COMPARISON OF OPERATING SPEEDS ON DRY AND WET PAVEMENTS OF TWO-LANE RURAL HIGHWAYS

The impact of design parameters and traffic volume on operating speeds of passenger cars is evaluated under free-flow conditions on 322 curved roadway sections of two-lane rural highways in New York state. The design parameters considered are degree of curve, length of curve, superelevation rate, gradient, sight distance, lane width, shoulder width, posted speed, and average annual daily traffic. For the evaluation of the quantitative effects of these factors on operating speeds, expressed herein by the 85th-percentile speeds, the multiple linear stepwise regression technique was used. The various stages of analyses revealed that degree of curve was the best available single-variable predictor of operating speeds on dry pavements. Other variables helped the regression model, but the equation did well even without them. Effect of wet pavements on 85th-percentile speeds of passenger cars were also examined. Analyses were performed using data from a total of 24 curved roadway sections. Ample evidence exists to indicate that wet pavement does not have a great effect on operating speed, and that drivers will not adjust their speeds sufficiently to accommodate inadequate wet pavement on curves in particular. Furthermore, results of the statistical analyses indicate that the relationship between operating speed and degree of curve, developed from speed data collected on dry pavements, is also valid for wet pavement conditions so long as visibility is not affected appreciably by heavy rain. It is obvious that the drivers do not recognize the fact that friction supply is significantly lower on wet pavements as compared with dry. For the implementation of the results for design purposes, recommendations for achieving consistency and detecting inconsistencies in horizontal alignment, as well as recommendations for harmonizing design speed and operating speed, as related to wet pavement conditions, were given for good, fair, and poor design practices. These tasks are important in modern highway design and redesign strategies for improving traffic safety.