Document Type



This report is an epidemiologic characterization of motor vehicle crashes on and an evaluation of geometric design criteria for U. S. Highway 666, milepost 92.7 to 93.6. Shiprock, New Mexico, prepared by the District Sanitarian, Office of Environmental Health, Indian Health Service. Motor vehicle related injuries are the leading cause of death on the Navajo Reservation and a topic of major public health concern. A previous motor vehicle crash study which examined a sample of all motor vehicle crashes occurring in 1986 in the Shiprock District, Navajo Division of Public Safety, identified a motor vehicle crash cluster area along a 0.9 mile section of U.S. Highway 666 in Shiprock, New Mexico. Specifically, 23% of all crashes involving injury during this time occurred between milepost points 92.7 and 93.6. This study was intended as a follow-up on a 1986 study, and was designed to more closely focus on the motor vehicle crashes and the road environment on the 0.9 mile section of highway. The objectives of the study were two-fold: 1) to epidemiologically describe the crashes in order to better define their etiologies and 2) to identify particular risk factor. and to determine if four preselected geometric highway design features met appropriate geometric design standards and if the design features contributed to any of the crash occurrences.This study was designed to provide a more detailed description of specific types of motor vehicle crash problems occurring on this section of highway by examining all crashes over a two-year period. The study was also designed to compare four geometric highway design features, which were hypothesized as possible motor vehicle crash contributing factor, against appropriate design standards. The data for the epidemiologic characterization were acquired through the Records and Data Control Section, Navajo Nation Division of Public Safety, Toyei, Arizona. The data were acquired by reviewing all individual police crash reports on file for motor vehicle crashes that occurred on this stretch of U.S. Highway 666 from January 1, 1987 to December 31, 1988. The information from the data collection forms was entered into a computer database management program. The crash data were described by all crashes and by crashes involving injury. The four geometric highway design features selected for comparison to appropriate standards were subjectively chosen by the author based on direct observation of actual crashes, ""near-miss crashes,"" patterns of unusual or unsafe traffic maneuvers, and patterns of traffic conflicts at locations having one or more of the design features. The appropriate standards against which the existing geometric design features were compared were derived from two sources: 1) the American Association of State Highway and Transportation Officials; and 2) the Federal Highway Administration. Three additional measurements were included as part of the study. This study has identified a number of causative and contributing factors associated with the motor vehicle crashes occurring along the 0.9 mile crash cluster area. It is necessary to identify those important risk factors and specific circumstances that can be changed so that remedial efforts can be appropriately prioritized. A considerable problem was identified with rear-end collisions and with collisions involving left turns in front of an oncoming vehicle. Both types of collisions displayed clusters at specific locations again lending themselves to correction by modification of geometric design. Other significant risk factors identified included: 1) a high incidence of crashes between noon and 6:00 PM, particularly on Wednesday; 2) a problem with speeding; 3) a problem with alcohol involvement; and 4) a problem with some drivers having no drivers license. Some of these problems may be alleviated to a certain extent by changes in geometric design. Evaluation of features of the geometric design criteria included: median width, the location and control of side-street cross-traffic, left turn lane availability and positions, median crossover positioning.The majority of the risk factors or design problems are problems with the roadway environment and the proposed solutions to the problems are through environmental modifications. Several problems involve factors intrinsic to the vehicle driver, and as such, the proposed solutions are more in the realm of enforcement modifications or policy changes.

Publication Date



Indian Health Service, Staff Office of Planning, Evaluation and Research, Rockville, MD 20857 (E-93).