Rounding On a Solution
Highway officials have identified the installation of roundabouts—circular intersections in which drivers enter and exit through right turns—as another way to improve traffic safety. According to Tom Mannino, project manager with St. Louis-based engineering firm Horner & Shifrin Inc., roundabouts offer a range of benefits.
“In a roundabout, cars are approaching at a slower speed; therefore, drivers have more time to react to pedestrians, and pedestrians also have more time to react to vehicles,” said Mannino. Collisions at roundabouts typically are less severe and frequent than at traditional intersections. In a December 2002 report, the Maryland Highway Administration indicated that 15 single-lane roundabouts had significantly improved safety at those intersections; analysis showed a 100% decrease in fatal crashes, 82% decrease in injury crashes, and a 60% reduction in overall crashes.
In O'Fallon, Ill., town officials harbored concerns about the intersection of State Street, an arterial roadway, and North Green Mount Road, a collector roadway.
“The location is problematic,” said Dennis Sullivan, director of public works and engineering. “State Street, east and west, has cars traveling in a train-like pattern. As a result, motorists heeding the stop on the north-south road often become impatient and pull out in front of cars on State, causing accidents.”
The city looked at a number of options, most of which were dismissed.
“A four-way stop would cause excessive stopping of traffic on State Street most hours of the day,” said Sullivan. “A traffic light configuration would solve this problem, but is expensive to build and maintain, as well as coordinate with other signalized intersections nearby; it would also cause excessive delays in traffic most of the day based on the traffic distribution.”
According to Mannino, a traffic study prepared by another engineering firm concluded that traffic signals should be installed with left turn lanes. The city council instead voted to pursue an overpass over the CSX railroad, but would need to defer the project until funding could be established. Engineers from Horner & Shifrin brought the idea using a roundabout as an interim improvement to officials, who gave the thumbs up.
“The roundabout best accommodates traffic for now and into the next 20 years, saving motorists time and minimizing pollution,” said Sullivan.
Engineers performed a computer simulation that demonstrated installation of a roundabout at the location would lead to a marked reduction in air pollutants due to improved traffic flow and minimal traffic delay and congestion. Based on Horner & Shifrin's recommendation, the city applied for and received a Congestion Management and Air Quality (CMAQ) grant for $365,000 toward the projected total cost of $500,000. In addition, because the intersection averages more than one crash per month, many involving multiple vehicles, the city is applying for Highway Safety Improvement Program funds to supplement the CMAQ funding already received. This project is scheduled to begin construction this summer and wrap up by September. The city expects a 73% reduction in crashes and improved side street access.