GEOMETRICALLY INDUCED ROUGHNESS AT GRADE BREAKS

Grade breaks are used to make the transition from a secondary roadway elevation to the elevation of the main highway. At the design stage, wrong decisions regarding the geometrical composition of the transition profile may produce an undesirable level of roughness that may be unsafe. Although a variety of instruments can be used to physically measure the road roughness, there are no known standards for analyzing and estimating the level of profile roughness during the design stage. A methodology that uses the International Roughness Index (IRI) as the performance measure of profile roughness is presented. A series of simulation experiments was performed for six types of intersections to examine the relationship between roughness and the intersection design parameters. The results of the experiments show that profile roughness is affected by the transition curve parameters as well as the elevation difference between the main highway and the intersecting secondary roadway. A decision support system called SIDRA (System for Intersection Design with Roughness Analysis) was developed to estimate the roughness of an existing profile or to generate an alternate profile with a lower level of roughness. Statistical analysis showed a close correlation between SIDRA-generated results and data measured in the field. SIDRA-produced values of IRI were also correlated with the serviceability index, a commonly accepted roughness measure.