Snellville’s Displaced Left Turn Intersection- The Real Story

In 2012, I started writing the “Council Confidential” series of articles as a means of providing information about council actions that were being inaccurately presented to the public. Since the 2015 elections, that has no longer been an issue, so “Council Confidential” has been quietly resting on the sidelines.

However, (you knew there was going to be a “however”) as the number of projects in the works has increased, it seems that confusion and misunderstanding have come along for the ride- primarily as a result of activity on social media. In spite of the city’s new web site, and the continual flow of new, accurate information it presents, we still see an abundance of rumors and misinformation presented as fact on various social media sites.

The upcoming redesign of the intersection of Highways 78 and 124 is a prime example. News articles, postings on social media and statements during town hall meetings have all clearly stated that the revised intersection is a Georgia Department of Transportation (GDOT) project. Yet, some people continue to ask why the Snellville City Council has chosen to start the project, and how the Council will handle traffic during construction.

Although the intersection modification is strictly a GDOT project, the agency’s personnel have been extremely willing to listen to the city’s concerns, and have taken steps to accommodate them. One of those concerns is detour routing during construction. When city representatives discussed the issue with GDOT, the agency formulated a plan to deter traffic from cutting through residential neighborhoods. GDOT has also been very agreeable to working with the city to make the new intersection as visually appealing as possible. However, that doesn’t alter the fact that design and construction are under GDOT direction.

Another point of misconception is the design of the intersection. Contrary to some posts on social media, the new intersection will not include a round-about, a “fly-over” bridge, or a combination of both. Although a “fly-over” design (with one highway passing over the other) was entertained during the initial evaluation of the project, it was never seriously considered. At a cost of four times that of the chosen design, ($43 million versus $10 million) GDOT rejected the “fly-over” concept in favor of an “at grade” design, well before any actual planning work began. In addition to its expense, the “fly-over” was rejected because it would have turned that section of Snellville into a concrete and asphalt desert.

As for the chosen option, it was initially called a “Continuous Flow Intersection”, (CFI) which led to a good deal of confusion. “Continuous Flow” infers that no traffic will not stop at the intersection. That is an impossible situation when two roads cross at grade. In the case of a CFI, “Continuous Flow” refers to straight-through traffic moving at the same time a left-turning traffic. To accomplish that, traffic turning left must enter a turn lane on the left side (as opposed to in the center) of the roadway, hence the subsequent name change to Displaced Left Turn (DLT) Intersection. Although it may seem like a new idea, the DLT concept is at least 20 years old; the first such intersection opened in New York state in 1996.

The DLT design functions somewhat like the Diverging Diamond intersections such as the ones at Pleasant Hill Road and Interstate 85, Jimmy Carter Blvd and Interstate 85, Windy Hill road and Interstate 75, and Ashford-Dunwoody Road and Interstate 285. (There are currently 87 Diverging Diamond interchanges throughout the United States.) Although the Diverging Diamond and the Diverted Left Turn intersections are similar in concept, there is a significant difference in execution; the DLT accommodates cross traffic, the Diverging Diamond does not.

In both instances, left-turning vehicles move to turn lanes on the left side of the roadway well before the point at which they turn. When the light turns green at a Diverging Diamond, straight-through traffic, as well as traffic turning left to enter the interstate, move at the same time. Since there is no cross traffic, signal timing must only accommodate traffic flowing in opposite directions. That is, northbound traffic is stopped while southbound traffic moves and vice versa.

Signal timing at DLT intersections is somewhat more complex.  As with the Diverging Diamond, left-turning traffic has to move across oncoming traffic lanes prior to reaching the intersection. This is accomplished by having vehicles enter a center turn lane, just as in a conventional intersection. The difference is, with a DLT configuration, the center left turn lane is well back from the intersection itself. When the lights at the intersection turn red and traffic on Highway 78 stops, vehicles cross from the center turn lane to the left side of the. (As is the case with the existing traditional intersection, traffic on Highway 78 will stop to allow traffic on Highway 124 to proceed.) When the lights at the intersection turn green, allowing Highway 78 traffic to proceed, vehicles turning left onto Highway 124 will have already crossed the roadway, so straight-through and left turns can move at the same time. In addition to improving traffic flow, (by eliminating wait time for left-turning traffic) this design also reduces accidents as it eliminates left-turning vehicles crossing in front of on-coming traffic.

Another flow improvement with the DLT intersection can be achieved through diverting some traffic away from the intersection. Unlike some DLT designs in other parts of the country, which accommodate left-turning traffic on both roadways, Snellville’s new intersection will not include any provision for vehicles turning left from Highway 124 onto Highway 78. Consequently, the DLT project includes improvements to Henry Clower Blvd to better accommodate drivers traveling north on 124 wishig to turn left onto Highway 78. These drivers will turn on Henry Clower Blvd, and then left from Henry Clower onto Highway 78. Vehicles traveling south on 124 can turn left on Oak Road or Wisteria Drive and then left when these roads intersect Highway 78.

According to information from people in other parts of the country, where DLT intersections are in operation, they unquestionably improve traffic flow. Undoubtedly, it will take a bit of getting used to the operation of a DLT intersection, but it will be easy to get used to smoother flowing traffic.




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