Analyzing the effect of passing-lanes on rural two-lane highway operations

Abstract

Two-lane highways make up 80% of roads in this country. Rural two-lane highways are unique in they are typically characterized by high-speeds and low volumes. Level-of-service is associated with the proportion of vehicles stuck in platoons, or the percentage of vehicles impeded by a slow-moving vehicle. The Highway Capacity Manual 2000 (HCM 2000) recommends determining the percentage of vehicles stuck in a platoon using a performance measure called Percent Time Spent Following (PTSF). Because PTSF is nearly impossible to measure in the field, the HCM has proposed two methods for estimating its value. However, there are inconsistencies between the two methods. This limitation has led researchers to seek more appropriate performance measures for two-lane highways. When performance on these highways deteriorates to the point of needing an upgrade, the addition of a passing-lane is considered. A passing-lane is an extra lane in one or both directions of travel. The current design standards for passing-lanes (length and frequency) are based on simulation research conducted more than 20 years ago. The purpose of this thesis is to analyze traffic stream characteristics within and around a passing-lane as to improve design standards. Three appropriate performance measures were used to analyze data from two passing-lane sites in Montana. The purpose of this analysis was to determine passing lane effective length. The results showed this value may be greater than 6.6 miles. Lane use within a passing-lane section was also investigated. Traffic counters were placed at pre-determined locations within a passing-lane and data were recorded. Analysis of the data showed that aggressive drivers in the left lane are able to drive at 10 miles-per-hour faster than they were traveling as they entered the three-lane section. Further analysis showed that nearly all passing maneuvers of slower vehicles were conducted in the first 1/2-mile of the passing-lane. This thesis found the most appropriate performance measures for investigating passing-lanes are Percent Followers, Percent Impeded, and Follower Density. Also, the required spacing of passing-lanes on a two-lane highway may be farther than the current standards recommend. Lastly, for the site investigated a 3/4-mile long passing-lane would have been adequate.