Missouri Route 109 and Route CC Intersection Improvements in St. Louis

What is the current situation?

The intersection of Route CC and Route 109 in St. Louis County is getting congested.  With ongoing and anticipated developments in Wildwood, Franklin County and the Chesterfield Valley, that congestion will continue to get worse as times goes by.

MoDOT engineers studied the area to see what possible improvements could be made to the intersection to improve the flow of traffic and help reduce congestion. The department also applied for, and received, Federal funds for this intersection that are specifically set aside for projects that address congestion mitigation and air quality (CMAQ).

Information shared during the virtual public hearing held on December 3, 2020, is available on this page.

What is the current plan?

Based on that study, MoDOT determined that the best solution to keep traffic flowing in the area would be a two-lane roundabout, similar to those constructed further south on Route 109 between Clayton Road and Manchester. This would reduce backups and improve the movement of traffic in the area.

Can Route 109 be taken over Route CC (or vice versa)?

Traffic levels on both Route 109 and Route CC are not high enough to make separating the two roadways an effective use of available money. Our goal here is to improve the flow of traffic now and into the future, while minimizing the impacts to local property owners. All planned road projects need to be able to keep up with anticipated traffic growth – putting one road under the other is typically used with roadways with much higher (and more consistent) levels of traffic, which we don’t anticipate to be the case here, even with the expected growth in the area.;

Why a roundabout? Why not a signal?

When traffic levels range widely over the course of a day, roundabouts can help move traffic more effectively than a traffic signal. Traffic ebbs and flows at the intersection of Route CC and Route 109.  There are some peak traffic times during the morning and evening rush, but most of the remainder of the time, traffic is generally light, but steady in all directions.  A signal, even one with detection, cannot adjust quickly to varying levels of traffic.

As a whole, roundabouts are also safer.  Since traffic heads in the same direction around the roundabout, there are fewer opportunities for crashes, and those that do occur are potentially much less serious than the crashes at a four-way stop (as it is currently) or a signalized intersection.

How do you use the roundabout?

The current plan calls for a partial two lane roundabout.  Just like any other roundabout, traffic entering the roundabout would yield to any traffic already in the roundabout. Traffic on the outside lane would generally be turning right, while traffic on the inside lane would be heading though the roundabout or heading around the roundabout to turn left. 

What is currently underway?

Work has started at the intersection of Route 109 and Route CC to construct a new roundabout.  The project was awarded late last year to Millstone Weber construction company for $4.7 million. Crews have been working on removing trees and grading, and will begin work on some of the temporary pavement needed to keep traffic flowing during construction.  When they work on that temporary paving, which we anticipate starting in late April or early May, they will also work on temporary driveway connections, as appropriate, to that new pavement. 

This will be a long project – we anticipate it to be complete in early 2026, and there will be some long-term closures on South Eatherton and Wild Horse Creek Road. Specifics on timing haven’t been determined on that work, but we will share information on the closures when we get closer. 

During construction, please pay attention to the changing traffic patterns.  We plan to keep at least one lane open on Routes 109 and CC during peak traffic periods. You can follow the anticipated lane closures on our road construction report, which is updated weekly each Wednesday. We thank you for your patience as we work together through this. As you travel through the work zones, please pay attention to the speed limits, even if no workers are in sight; please put your phone down and avoid other distractions; and please be kind to other drivers who may be experiencing the changes through this intersection for the first time.

What is the current timeline?