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No Left Turn: ‘Superstreet’ Traffic Design Improves Travel Time, Safety

The so-called “superstreet” traffic design results in significantly faster travel times, and leads to a drastic reduction in automobile collisions and injuries, according to North Carolina State University researchers who have conducted the largest-ever study of superstreets and their impacts.

Superstreets are surface roads, not freeways.  It is defined as a thoroughfare where the left-hand turns from side streets are re-routed, as is traffic from side streets that needs to cross the thoroughfare. In both instances, drivers are first required to make a right turn and then make a U-turn around a broad median. While this may seem time-consuming, the study shows that it actually results in a significant time savings since drivers are not stuck waiting to make left-hand turns or for traffic from cross-streets to go across the thoroughfare.

"Superstreet" traffic designs result in faster travel times and significantly fewer accidents, according to the new study.

“The study shows a 20 percent overall reduction in travel time compared to similar intersections that use conventional traffic designs,” says Dr. Joe Hummer, professor of civil, construction and environmental engineering at NC State and one of the researchers who conducted the study. “We also found that superstreet intersections experience an average of 46 percent fewer reported automobile collisions – and 63 percent fewer collisions that result in personal injury.”

The researchers assessed travel time at superstreet intersections as the amount of time it takes a vehicle to pass through an intersection from the moment it reaches the intersection – whether traveling left, right or straight ahead. The travel-time data were collected from three superstreets located in eastern and central North Carolina, all of which have traffic signals. The superstreet collision data were collected from 13 superstreets located across North Carolina, none of which have traffic signals.

The superstreet concept has been around for over 20 years, but little research had been done to assess its effectiveness under real-world conditions. The NC State study is the largest analysis ever performed of the impact of superstreets in real traffic conditions.

A paper on the travel time research is being presented Jan. 24 at the Transportation Research Board Annual Meeting in Washington, D.C. The paper is co-authored by Hummer, former NC State graduate students Rebecca Haley and Sarah Ott, and three researchers from NC State’s Institute for Transportation Research and Education: Robert Foyle, associate director; Christopher Cunningham, senior research associate; and Bastian Schroeder, research associate.

The collision research was part of an overarching report of the study submitted to the North Carolina Department of Transportation (NCDOT) last month, and is the subject of a forthcoming paper. The study was funded by NCDOT.

NC State’s Department of Civil, Construction and Environmental Engineering is part of the university’s College of Engineering.

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Note to Editors: The abstract of the travel time paper follows.

“Operational Effects of Signalized Superstreets in North Carolina”

Authors: Rebecca L. Haley, Sarah E. Ott, Joseph E. Hummer, Robert S. Foyle, Christopher M. Cunningham, Bastian J. Schroeder, North Carolina State University

Presented: Jan. 24, 2011, at the Transportation Research Board Annual Meeting in Washington, D.C.

Abstract: Arterials across the US are operating inefficiently and are becoming increasingly congested due to growing traffic demand. Agencies tasked with improving these arterials are running out of good solutions. Superstreets, called restricted crossing u-turns by the FHWA, are part of a menu of unconventional arterial designs that may provide promising solutions. Up to this point, there is little valid information available on the operational effects of superstreets, as study results have been from macroscopic analyses, and simulations of hypothetical arterials. The purpose of this research was to determine the operational effects of the superstreet treatment on existing signalized arterials in North Carolina. The operational analysis involved calibrating and validating VISSIM models of three existing signalized superstreets in North Carolina – two isolated intersections and one five-intersection superstreet corridor. After some adjustments, the team produced VISSIM models that provided travel times within an acceptable range of the field data collected. Results from the three calibrated VISSIM models were compared to results from models of equivalent conventional sites at various volume levels using travel time as the primary measure of effectiveness. The superstreet outperformed the conventional design at each location studied, reducing the overall average travel time per vehicle traveling through the intersection. The travel time savings and extra capacity at higher volumes can buy agencies more years of acceptable operation before intersection improvements, or even interchanges, are necessary.

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