Medlock Bridge Road (SR 141) and State Bridge Road (TSPLOST) Project
The most congested intersection in Johns Creek is SR 141/Medlock Bridge Road and State Bridge Road (the intersection). These two major roadways are largely responsible for carrying traffic North/ South and East/West across the City and to neighboring regions. The intersection is also critical as it includes a high school, entertainment centers, borders residential subdivisions, and is a main commercial hub in the City. The intersection currently carries approximately 100,000 cars per day and presently has three travel lanes on the southern and eastern legs and two travel lanes on the northern and western legs.
The goal of the intersection improvement project is to develop an implementable long-term solution to improve congestion throughout the area.
Q. What has been done to identify the intersection issues?
A. The intersection was studied in 2015 and 2016 including an assessment of nine alternatives which were reviewed for traffic improvement, impacts to the community, and cost. In 2016, the intersection was included as a Tier-1 TSPLOST project. Following the passage of the TSPLOST, City Council put the project on hold in order to establish a process by which further community input and involvement could be gathered to improve the concepts to final plans. In 2018, 35 alternatives were examined as part of an updated traffic study, in order to better understand a magnitude of long term solutions.
Q. What is the process for the intersection improvement?
A. The project begins with concept/engineering/design. The second phase is right-of-way and finally construction and maintenance. City Council approved entry into the Concept/Design Phase for the intersection on March 26, 2018. The authorization included the request to consider a complete range of solutions, perform an updated traffic study / modeling, and understand the potential alternatives.
Q. What has the city done to date to move the process forward?
A. For this project, the first step has been to analyze and review traffic patterns through a traffic study at the intersection and immediately surrounding area known as the “study footprint.” [PDF, 9.5MB]
The City worked with Wolverton & Associates to perform a traffic study for the study footprint. The team reviewed information from past studies, including traffic volumes. To analyze design improvements, the GDOT standard traffic growth rate of 1% per year was utilized.
Q. How did the city understand the overall traffic and not just push the issues to the next intersection?
A. The study footprint spanned SR 141/Medlock Bridge Road from the south at Old Alabama Road to the north at St. Ives Country Club Parkway/Grove Point Road and along State Bridge Road from the west at Johns Creek High School to the east at St. Georgen Common. These additional areas allowed for a more cumulative study than just the one intersection and allowed us to better understand traffic flow throughout this important area.
Q. How many alternatives did the city study?
A. In the current study, the city began with 35 innovative alternatives from the simplest to very complicated in order to provide a wide range of options and a data driven solution. Only the alternatives that maintained an acceptable operation for the long term (25 years) were considered for further modeling and analysis.
Q. What were the metrics the city chose to evaluate for consideration?
With a goal of aiding the flow of traffic through the intersection the 35 designs were evaluated using three metrics:
- Travel Time: shows the amount of time it takes drives to get from one point to another and improvement in seconds.
- Level of Service: shows the delay time and overall improvement in seconds.
- Throughput: the number of cars that are successfully able to exit the intersection and move through the area.
Q. What were the results of the traffic study and the recommendations of alternatives?
A. As a first step, the alternatives were overlaid on the current intersection footprint, thereby constraining them to the two travel lanes on the northern and western legs of the intersection. All 35 alternatives gridlocked in the model before the opening year of 2024. After adjusting the alternatives to include the third lane on the northern and western legs of the intersection, eight alternatives rose to the top with similar positive outcomes for the traffic.
Signalization and traffic signal adjustments were also considered, and optimized for each of the final eight alternatives. Three of the eight designs relied heavily on left turn movements. This minimized the long term effectiveness of the options because of the time necessary for the turning movement. These movement also increased conflict points and potential crash sites. The remaining five alternatives were considered as viable alternatives for further discussion.
The five alternatives with the most positive traffic results per our metrics are noted below:
||Converting the SR 141 at State Bridge intersection to a Tight Urban Diamond Interchange
||Converting the SR 141 at State Bridge intersection to a Continuous Flow Intersection on SR 141 and a Median U-Turn on State Bridge
||Converting the SR 141 at State Bridge intersection to a Median U-Turn on all legs
||Creating a quadrant roadway on the SW and NE Quadrants to remove left turns at the SR 141 at State Bridge intersection
||Each leg of the intersection differs in its layout: a Continuous Flow Intersection for the SB Left, an Median U-Turn for the EB Left and a Quadrant Roadway for the NB and WB Lefts
Q. What were the traffic improvements of the top five alternatives in the design year + 2 years?
A. As measured against the three metrics, the study revealed the following traffic improvements of the top five alternatives in the design year + 2 years:
- Travel time: Improved average peak hour travel time between 72% and 85%.
- Level Of Service / Average Delay: Improved between 54% and 62%.
- Throughput: Improved between 19% and 26%.
Q. How else did the city evaluate the top five alternatives?
A. The city reviewed the top five alternatives against travel time, level of service and throughput to assess the greatest positive traffic outcomes. In addition, the city introduced three other important metrics to further evaluate the designs:
- Cost - a combination of both anticipated right of way and overall construction costs
- Construction Schedule - a schedule was associated with a technical understanding of the needs of each alternative and anticipated procurement and contractor timetables
- Development and Economic Impacts - based on anticipated property acquisitions based on conceptual design and discussion with the Community Development staff on potential economic issues associated with each alternative
Q. What are the estimated costs of the five alternatives?
Each of the top five alternatives have variable costs. The major costs are a combination of construction and right of way costs; overall estimates for each project can be seen below:
||Total Estimated Cost ($)
This project is currently budgeted as TSPLOST project TS0103 and has $8,000,000 allotted. Additional state/federal funding could also be considered.
Q. What was the recommended alternative which the city seeks to implement?
A. Following extensive review and analysis of the top five alternatives to address the long term intersection congestion issues, the city examined the addition of cost, impact, and schedule metrics. Based on the analysis, the city recommends constructing the Thru-U to improve the intersection at SR 141/Medlock Bridge Road and State Bridge Road.
Q. Will design plans for the implementation of a Thru-U at Medlock Bridge Road and State Bridge Road create a scenario where the intersection will be almost 12 percent over capacity by the year 2024?
A. No, the plans for a Thru-U at SR 141/Medlock Bridge Road at State Bridge Road will actually improve Intersection Capacity by 2024. With a Thru-U the Volume/Capacity Ratio of AM Peak travel would be 80%, and PM Peak travel would be at 95% (Pages 3112 and 3124, respectively). For perspective, the capacity today (2019) is at 126% in the AM Peak and 132% in the PM Peak.
Q. Are the traffic signals at the intersection and along the corridor synchronized?
A. Yes, the lights ARE synchronized today, they will be synchronized in the future. The concept that because the northbound and southbound splits aren’t exactly the same number doesn’t mean the signals aren’t synchronized.
Q. Will design plans for the implementation of a Thru-U at Medlock Bridge Road and State Bridge Road create a scenario where the intersection will be 18 percent over capacity by the year 2024?
A. No, the plans for a Thru-U at Medlock Crossing at State Bridge Road will actually improve Intersection Capacity by 2024. With a Thru-U the Volume/Capacity Ratio of AM Peak travel would be 89%, and PM Peak travel would be at 94%. For perspective, the capacity today (2019) is at 161% in the AM Peak and 100% in the PM Peak. During the design alternative exploration process, the engineers and staff examined hundreds of alternatives and brought the best ones back to the table for consideration.
Q. Will this recommended solution bring future congestion at other intersections in the area?
The city recommends fixing the weakest link first and is also the best place to start. The weakest link is SR 141/Medlock Bridge Road at State Bridge Road. If the city were to give SR 141/Medlock Bridge Road more than 65% of the green time now, that would mean that State Bridge Road will have its green time portion reduced. When examining total green time on SR 141/Medlock Bridge Road (through time + left time), SR 141/Medlock Bridge Road is receiving 59% of the cycle in the AM Peak and 64% of the cycle in the PM Peak right now. Right now the timing is managed to balance delay between the corridors. It would be incorrect to imply that not having the same green times translates into the current signals are only synchronized occasionally. The signals are monitored and constantly adjusted to ensure they are synchronized for the entire corridor and area. Staff and Council are aware that the long term solution for State and Medlock will not fix the issues north and south of the intersection.
Q. Will design plans for the implementation of a Thru-U at Medlock Bridge Road and State Bridge Road create a scenario where Old Alabama Road will be 10 percent over capacity by 2024?
While it is certain that the SR 141/Medlock Bridge Road at Old Alabama Road is and will be an issue, the implementation of a Thru-U at SR 141/Medlock Bridge Road at State Bridge Road will not directly cause a 10 percent overcapacity issue at Medlock Bridge and Old Alabama Road. The city originally had this intersection as part of the study area, but it was removed since the purpose of this study was to mitigate the SR 141/Medlock Bridge Road at State Bridge Road intersection. Old Alabama will also need to be addressed in the future.
In 2024, based on the city study, the AM Peak southbound green on SR 141 will be 58.1 seconds plus the 6.9 seconds of yellow and red time for a total of 65 seconds (54% of the cycle). The northbound green will be 58.1 seconds plus the 6.9 seconds of yellow and red time for a total of 65 seconds (54% of the cycle). The PM Peak southbound green on SR 141 will be 70.1 seconds plus the 6.9 seconds of yellow and red time for a total of 77 seconds (64% of the cycle). The northbound green will be 70.1 seconds plus the 6.9 seconds of yellow and red time for a total of 77 seconds (64% of the cycle).
For comparison, the existing AM Peak has a southbound green on SR 141 will be 66.1 seconds plus the 6.9 seconds of yellow and red time for a total of 73 seconds (46% of the cycle). The northbound green will be 59.8 seconds plus the 6.9 seconds of yellow and red time for a total of 67 seconds (42% of the cycle). The PM Peak southbound green on SR 141 will be 75.1 seconds plus the 6.9 seconds of yellow and red time for a total of 82 seconds (51% of the cycle). The northbound green will be 65.1 seconds plus the 6.9 seconds of yellow and red time for a total of 72 seconds (45% of the cycle).
This means that with the implementation of a Thru-U at the intersection it is anticipated that by 2024 there will be the following results:
- The SR 141 Green time will increase in the AM Peak from 46%/42% (SB/NB) as it is now, to 54%/54% (SB/NB) in 2024 with the Thru-U.
- The SR 141 Green time will increase in the PM Peak from 51%/45% (SB/NB) as it is now, to 64%/64% (SB/NB) in 2024 with the Thru-U.
- The State Bridge Road Green time will increase in the AM Peak from 23%/29% (EB/WB) as it is now, to 46%/46% (EB/WB) in 2024 with the Thru-U.
- The State Bridge Green time will increase in the PM Peak from 21%/26% (EB/WB) as it is now, to 36%/36% (EB/WB) in 2024 with the Thru-U.
Q. Would there be some options for a short-term project to bring relief during the waiting period of the long-term solution?
The current configuration of the intersection includes 2 four lane legs (west State Bridge and north Medlock) and 2 six lane legs (east State Bridge and south Medlock). In every alternative, the intersection gridlocks unless the intersection has six lanes on each approach. Preventing gridlock can be accomplished in the short-term by restriping the north leg of Medlock to St Ives to 6 lanes and adding additional pavement along the west leg of State Bridge to Johns Creek High School. Attachment 1 shows the traffic impacts as well as information on the schedule and cost of this short-term / initial improvement to the intersection. This short term phase of the project was approved by Council on January 11, 2019.
Q. Are there additional risks/concerns (e.g., off peak impact, bike/ped safety, signage/aesthetics, education of populace of unique intersection, accident volume, etc.) that should be considered when weighing the options?
Please see the following table for additional information on risks associated with each of the options.
Q. Isn’t a Thru-U more fitting for two different sized roads with the smaller street having less volume?
Not necessarily. As one can see in table below, the three examples of existing Thru-Us that carry close to or over 100,000 vehicles and operate in a balanced manner between the two roads (similar to the volume experienced at Medlock Bridge Road at State Bridge Road).
||Total Vehicles (ADT)
||Primary Road vehicles
||Secondary Road vehicles
|M-53 (Van Dyke) and Metropolitan Parkway (Sterling Heights, MI)
||All approaches have left-turn crossovers.
Volumes are near equal from all approaches.
All approaches are 6 lane.
|Metropolitan Parkway and Mound Road (Sterling Heights, MI)
||All approaches have left-turn crossovers.
Volumes are near equal from all approaches.
All approaches are 6 lane.
|Telegraph Road and Square Lake Road (Bloomfield Hills, MI)
||Eight lanes on primary road and six on secondary.