PREDICTING TRAFFIC ACCIDENTS FROM ROADWAY ELEMENTS OF RURAL TWO-LANE HIGHWAYS WITH GRAVEL SHOULDERS

THE INVESTIGATION DESCRIBED IN THIS REPORT REPRESENTS RESEARCH BY THE OREGON STATE HIGHWAY DEPARTMENT TO DEVELOP EQUATIONS WHICH CAN BE USED TO PREDICT ACCIDENTS ON RURAL 2-LANE HIGHWAYS FROM ROADWAY ELEMENTS SUCH AS ADT, LANE WIDTH, SHOULDER WIDTH, SLIGHT DISTANCE RESTRICTIONS, COMMERCIAL AND RESIDENTIAL DRIVEWAYS, AND INTERSECTIONS. A SAMPLE OF NEARLY 1,400 MILES OF 2-LANE HIGHWAYS WAS UTILIZED. THE DATA WERE ANALYZED THROUGH THE USE OF MULTIPLE CORRELATION TECHNIQUES. THE RESULT OF THE ANALYSIS IS A SERIES OF EQUATIONS WHICH CAN BE USED TO PREDICT TOTAL ACCIDENTS ON RURAL 2-LANE HIGHWAYS IN OREGON. THE IMPORTANT CONCLUSIONS WHICH CAN BE DRAWN FROM THE FINDINGS OF THE STUDY ARE AS FOLLOWS: (1) MOTOR VEHICLE ACCIDENTS ARE DIRECTLY RELATED TO VEHICLE VOLUMES AND CERTAIN PHYSICAL FEATURES OF THE HIGHWAY. THIS RELATIONSHIP IS STRONG ENOUGH IN THE HIGHER ADT RANGES TO MAKE POSSIBLE REASONABLE ACCURATE PREDICTIONS OF TOTAL ACCIDENTS ON THE BASIS OF KNOWN PHYSICAL FEATURES. (2) ACCESS TO THE HIGHWAY THROUGH DRIVEWAYS OR INTERSECTIONS IS DIRECTLY RELATED TO ACCIDENTS AT ALL ADT LEVELS. THE NUMBER OF ACCESS POINTS IS A REASONABLY GOOD PREDICTIVE INDEX OF THE NUMBER OF POTENTIAL ACCIDENTS WITHIN AN ADT GROUP. (3) ALTHOUGH THE HIGHWAY DESIGN ELEMENTS SUCH AS LANE WIDTH, SHOULDER WIDTH, AND SIGHT DISTANCE RESTRICTION ARE RELATED TO ACCIDENTS, THEY DO NOT ORDINARILY SERVE AS GOOD PREDICTORS OF THE NUMBER OF ACCIDENTS. GENERALLY SPEAKING, WIDER LANES, WIDER SHOULDERS, AND UNIMPAIRED SIGHT DISTANCE RESULT IN A SAFER HIGHWAY. (4) AN ANALYSIS OF THE DATA PRESENTED IN THIS REPORT CONFIRMS THE THEORY THAT ACCIDENTS ARE ESSENTIALLY CHANCE OCCURRENCES RESULTING FROM ERRORS IN JUDGMENT. THE NUMBER OF ACCIDENTS INCREASES WITH THE NUMBER OF SITUATIONS PRESENTING A CHANGE IN CONDITIONS, AND THEREFORE REQUIRING A DECISION ON THE PART OF THE MOTOR VEHICLE OPERATOR. THESE DATA CONFIRM THIS THEORY IN THE FOLLOWING WAYS: (1) ACCIDENTS ON LOW VOLUME ROADS DO NOT APPEAR TO BE RELATED TO ANY ROADWAY FEATURE, (2) ACCIDENTS INCREASE WHEN: (1) VEHICLE VOLUME INCREASES (2) ACCESS POINTS INCREASE, (3) SIGHT DISTANCE IS IMPAIRED, (4) THE CROSS-SECTION IS REDUCED. /AUTHOR/