Operational effects of a paved shoulder in high speed curb sections

Abstract

Right of Way for many suburban highways which require capacity expansion is restricted by adjoining commercial and residential development. This type of restriction sometimes necessitates the use of curb and gutter drainage facilities adjacent to high speed through traffic lanes, eliminating the need for parallel drainage ditches and thus reducing the right of way requirement for the project. These roadways are usually associated with high travel speeds and a high level of access. Currently, no design criteria exist for this type of situation. It was the purpose of this thesis to investigate operational effects of a paved shoulder in high speed curb and gutter sections, in order to establish design recommendations. The operational effects to be evaluated include conflict rates, lane distributions and free flow speeds. Sixteen sites were selected from various geographic locations in the state of Texas for study. These sites included locations both with and without paved shoulders. By observing traffic during the morning and evening peak periods, traffic volumes, conflict rates and lane distributions were determined. Data from sites without paved shoulders was then compared to data from sites with paved shoulders to determine whether the shoulder would have any effect on various measures of effectiveness. These measures included conflict rates, lane distributions and free flow speeds. The results of the thesis indicate a higher conflict rate in those sites without a paved shoulder. Conflict rates in these sites were also much more sensitive to increases in traffic volume than in those sites with a paved shoulder. The data also indicated, for the entire range of traffic volumes, a significantly higher proportion of vehicles in the right lane in those locations with a paved shoulder indicating that the shoulder might act as a buffer from obstructions. Finally, there was no significant difference in free flow speed from one type of cross section to the next. It was recommended that a paved shoulder be provided for average daily traffic volumes in excess of 5000 vehicles per day and that further study be conducted into design procedures as related to lane distributions.