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5 CHAPTER 2 LITERATURE REVIEW INTRODUCTION between redirective behavior and mounting behavior. Com- binations of impact speed and angle falling to the left of the Assessing the safety effectiveness of curbs attracted a con- curve would result in redirection, and those falling to the siderable amount of attention in the early decades of roadside right would result in mounting the curb. safety research. Curbs were thought to be a low-cost method The boundary between redirection and mounting can be of keeping vehicles on the roadway for at least some impact described by K = V sin where V is the impact velocity and conditions. In 1953 the California Division of Highways per- is the impact angle. In essence, this expression indicates that formed a series of 149 full-scale tests on 11 different types of a given curb will redirect the vehicle when the lateral compo- curb geometries in order to assess the safety effectiveness of nent of the impact velocity is less than some characteristic curbs (4). This test series was followed in 1955 by another value. In his 1973 study of barrier curbs, Dunlap found that series of tests using the four best-performing curbs from the the characteristic lateral component of velocity for the Trief first series (7). The conclusion of the researchers was that bar- curb was 5 km/h and for the Elsholz curb was 14.6 km/h; thus, rier curbs, i.e., vertical curbs, should be at least 10 inches high, the Elsholz curb was more effective at redirecting vehicles have undercut faces, and have a relatively smooth surface tex- than the Trief curb (11). ture. Other similar but less extensive studies were performed Dunlap attempted to extend this basic methodology by in Canada, Germany, and the United Kingdom (8, 9, 10). treating the impact speed and angle as a random probabilis- These early crash tests formed the basis of the AASHTO tic variable along with the vehicle type. If the distribution policy described in Chapter 1. Although the vehicle fleet has of encroachment angles and vehicle speeds for a particular changed considerably since the time of these early studies, roadway is known, the percent of vehicles that would be redi- the current version of the AASHTO Green Book contains rected by each type of curb can be estimated (11). Dunlap substantially the same recommendations as the 1965 Green used data from a specific roadway in Michigan for the speed Book regarding the use of curbs. distribution and the Hutchinson-Kennedy encroachment data The methods that have been employed for analyzing the for the impact angle distribution (12). For the specific site in safety effectiveness of curbs in earlier research included ana- Michigan, Dunlap found that the Elsholz curb could be lytical methods, vehicle dynamics codes, and full-scale crash expected to redirect 70% of the impacting vehicles and the testing. Each of these methods is discussed in the following Trief curb could only be expected to redirect 27%. sections. Information from selected studies from previous Unfortunately, the curb characteristic lateral component of research on curbs and curbbarrier combinations are also pro- velocity is also a function of the characteristics of the vehi- vided, followed by a summary of the literature review. cle that strike the curb and the type of curb. Some vehicles will have geometric, suspension, and handling characteristics more prone to mounting the curb than other vehicles. A curb's ability to redirect a vehicle depends not only upon the speed ANALYSIS METHODS APPLIED IN and angle of impact, but also upon the dimensions of the THE STUDY OF CURB SAFETY curb, the surface material of the curb, if it is wet or dry, and Analytical Methods the radius of the impacting tire. The boundary line between mounting and redirection shown in Figure 3, therefore, is Most analytical work regarding vehicle impact into curbs only valid for a single type of test vehicle impacting a spe- has been concerned with either redirectional capabilities of cific type of curb. The dramatic changes in vehicle charac- vertical face curbs or their potential to cause rollover. If the teristics over the past decade seriously limit the validity of impact speed and angle are plotted on a graph and different the findings of these early studies. The vehicles of today are symbols used to denote redirection and mounting, then a lighter, have higher centers of gravity, and have lower pro- curve like Figure 3 can be developed. Figure 3 shows the file tires. In addition, the passenger vehicle population has characteristics of two particular experimental curbs, the Trief become much more diverse, now including pickup trucks, and Elsholz curbs (9, 10). The line describes the boundary large and small sport utility vehicles (SUVs), and minivans,

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6 1 N 3.5 Vr sin CD h = r 50 where h is the height of the curb required to redirect the impact- ing vehicle, r is the radius of the tire in millimeters, Vr is the speed at redirection, is the impact angle, N is the coefficient of friction of smooth rubber on test surface, and CD is the coefficient of friction of smooth rubber on dry concrete. Note that the required height of the curb increases as the radius of the tires increases, the velocity of the vehicle increases, the angle of impact increases, or the friction coefficient increases. Vehicle Dynamics Codes The first computer simulation program used for the analy- sis of vehicle-curb impacts was the Cornell Aeronautical Lab- oratory Single Vehicle Accident program (CALSVA), devel- oped in the 1960s (14). It was used in the early 1970s by Wayne State University and the Highway Safety Research Institute (HSRI) at the University of Michigan to determine the redirection capability of various curb configurations (15). The CALSVA program, developed by Cornell Aeronautical Laboratory, was only capable of simulating a limited range of impact scenarios because of the simplicity of the program; however, it did serve as a precursor to more advanced com- puter simulation codes. The second generation version of CALSVA was the High- way Vehicle-Object Simulation Model (HVOSM) (16). This Figure 3. Performance characteristics of the Trief program has been used extensively in conjunction with full- and Elsholz curbs (9). scale crash testing to study vehicle dynamics during impact with curbs. A comprehensive review of these studies will be presented in subsequent sections of this chapter. The vehicle dynamics code VDANL (Vehicle Dynamics as well as the traditional passenger car. Some of these vehi- Analysis, Non Linear) was developed in the 1980s by the cle types have proven to be less stable in collisions with traf- NHTSA and Systems Technology, Inc. (STI). It is a com- fic barriers than traditional passenger cars. While the testing prehensive vehicle dynamics simulation program that runs done over the past 40 years provides some interesting insights, on a PC in a Windows environment. It was designed for the the results must be viewed carefully since the vehicle popu- analysis of passenger cars, light trucks, articulated vehicles, lation of today is much different than it was during the 1960s. and multipurpose vehicles, and it has been upgraded over the An analytical study on the safety of roadside curbs was years to expand and improve its capabilities. It now permits conducted by Navin and Thomson at the University of British analysis of driver-induced maneuvering up through limit per- Columbia in 1997 (13). They developed the following empir- formance conditions defined by tire saturation characteris- ical relationships to estimate the ability of a dry concrete curb tics, as well as driver feedback control features. to safely redirect a vehicle based on the findings produced in VDANL was chosen by the FHWA for use in the Interactive previous research: Highway Safety Design Model (IHSDM) (17). The IHSDM