FRICTION COEFFICIENTS. ANALYSIS OF A STRETCH OF HIGHWAY WITH REGARD TO ACCIDENT FREQUENCY. A STUDY BY BETONSTRASSEN AG

IN 1974 ON THE ZURICH-BERNE MOTORWAY BETWEEN ROTHRIST AND DIETIKON, SRT (FRICTION) COEFFICIENTS WERE OBTAINED AND BETWEEN 1970 AND 1977 DYNAMIC MEASUREMENTS WERE MADE WITH THE LOCKED WHEEL TRAILER. THESE MEASUREMENTS WERE CARRIED OUT ON CONCRETE SURFACES FOR A 38.9 KM LENGTH AND ON A BITUMINOUS SURFACING FOR 8.5 KM. THE ACCIDENTS ON WET CARRIAGEWAYS WERE COMPARED WITH THE GROWTH IN THE TRAFFIC VOLUME. THE AIM WAS TO COMPARE THE CHANGE OVER A PERIOD OF TIME IN THE FRICTION COEFFICIENTS AND THE ACCIDENT FREQUENCY. THE FRICTIONAL COEFFICIENT OBTAINED FIRST DROPPED, AND AFTER REMAINING AT AN ALLOWABLE MINIMUM VALUE, SLOWLY INCREASED. THE GRADIENTS (EXPRESSED AS DELTA MY/YEAR OR DELTA MY/MILLION VEHICLES) ARE DEPENDENT ON THE INITIAL ROUGHNESS DEPTH, THICKNESS OF WEARING COURSE, TRAFFIC DENSITY AND AVERAGE SPEED. THE RELATION BETWEEN THE ACCIDENT FREQUENCY AND THE SURFACE ROUGHNESS AND THE TRANSITION BETWEEN TWO TYPES OF SURFACING CANNOT BE ESTABLISHED. HOWEVER, IN THE WESTERN DIRECTION FOUR ACCIDENT BLACK SPOTS AND IN THE OPPOSITE DIRECTION FIVE BLACK SPOTS WERE IDENTIFIED AT POINTS WITH CHANGES IN CROSS FALL; ON THE OTHER HAND THERE WAS NO DIFFERENCE IN THE TYPE OF SURFACING IN THE CASE OF ACCIDENT FREQUENCY ON WET PAVEMENTS. PLASTIC RIDGES FOR SURFACE DEWATERING SHOULD CONTRIBUTE TO A REDUCTION IN ACCIDENTS, EVEN WHEN DRIVING COMFORT IS REDUCED IN FAVOUR OF TRAFFIC SAFETY. SOME EXPERIENCE WAS ALSO REPORTED REGARDING GROOVES IN THE LONGITUDINAL OR TRANSVERSE DIRECTION. THE RESTRICTION IN MAXIMUM SPEED (100 KM/HR FROM NOVEMBER 1973 AND 130 KM/H FROM MARCH 1974) HAS OBVIOUSLY CONSIDERABLY REDUCED THE DANGER OF ACCIDENTS. IN ADDITION, IT WAS ESTABLISHED THAT THE TRAFFIC SAFETY OF A STRETCH OF HIGHWAY SHOULD NOT BE DETERMINED FROM A COLLECTION OF MEASUREMENT DATA, BUT THAT THE EXPERIENCES OF THE TRAFFIC POLICE SHOULD BE CALLED UPON.