STATISTICAL DETERMINATION OF EFFECT OF PAVED SHOULDER WIDTH ON TRAFFIC ACCIDENT FREQUENCY

THIS INVESTIGATION REPRESENTS RESEARCH BY THE OREGON STATE HIGHWAY DEPARTMENT IN THE USE OF STATISTICS TO EXPLAIN HOW THE WIDTH OF PAVED SHOULDERS ON LEVEL AND TANGENT RURAL TWO- LANE HIGHWAYS AFFECTS ACCIDENT FREQUENCY. TWO DIFFERENT APPROACHES WERE TAKEN. CORRELATION PROCEDURES WERE USED TO EVALUATE THE RELATIONSHIP BETWEEN PAVED SHOULDER WIDTH AND ACCIDENT OCCURRENCE, AND VARIANCE MEASURES WERE EMPLOYED TO ANALYZE THE DIFFERENCE BETWEEN THE AVERAGE ACCIDENT FREQUENCY ON SECTIONS WITH NARROW PAVED SHOULDERS (4 FT OR LESS) AND THE AVERAGE ACCIDENT FREQUENCY ON SECTIONS WITH WIDE PAVED SHOULDERS (8 FT OR MORE). THE PARTIAL CORRELATION TECHNIQUE ESTABLISHED THAT WHEN THE EFFECTS OF OTHER ROADWAY ELEMENTS WERE ELIMINATED AND THE SECTIONS GROUPED IN VARIOUS ADT RANGES, NO SIGNIFICANT RELATIONSHIP BETWEEN ACCIDENT FREQUENCY AND PAVED SHOULDER WIDTH WAS EVIDENT EXCEPT IN THE 2,000-2,999 ADT RANGE WHERE PROPERTY DAMAGE AND TOTAL ACCIDENTS SHOWED A SIGNIFICANT TENDENCY TO INCREASE IN FREQUENCY AS PAVED SHOULDER WIDTH INCREASED. THE ANALYSIS OF CO-VARIANCE PROCEDURE ESTABLISHED THAT WHEN THE EFFECT OF ADT WAS CONTROLLED THERE WAS A SIGNIFICANTLY HIGHER MEAN NUMBER OF PROPERTY DAMAGE AND TOTAL ACCIDENTS ON SECTIONS WITH WIDEPAVED SHOULDERS THAN THERE WAS ON SECTIONS WITH NARROW PAVED SHOULDERS IN THE 1,000-5,600 ADT RANGE. THE RESULTS OF THIS STUDY SHOULD BE INTERPRETED WITH EXTREME CAUTION, INASMUCH AS THE TRAFFIC VOLUMES ON THE BULK OF THE SECTIONS WERE LESS THAN 5,000 VEHICLES PER DAY. FOR THIS REASON, IT WOULD BE ERRONEOUS TO GENERALIZE THAT IN ALL CASES THE NUMBER OF ACCIDENTS WILL BE HIGHER ON SECTIONS WITH WIDE PAVED SHOULDERS THAN ON THOSE WITH NARROW PAVED SHOULDERS. ON THE OTHER HAND, IT CANNOT BE SHOWN THAT INCREASING THE WIDTH OF THE PAVED SHOULDER IS ACTUALLY HELPFUL IN REDUCING THE ACCIDENT FREQUENCY ON LEVEL AND TANGENT RURAL TWO-LANE HIGHWAYS WITH TRAFFIC VOLUMES IN THE RANGES STUDIED. /AUTHOR/