National Academies Press: OpenBook

Recommended Guidelines for Curb and Curb-Barrier Installations (2005)

Chapter: Chapter 1 - Introduction

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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2005. Recommended Guidelines for Curb and Curb-Barrier Installations. Washington, DC: The National Academies Press. doi: 10.17226/13849.
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Page 1
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2005. Recommended Guidelines for Curb and Curb-Barrier Installations. Washington, DC: The National Academies Press. doi: 10.17226/13849.
×
Page 2
Page 3
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2005. Recommended Guidelines for Curb and Curb-Barrier Installations. Washington, DC: The National Academies Press. doi: 10.17226/13849.
×
Page 3
Page 4
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2005. Recommended Guidelines for Curb and Curb-Barrier Installations. Washington, DC: The National Academies Press. doi: 10.17226/13849.
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Page 4

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

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

potential deflection. Other measures that usually prove satis- factory are bolting a W-beam to the back of the posts, reduc- ing post spacing, double nesting the rail, or adding a rubrail. On lower speed facilities, a vaulting potential still exists, but since the risk of such an occurrence is lessened, a design change may not be cost effective. A case-by-case analysis of each situation considering anticipated speeds and conse- quences of vehicular penetration should be used (2). The AASHTO policy quoted above is used by most states. For example, the Iowa Department of Transportation Design Manual states: It is not desirable to use guardrail alongside curbs. Every effort should be made to remove fixed objects or relocate them outside the clear zone, instead of using guardrail. If there is no other alternative to using guardrail, it may be used alongside a 4-inch sloped curb, normally with the installation 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- tioned to a 4-inch sloped curb throughout the guardrail instal- lation (3). At first consideration, combining a curb and a traffic barrier might seem to be a reasonable strategy for redirecting errant vehicles. Curbs, as discussed above, possess some capacity to 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. There are three basic types of longitudinal traffic barriers: rigid, semirigid, and flexible. Rigid barriers are often shaped 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 probably no significant reasons why a curb would be neces- sary in conjunction with a concrete barrier. Semirigid barriers include the widely used strong-post W-beam guardrails, which usually deflect laterally less than a meter in NCHRP Report 350 Test Level Three (TL-3) crash tests (2). These barriers are used in nearly every state and account for the vast majority of the installed inventory of road- side hardware (6). These types of barriers are also widely used in many states in conjunction with curbs. The use of curbs and strong-post W-beam guardrails was a major issue in this research. The flexible barriers include such systems as the weak-post three-cable guardrail, the weak-post W-beam guardrail, and the weak-post box-beam guardrail. These systems are designed to accommodate lateral deflections of as much as 3 m. Because these systems allow large lateral deflections, most vehicles would mount the curb while interacting with the barrier. For this reason, the authors believe that it is relatively unusual for states to use curbs in conjunction with weak-post guardrails. The issue of combining weak-post barriers and curbs relates to how far the barrier should be located behind the curb. If the barrier is located far enough behind the curb, the vehicle can stabilize prior to striking the barrier. An important issue Figure 2. Sloping curb installed flush with a strong-post W-beam guardrail on a 90 km/h two-lane rural roadway in Maine.

in this research was the lateral encroachment distance neces- sary for a vehicle to stabilize after impacting a curb at high- way speeds. PROJECT OBJECTIVES The primary goal of this research was to develop design guidelines for using curbs and curb–barrier combinations on roadways with operating speeds greater than 60 km/h. The guidelines took into account the following factors: • Curb type, height, configuration, material, vertical reveal, and distance from edge of traveled way. • Purposes of curb: aesthetics, hydraulics, delineation, access control, pedestrian refuge, protection of local envi- ronment, water quality, and historical preservation. • For curb–barrier combinations, barrier type (i.e., flexi- ble, rigid, and semirigid), height, configuration, distance from edge of traveled way, distance from curb, and end treatment. • Roadside characteristics, including the surface behind curbs, such as grass, sidewalks, pavement, or sideslope. • Environment. • Area characteristics (e.g., suburban or rural). • Climatic conditions (e.g., snow or heavy rains). 4 • Traffic characteristics, including speed, vehicle mix, and volume. • Roadway alignment. • Facility type (e.g., parkway, arterial, or freeway). • Cross-section (e.g., median, number of lanes, shoulder, and roadside). There were essentially two complementary objectives of this research: (1) determining the safety effectiveness of dif- ferent types of curbs and (2) determining the proper combi- nation and placement of curbs and barriers such that traffic barriers remain effective. The first phase of the project involved an in-depth review of published literature in order to identify information perti- nent to the design, safety, and function of curbs and curb– barrier combinations on roadways with operating speeds greater than 60 km/h (37 mph). Computer simulation meth- ods were used in a parametric investigation involving vehi- cle impact with curbs and curb–barrier combinations to deter- mine which types of curbs are safe to use on higher-speed roadways and to determine proper placement of a barrier with respect to curbing such that the barrier remains effective in safely containing and redirecting the impacting vehicle. The results of the study were then synthesized and guidelines for the use of curbs and curb-and-barrier systems were developed.

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Recommended Guidelines for Curb and Curb-Barrier Installations Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 537: Recommended Guidelines for Curb and Curb–Barrier Installations presents the findings of a research project to develop guidelines for the use of curbs and curb–guardrail combinations on high-speed roadways. The report includes recommendations concerning the location of curbs with respect to the guardrail for various operating speeds.

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