Background
Diverging Diamond in St. Louis Missouri
In 2007, the district completed a corridor study of Hwy 15 from I-94 to Hwy 10 in St. Cloud. Future traffic volumes showed the need to convert the current expressway design (at-grade signalized intersections) to a freeway design. The cost for the whole corridor was over $300 million, but some of the interchange locations were isolated enough (like this project) that they could be built separately as need arose and funding became available.
Why a Diverging Diamond
The Greater Minnesota Interchange project selection criteria was weighted towards business development and increased employment (75%), safety (15%), and congestion relief (10%).
- Business Development and Increased Employment
The area around the intersection has been rapidly developing. The CentraCare Plaza healthcare facility is a major traffic contributor west of Hwy 15. The city of Sartell had over $70 million in new office and medical facility construction in 2009, with more planned. The retail development east of Hwy 15 (Sam’s Club, WalMart, etc.) was just beginning construction and was expected to bring large amounts of new traffic to the area. The existing intersection already had a high crash rate and experienced congestion during the peak hours.
- Safety
The project received Destination Innovation Program funding because the design is innovative and it improves safety by eliminating standard intersection geometrics and conflict points. The signal cycles are very short because there are no left turns, providing better mobility. The bridge is similar in size to a standard diamond interchange, and much smaller than the bridge needed for a single point diamond. The DDI is expected operate at a high level of service with few crashes and improved signal efficiency.Highway 15 St. Cloud/Sartell Location: Intersection of Hwy 15 and Stearns County Rd 120 in Sartell/St. Cloud Description: Construct a diverging diamond interchange Construct: 2012-2013 Communities: Sartell, St. Cloud, Stearns County, Le Sauk Township Detour: TBD Cost: $17.5 million
- Congestion Relief
When the original interchange application was submitted, District 3 Project Development staff expected to construct a standard diamond interchange, with the only real question being would there be signals at the ramps, or roundabouts. A consultant was hired to perform traffic modeling to determine which traffic control method would provide the best level of service for the longest period of time. About the time the traffic model showed problems with the volumes of left turning traffic, information about a new interchange concept being built in Missouri – the Diverging Diamond Interchange – started circulating around the state. Our staff was intrigued, and had the DDI added to the traffic modeling contract. Ultimately, the traffic model showed that DDI provided better traffic operations than even a single point diamond, and shows some excess capacity 20 years in the future. District staff reviewed the concept with Central Office Geometrics and the Federal Highway Administration’s resource group, and were given approval to proceed.
Neighborhood Noise Impacts
Three of the interchange quadrants are filled with office and commercial uses. The area southeast of the existing intersection includes residential neighborhoods. Limiting noise impacts to the neighborhoods was a primary objective. Early in the process it was decided to depress Hwy 15 under Stearns County Hwy 120, taking the highway noise below the existing ground line. A screening berm along the southeast ramp may be constructed to separate the interchange from the neighborhood.
Funding
In 2009, the legislature (through Chapter 36 and Chapter 152 bond funds) made a limited amount of funding available for interchange construction in greater Minnesota. The district applied for Greater Minnesota Interchange Funding and was awarded $10 million in bond funds towards the project.
In 2010, a separate application was made for Destination Innovation Program (DIP) funding, recognizing the district’s preferred alternative was a new concept called a Diverging Diamond Interchange. The project was awarded an additional $4.5 million in DIP funding. Stearns County, Sartell and St. Cloud will be responsible for the remainder of the funding.
History
The DDI roadway innovation design originated from France and cuts overall traffic delays up to 60 percent.