Driving Radius Impact on Interchange Ramps

Abstract

The geometric elements of interchange ramps are selected to provide comfortable and safe passage. In previous studies, field measurements on interchange ramps revealed that the lateral acceleration experienced by drivers was lower than the design value derived from the basic curve equation for a specific radius, superelevation, and speed. This study aims to explain the deviations using the geometric criteria, noting that the discrepancy between the recorded lateral acceleration and the design value is because of the driving (vehicle path) radius and the critical geometric and cross-sectional elements of the horizontal curve (radius, superelevation, deflection angle, curvature change rate, lane width, number of lanes and directions). The sample consists of over 160 drivers with various characteristics (e.g., age group, experience, gender). The measurements were conducted on eight interchange ramp curves. The presented diagrams correlate either the driving or the differential radius with the critical geometric elements of the curves and highlight the significance of each. New equations are established for direct computation of the driving and the differential radius, which can contribute to the selection of the minimum radius values and the speed limit in dry pavement conditions for each new interchange ramp and the differentiation of the current values when incorporated into the design guidelines. The comfort, tolerance, and safety limits of driving and differential radius are established based on design radius, complementing established limits for lateral acceleration and longitudinal deceleration in interchange ramps, to encourage design guidelines to revise the current manuals under the framework of performance-based design.