Modelling operating speed changes between two successive geometric elements along two-lane rural roads
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The study was aimed at developing models to predict the speed changes between two successive geometric elements along two-lane rural roads. This was necessary to operationalize the criteria used in the technical audit of alignment design in Uganda’s Geometric Design Manual, (2010) as adapted from the American Association of State Highways and Transportation Officials (AASHTO), (2004). The data used in model development were collected at 21 transition sections along Kampala-Mityana-Mubende, Kampala-Luwero-Kafu, and Mbale-Sironko highways. The speed was collected by using the MH Corbin 350 Blue-star Automatic Traffic Analyzer device placed in the center of the lane. The geometric characteristics were; curve length, curve radius, approach tangent length, curve grade, tangent grade, shoulder width, and direction of curvature which were obtained by either physical measurement and/or existing geometric designs acquired from the Uganda National Roads Authority (UNRA). The sample size of individual vehicle speeds at respective transition sections ranged between 90–120 vehicles for Passengers and 30–45 for Heavy Vehicles from which the 85th percentile change in speed was determined and used to estimate the model using data from 16 out of the 21 transition sections. The Heavy vehicles model had an adjusted R-squared of 0.556 and a standard error of 4.7kph with curve length, tangent length, curve grade, shoulder width, lane width, and curve direction as the significant variables at a 95% confidence level. On the other hand, the model for Passenger vehicles had an adjusted R-squared of 0.891 and a standard error of 2.4kph with curve length, curve radius, tangent grade, tangent length, curve grade, shoulder width, lane width, and curve direction as the significant variables at 99% confidence level. It was observed that clockwise curves lead to a speed reduction of 9.12kph and 5.07kph more than the reduction required by a vehicle along anti-clockwise curves for Passenger and Heavy vehicles respectively. Also, the speed reduction reduces with an increase in the shoulder and lane width. However, increasing the tangent and curve were found to increase the reduction in the 85th percentile speed. The study recommends varying lane and shoulder width as the roadway approaches a curve to provide additional space requirements to avoid encroaching on shoulders meant for pedestrians, cyclists, and emergency stoppage. Finally, the study also suggests that studies to investigate the friction characteristic be undertaken to enable practitioners to fully operationalize all the three consistency criteria stated in the Geometric Design Manuals.