In addition, a
111 detailed discussion of assessing structural adequacy, occupant risk, and deformation and post- impact vehicular response is given in MASH Sections 5.2.1 through 5.2.3, respectively. A discussion of the supporting research of the same is presented in MASH Appendices A5.2.1 through A5.2.3. Anything done to change the way the support is anchored into or onto the ground can greatly affect the sign installationâs ability to yield and can cause excessive occupant risk values including excessive deformation and intrusion into the vehicle occupant compartment. This appendix provides a general discussion of the performance of two types of highway safety appurtenances. The key themes are: ⢠Designers seek to provide a forgiving roadway environment. ⢠Highway safety appurtenances should be crash tested and evaluated in accordance with FHWA/AASHTO crash testing guidance (currently MASH). ⢠All crash-tested appurtenances are evaluated with regard to their structural adequacy, occupant risk, and post-impact trajectory performance.
