Development of design guidelines for two-lane, two-way rural highways in Texas

Abstract

Passing lanes on two-lane, two-way rural highways have a significant effect on their operational performance. The operational performance of a two-lane, two-way rural highway is often measured with percent time delay. At moderate to high Average Annual Daily Traffic (AADT) values, two-lane, two-way rural highways with no passing lanes often tend to experience operational difficulties due to inadequate passing opportunities leading to an increase in the percent time delay and also increasing the risk of crashes due to potential dangerous overtaking maneuvers attempted by some motorists. This research focused on obtaining the optimal design length and spacing of passing lanes on two-lane, two-way rural highways, for various two-way volumes. Research was conducted using the microscopic computer simulation package TWOPAS, an FHWA software that simulates two-lane, two-way highways under a wide variety of conditions. The measure of effectiveness (MOE) used for this research was percent time delay. The simulation results indicated that percent time delay increased with increasing two-way volume and spacing between passing lanes, and the percent time delay decreased with increasing passing lane length and percentage of roadway four-laned. However, there seemed to be no specific relationship between percent time delay and truck percentage in the traffic stream. During the research it was also discovered that TWOPAS did not perform as expected with regards to increasing truck percentage in the traffic stream. It was observed that at high volume conditions and certain road configurations the percent time delay value actually decreased with increasing truck percentage. A discussion of this finding is also included in the thesis. Having established the relationship between percent time delay and two-way volume, percent trucks in the traffic stream, length of passing lane section, spacing of passing lane section and percentage of roadway four-laned, two models were developed which were then used to arrive at the optimal length and spacing of passing lanes for given two-way volumes. This research dealt with passing lanes on two-lane roads in flat terrain and did not consider the effects of grades, both horizontal and vertical, and as such the percent time delay values were found to be lower than might be expected in field. This thesis also includes recommendations for issues to be addressed in future research.