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1 CHAPTER 1 INTRODUCTION BACKGROUND The Green Book defines two basic types of curbs, as shown in Figure 1: vertical curbs and sloping curbs. Vertical curbs There has long been concern over the use of curbs on road- usually have a vertical or nearly vertical face. Such curbs ways because of their potential to cause drivers to lose con- usually serve several purposes, including discouraging vehi- trol and crash. Curbs extend 75 to 200 mm above the road cles from leaving the road, drainage, walkway edge support, surface for appreciable distances and are located very near and pavement edge delineation. the edge of the traveled way; thus, they present a possible Vertical curbs have some ability to redirect errant vehicles hazard for motorists who may encroach on the roadside at since the impacting wheel is steered by the curb in a direction any point within the length of the curb. This project focused parallel to the traveled way. If the impact velocity and angle on the use of curbs on higher-speed roadways, defined as are modest, this steering action is all that may be required to roadways with design speeds of 60 to 100 km/h. AASHTO prevent the vehicle from leaving the roadway. If the speed highway design policy discourages the use of curbs on and encroachment angle are higher, then the steering action higher-speed roadways because of their potential to cause of the curb alone is not sufficient to redirect the vehicle. drivers to lose control and crash. Curbs can also cause a lat- Since the vehicle center of gravity is much higher than the erally skidding vehicle to roll over upon striking the curb, a top of the curb, a high-speed impact with the curb will intro- situation referred to as tripping. While the use of curbs is dis- duce a roll moment. This roll moment will in turn introduce couraged on higher-speed roadways, they are often required instability into the vehicle trajectory and may even be large because of restricted right-of-way, drainage considerations, enough to cause the vehicle to roll over. Since curbs are often access control, delineation, and other curb functions. used primarily for drainage purposes, they are often found in In some cases, a barrier is placed in combination with a curb conjunction with steep sideslopes where a rollover would be and an inadequate design can result in vehicles vaulting or even more likely. For these reasons, vertical face curbs are underriding the barrier. Such installations are currently being constructed without a clear understanding of the effects that usually restricted to low-speed facilities where vehicles are these combinations will have on the ability of the barrier to to be discouraged from leaving the roadway. safely contain and redirect an errant vehicle. There have been Sloping curbs, as illustrated in Figure 1, have a sloped face a very limited number of full-scale crash tests on curbbarrier and are configured such that a vehicle can ride up and over combinations and a large percentage of those tests involving the curb. These curbs are designed so that they do not signif- the larger class of passenger vehicles such as the 2000-kg icantly redirect a vehicle. They are usually used in situations pickup truck were unsuccessful. Even the cases involving where redirecting a possibly damaged and out-of-control the 2000-kg pickup truck that satisfied the requirements of vehicle back into the traffic stream is undesirable. Sloping NCHRP Report 350 resulted in excessive damage to the curbs are often used primarily for drainage purposes but are barrier system or extreme trajectories and instability of the also used on median islands and along shoulders of higher- vehicle. speed roadways for delineation and other reasons. Sloping Policy on the design and use of cross-sectional highway curbs provide drainage control while also allowing vehicles features, including curbs, is contained in AASHTO's Policy access to the roadside in emergency situations. on Geometric Design of Highways and Streets (i.e., the Green It is often necessary to use a curb for drainage or other rea- Book) (1). The purposes of curbs are to provide drainage, sons at a particular location that also warrants a traffic bar- delineate the edge of the pavement, support the pavement rier. For example, approaches to bridge structures (e.g., over- edge, provide the edge for a pedestrian walkway, and possi- passes) are often built on fills with steep slopes. An approach bly provide some redirective capacity for low-speed impacts. guardrail is required both to shield the end of the bridge rail- On higher-speed roadways, the subject of this study, the pri- ing and to shield errant motorists from the steep sideslope mary function of curbs is to provide drainage, especially in approaching the structure. If surface water were allowed to the area of a bridge approach or other location where the risk drain from the roadway down the steep slope next to the of erosion is high. bridge, an erosion problem could develop. A curb is usually

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2 150mm 10mm (A) Vertical Curb 150mm 50mm 130mm 50mm 125mm R75mm (B) (C) (D) 225mm 300mm 300mm 100mm (E) (F) (G) Sloping Curbs Figure 1. Typical AASHTO highway curbs (1). required to channel the runoff into a catch basin or some other When using curbs in conjunction with traffic barriers, such drainage structure. Both the curb and the traffic barrier are as on bridges, consideration should be given to the type and height of barrier. Curbs placed in front of traffic barriers can important functional features of the roadside in this situation. result in unpredictable impact trajectories. If a curb is used in Another similar situation occurs on roadways where a conjunction with a traffic barrier, the height of a vertical curb guardrail is needed to shield a steep roadside slope. Figure 2 should be limited to 100 mm or it should be of the sloping shows a 100-mm high, sloped-face, asphalt curb installed type, ideally, located flush with or behind the face of the bar- rier. Curbs should not be used with concrete median barriers. just in front of the posts of a G4(1S) W-beam guardrail. The Improperly placed curbs may cause errant vehicles to vault site is a 90 km/h rural two-lane roadway in Maine. The curb the concrete median barrier or to strike it, causing the vehi- is placed at this site to provide drainage away from the steep cle to overturn (1). sideslope behind the guardrail and thereby prevent erosion. The erosion would likely weaken the edge of the road, erode AASHTO's policy regarding the use of roadside barriers is the soil from around the guardrail posts and cause slope sta- contained in the Roadside Design Guide (2). The use of curbs bility problems. The curb is therefore necessary for proper in conjunction with traffic barriers is addressed in section drainage. Likewise, the guardrail is necessary for shielding 5.6.2.1 of the Roadside Design Guide: errant motorists from the steep embankment. In such a situ- ation there are few alternatives but to use a curb and traffic Crash tests have shown that use of any guardrailcurb com- bination where high-speed, high-angle impacts are likely barrier combination. should be discouraged. Where there are no feasible alterna- The Green Book limits its guidance on the use of vertical face tives, the designer should consider using a curb no higher than curbs and traffic barriers to the following statement (p. 327): 100 mm (4 in.) and consider stiffening the guardrail to reduce

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3 redirect vehicles, and traffic barriers are designed specifically for that purpose. Combining the two, therefore, might pro- vide cumulative protection to motorists. Unfortunately, the curb's effect on the trajectory of the vehicle is complicated and can often involve transforming longitudinal kinetic energy into hard-to-control vertical and rotational kinetic energy. Researchers in an early California study called the tendency of the curb to launch the vehicle "dynamic jump" (4). Most of the current understanding of vehicle behavior during impact with curbs was developed in full-scale tests performed nearly 40 years ago (4). More recent testing of bridge railings and guardrail-to-bridge rail transitions has added to this knowledge somewhat (5). While the age, vari- ability between tests, and adequacy of the traffic barriers make it difficult to generalize about the results of these tests, it has been generally accepted that when a curb is used in conjunc- tion with a steel post-and-beam traffic barrier, the barrier must be stiffened in some manner to prevent large barrier deflec- tions. In essence, if the barrier deflects too much, the curb can initiate a vertical component of vehicle motion that may launch the vehicle over the barrier. Common methods of stiff- ening the barrier include nesting two sections of W-beam, adding a W-beam on the back of the barrier, adding a rub rail, and reducing the post spacing. The basic objective is to keep the vehicle from contacting the curb by placing the curb behind the barrier face and limiting the deflection of the barrier. Figure 2. Sloping curb installed flush with a strong-post There are three basic types of longitudinal traffic barriers: W-beam guardrail on a 90 km/h two-lane rural roadway rigid, semirigid, and flexible. Rigid barriers are often shaped in Maine. concrete barriers like the F-shape median barrier, the New Jersey barrier, the Ontario tall wall, and so forth. These types of barriers can also function as drainage devices, so there are potential deflection. Other measures that usually prove satis- probably no significant reasons why a curb would be neces- factory are bolting a W-beam to the back of the posts, reduc- ing post spacing, double nesting the rail, or adding a rubrail. sary in conjunction with a concrete barrier. On lower speed facilities, a vaulting potential still exists, but Semirigid barriers include the widely used strong-post since the risk of such an occurrence is lessened, a design W-beam guardrails, which usually deflect laterally less than change may not be cost effective. A case-by-case analysis a meter in NCHRP Report 350 Test Level Three (TL-3) crash of each situation considering anticipated speeds and conse- quences of vehicular penetration should be used (2). tests (2). These barriers are used in nearly every state and account for the vast majority of the installed inventory of road- The AASHTO policy quoted above is used by most states. side hardware (6). These types of barriers are also widely used For example, the Iowa Department of Transportation Design in many states in conjunction with curbs. The use of curbs Manual states: and strong-post W-beam guardrails was a major issue in this research. It is not desirable to use guardrail alongside curbs. Every The flexible barriers include such systems as the weak-post effort should be made to remove fixed objects or relocate three-cable guardrail, the weak-post W-beam guardrail, and them outside the clear zone, instead of using guardrail. If the weak-post box-beam guardrail. These systems are designed there is no other alternative to using guardrail, it may be used to accommodate lateral deflections of as much as 3 m. Because alongside a 4-inch sloped curb, normally with the installation these systems allow large lateral deflections, most vehicles line at the face of the curb. If 6-inch curbs are being used throughout the rest of the project, the curb should be transi- would mount the curb while interacting with the barrier. For tioned to a 4-inch sloped curb throughout the guardrail instal- this reason, the authors believe that it is relatively unusual for lation (3). states to use curbs in conjunction with weak-post guardrails. The issue of combining weak-post barriers and curbs relates At first consideration, combining a curb and a traffic barrier to how far the barrier should be located behind the curb. If might seem to be a reasonable strategy for redirecting errant the barrier is located far enough behind the curb, the vehicle vehicles. Curbs, as discussed above, possess some capacity to can stabilize prior to striking the barrier. An important issue