Andrew Hoenig,Dustin O’Quinn 2015-03-12 01:51:37
For years, the northern “Top End” of Interstate 285 was one of the most congested roads in metro Atlanta. The 36-mile segment runs north of the I-20 interchanges through Fulton, DeKalb and Cobb counties. Countless traffic bottlenecks on the roadway have posed an ongoing threat to motorist safety. To make this crucial roadway section safer and more efficient, the Georgia Department of Transportation launched a variable speed limit (VSL) system and raised the maximum speed limit from 55 mph to 65 mph. While Georgia has long used variable speed limits to slow down traffic ahead of work crews, the I-285 project represents the first permanent VSL system in the state. Georgia chose to implement VSL on the Top End of I-285, rather than Southside, because the Top End carries an average of 50,000 more vehicles - 100,000 more in some segments. Top End I-285 also has nearly twice as many interchanges as the Southside segment, which leads to more congestion and a greater likelihood of crashes. Various studies have shown that VSL will decrease congestion, save motorists on gasoline, reduce accidents and cut travel time. In addition, VSL systems improve air quality by reducing the time that cars are idling. HOW IT WORKS A VSL system implements speed limits that change according to road, traffic and weather conditions. Through careful monitoring of the roadway, electronic variable speed limit signs slow down traffic ahead of congestion or bad weather to smooth out traffic flow and reduce the need for stop-and-go driving. In addition to decreasing traffic congestion, variable speed limits decrease the speed disparity between the fastest and slowest cars, which in turn reduces the likelihood of crashes. A good way to envision how a VSL system works is to think about rice flowing through a funnel, with rice as the cars and the funnel as the roadway. If the rice is poured into the funnel all at once, it gets congested at the bottom of the funnel and takes a long time to work its way through. But if the rice is poured slowly and steadily into the funnel, it moves through evenly and doesn’t get congested. Though the rice enters the funnel more slowly, it gets through the funnel faster. PROJECT DETAILS The I-285 VSL system went live in October of 2014, with two signs on either side of the road at 88 locations - a total of 176 electronic signs. The total cost of the program was $6 million, including about $4.5 million for the VSL technology and the balance for updated static signs that display the new 65 mph speed limit. Depending on road conditions, the system varies the speed limit from 35 to 65 mph in 10 mph increments. The expected speed limit remains at 65 mph most of the time. If there is an accident or congested area, the speed limit along that segment of roadway can be set as low as 35 mph, with the speed limit being reduced in 10-mph increments leading up to the affected area. Focus group sessions revealed that travelers were interested in receiving real-time updates related to the decreases in speed when driving on I-285. To meet this need, the system includes overhead electronic signs that display explanatory messages such as “Speed Limit Reduced Ahead” that are added to existing accident or travel time messages. Additional public support was gained by raising the overall speed limit along the Top End from 55 mph to 65 mph. Even prior to the adjustment, drivers typically reached speeds above 60 mph throughout uncongested segments of I-285, so raising the limit to 65 mph was met with minor opposition. LESSONS LEARNED AS CONCEPT SPREADS In Europe, VSL systems have helped reduce collisions, improve traffic flow and increase roadway capacity. In the United States, VSL projects have been undertaken in locales such as Washington State, Florida, Missouri, New Jersey and San Francisco. VSL is an evolving technology and different systems offer different features. For example, the VSL project in Washington allows the state DOT to post varying speeds in different lanes. If there’s an accident in a single lane, the system notifies motorists in advance. But the cost for that system was significantly more than the one that was installed on the Top End of I-285. Some variable speed limit systems have been more successful than others. In the St. Louis area, a VSL system along Interstates 270 and 255 was changed to an advisory only system in 2011 aft er a study by researchers from the Missouri University of Science and Technology found that nearly two-thirds of drivers wanted the signs taken down. Though the Missouri Department of Transportation said the variable speeds made the highways safer, the study concluded that police did not think the system was successful in getting motorists to comply. Then, in 2013, growing unpopularity led to removal of the advisory speed limit signs from the two St. Louis highways altogether. Aft er a review of other variable speed systems across the nation, including the St. Louis program, it was determined that, among the systems that did not operate properly, there was an issue with the spacing of signs. Th e Georgia VSL team took note and implemented half mile spacing between the signs on I-285, which appears to be having a positive impact. Currently, the system is operating at the level initially intended, however, GDOT continues to analyze incoming data to determine and maintain full effectiveness. COORDINATION IS CRUCIAL Depending on future assessments of the I-285 VSL system, VSL technology may expand to other Atlanta highways. And while a successful VSL system requires a great deal of technical expertise, the people-to-people aspects of such an endeavor are just as important. Th e I-285 VSL system goes through three counties and multiple cities. Coordination with these counties and cities, as well as multiple police departments and the highway patrol was crucial. Th ere were many moving parts involved in the unveiling of 176 VSL signs, and in replacing the 55 mph signs with 65 mph signs. Numerous meetings took place to prepare for active use of the system. The importance of reaching out to all impacted parties early in the process to work through potential problems is one considerable advisement from GDOT to any entity seeking to establish a VSL system in the future. Th e extensive communication and coordination that took place with the I-285 VSL installment helped create an consistent, uniform understanding of the overall plan and was a major factor in the successful implementation of this project.
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