Modeling Through Traffic Speed at Roundabouts Along Urban and Suburban Street Arterials

Al-Omari, Bashar H.; Ghuzlan, Khalid A.; Al-Helo, Lina B. · 2014 · Crossref

DOI: 10.5399/osu/jtrf.53.2.4234

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Summary

This study addresses the need to quantify the effectiveness of roundabouts as traffic calming measures by modeling through traffic speeds along urban and suburban street arterials. While roundabouts are widely used for their safety benefits and low cost, existing international models, such as those from the American Association of State Highway and Transportation Officials (AASHTO) and studies from Italy, have shown limitations in accurately predicting circulating speeds in different contexts. The research aims to develop empirical models that relate roundabout circulating speed to specific geometric and operational characteristics, thereby providing traffic engineers with more accurate tools for design and speed management. The researchers collected field data from 30 roundabouts located in three major cities in Jordan: Amman, Zarqa, and Irbid. To ensure data quality, sites were selected based on good pavement conditions, level alignments, and the absence of nearby traffic calming measures. Geometric variables measured included entry width, internal and external circle diameters, circulatory roadway width, super-elevation, entry deviation angle, and drive curve. Operational data included the upstream approach free flow speed (FFS) and area type (urban or suburban). Speed data were collected using laser radar guns during off-peak periods under free-flow conditions, measuring 100 leading passenger cars at the midpoint of the circulatory roadway for each roundabout. Stepwise regression analysis was then employed to identify significant predictors and develop models for both average and 85th percentile circulating speeds. The analysis revealed that entry width, internal circle diameter, entry deviation angle, upstream approach FFS, and drive curve were the significant determinants of circulating speed. The resulting models demonstrated high explanatory power, with adjusted R-squared values of 0.87 for the 85th percentile speed and 0.92 for the average speed. The average speed model was found to be statistically stronger than the 85th percentile model. The findings indicate that circulating speed is directly proportional to upstream FFS, entry width, internal diameter, and drive curve, but inversely proportional to the entry deviation angle. Variables such as exit width, circulatory roadway width, and super-elevation were not significant predictors. The significance of this research lies in its validation of locally derived models against established international standards. When compared to the AASHTO equation and the Bassani and Sacchi Italian model, the proposed models provided significantly more accurate predictions. Statistical tests confirmed that the AASHTO model underestimated speeds while the Italian model overestimated them, whereas the new models closely matched observed data. The study concludes that traffic engineers should utilize the average circulating speed model for more accurate estimates. It also highlights that differences in model performance may stem from varying driver behaviors between developed and developing countries, as well as the inclusion of specific geometric factors like the drive curve and entry deviation angle in the new models.

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• intersection design