Safety-Enhancing I-35 Interchange Design Reduces Traffic Conflicts in Twin Cities Metro Area
Reducing Twin Cities Congestion: Minnesota’s Newest Diverging Diamond Interchange Enhances Safety, Decreases Traffic in Metro Area
According to the Minnesota 20-Year State Highway Investment Plan (MnSHIP), roughly half of all roadway travel in the state occurs within the Twin Cities area. Given the fact that the region contains just 9 percent of Minnesota’s total roadway miles, it’s no wonder that congestion in this burgeoning Minneapolis-St. Paul metro community has reached a crisis point.
The Minnesota Department of Transportation (MnDOT) is working strategically to improve many of the state’s most critical, high-traffic corridors. Currently, one of its largest undertakings is the I-35 North Metro Split project, a $60 million design-build venture that includes continued road resurfacing and bridge replacement work along a 6-mile stretch of Interstate 35 near the cities of Forest Lake, Lino Lakes and Columbus.
One of MnDOT’s biggest hurdles was finding adequate funding for this two-year construction effort.
“As with any maintenance or preservation-type state roadway project, piecing together multiple funding sources is difficult,” says MnDOT East Area Manager Ryan Coddington. A combination of federal and state funding is earmarked for this venture, including bond monies, thanks to plans to construct a diverging diamond interchange (DDI) to optimize traffic operations and mitigate congestion.
Construction, led by Shafer Contracting Co., Inc. as the primary general contractor, began in early October 2017. Other key members of the project team include TKDA (lead designer), American Engineering Testing, Inc. (geotechnical design and quality management), Lunda Construction Co. (bridge construction), HNTB (design-build verification) and Zan Associates (public relations).
Congestion at All-Time High
In the Twin Cities area, traffic congestion is one of the biggest talking points among community members and media outlets. These zones of slow-moving vehicles and frustrating gridlock have peaked at their highest recorded level since 1993, when MnDOT first started tracking rush-hour traffic conditions.
“The I-35 North Metro Split project covers a major route for folks going to the cabin and other areas, and there are limited alternate routes, which can lead to heavy congestion,” Coddington explains.
He notes the project zone’s current AADT (annual average daily traffic) as follows: 39,000 (I-35E), 43,000 (I-35W), 82,000 (I-35 north of metro split) and 51,000 (north of Highway 8).
MnDOT’s latest annual Metropolitan Freeway System Congestion Report shows that Twin Cities freeway congestion slightly increased from 23.4 percent in 2015 to 23.7 percent in 2016. And in a study titled “Twin Cities Traffic Congestion: It’s No Accident,” transportation expert Randal O’Toole asserts that congestion costs the region nearly $4 billion per year in wasted time and increased business expenses.
A variety of factors can contribute to congestion, such as population growth, local economics and fluctuating gas prices. With a current population estimated at 3.6 million, MnDOT expects congestion to remain the same – or even increase – as the metropolitan area continues to grow.
Better Ride Quality, Safety, and Traffic Flow
The I-35 North Metro Split project entails resurfacing existing pavement stretching from the I-35W/I-35E split around Lino Lakes to a half mile north of Highway 8 near Forest Lake. Work also includes three bridge replacements and area drainage improvements.
Ultimately, this undertaking will provide a smoother roadway surface for better ride quality. It will also enhance safety and traffic flow at the Highway 97 interchange, benefitting travelers heading to some of the area’s top destinations, including the Running Aces Casino and Racetrack, the Forest Lake Rest Area and Broadway Avenue, a major downtown hub situated in the heart of the Forest Lake commercial district.
The project kicked off last fall with site preparation, including adding temporary pavement and removing trees and foliage within the construction zone. The principal construction activities will take place between 2018 and 2019 and are essentially split into two pieces – north and south – says Chris Tredinnick, Construction Project Manager, Shafer Contracting.
“The goal on the north half is to have most of the concrete pavement on the main line done by November 2018,” he notes further. “One piece – the bridge for Highway 8 and the new loop from Highway 8 to the main line – won’t be completed until 2019.”
This year, workers are resurfacing the north and southbound lanes of I-35 situated north of Highway 97 and realigning 235th Street North in Forest Lake. They are also constructing the north half of the new Highway 97 bridge over I-35 as well as the new I-35W flyover that will replace the existing bridge over I-35E. The flyover will open to traffic in 2019.
Next year, crews will resurface the north and southbound lanes of I-35 situated south of Highway 97 and begin constructing the remainder of the Highway 97 bridge. The north and southbound ramps to and from Highway 97 will also be resurfaced, and workers will reconstruct the Highway 8 bridge over I-35.
Roadway analyses confirm the existing pavement within the project limits is in poor condition, reports MnDOT. Beneath the present 4-inch bituminous surface lies a deteriorated continuously reinforced concrete pavement (CRCP) built in 1969. The last milling operation took place nearly a decade ago.
Approximately 50 to 75 days total will be needed to mill off the bituminous pavement and place an 8-inch concrete slab on top, says MnDOT Construction Engineer Eric Embacher. This will result in an overall 4-inch grade raise.
An unbonded concrete overlay was chosen to avoid reconstructing the entire pavement. Part of the bituminous pavement will be milled off and a new concrete surfacing will be placed on top of the remaining bituminous pavement and the old concrete pavement, helping to extend pavement life for 35 years with normal concrete maintenance. This durable pavement solution should not require concrete pavement rehabilitation for approximately 20 years, according to MnDOT officials.
Diverging Diamond Reduces Traffic Conflicts
The centerpiece of the I-35 Metro Split project is the new diverging diamond interchange. For those traveling westbound on Highway 97 toward southbound I-35, this innovative style of traffic switching enhances safety by reducing the number of vehicle conflict points.
Presently, drivers are forced to make hard left turns that slow traffic considerably. In a DDI, traffic makes a temporary shift to the left while crossing the freeway, allowing for direct left turns onto entrance ramps without waiting at a traffic signal. This timesaving design is a good fit for the growing Twin Cities area because it can handle higher-traffic volumes. It also promotes safety because drivers turning left do not have to intersect traffic.
MnDOT, in collaboration with city and county officials, considered four alternative intersection designs before deciding on the DDI model. DDIs – proven to cut down on traffic delays by up to 60 percent – address a wide range of operational needs, including queuing, synchronization, bottlenecks, lane balancing, weaving, lane widening and pedestrian crossing. More than 80 DDIs have been built in the U.S. since 2009, including two recent projects in Minnesota: the Elk Run Interchange on Highway 52 (between Pine Island and Oronoco) and the I-35W Interchange on Highway 96 in Arden Hills.
Other Timely Bridge Work
In Minnesota, bridges replacement typically occurs at 50 to 100 years. The other two bridges in this project were built in the 1960s, so their replacement is very timely.
The existing northbound I-35W bridge – which carries traffic over the southbound lanes of I-35E – is a three-span, continuous concrete box girder design. It is being replaced with a single-span, prestressed structure with high abutments located northwest of the existing alignment. The new overpass will have the same number of lanes and configuration as it does today, but the roadway would be shifted slightly to the north to allow the bridge to be staged while minimizing traffic impacts.
Crews will change out Highway 8’s semi-directional flyover ramp with an east-west aligned structure built perpendicular to I-35. A loop design is being used to provide access to I-35.
“With the existing two-lane flyover ramp, the design merges drivers down to one lane before merging them onto the highway, right before a curve. Many commuters try to make this weave happen at higher speeds, which poses safety risks,” says Coddington.
The new loop ramp design extends the weaving distance to more than 2,500 feet and forces commuters into a single-file line, allowing them to merge onto the freeway in a more orderly fashion, Coddington adds.
Traffic maintenance, says Embacher, is one of the project’s biggest coordination challenges. To minimize traffic impacts, the bulk of construction activities are scheduled from 7 a.m. to 7 p.m. during week days. Weekend and overnight work is scheduled as necessary.
In addition, Shafer Contracting’s traffic diversion solutions allow for more than the minimum required number of lanes to remain open during construction.
“For example,” Tredinnick shares, “we took the two northbound lanes and instead of having them side by side, we split them around the in-place median barrier. This allowed us to maintain three lanes on the southbound side, an extra lane beyond the contract requirement.”
Right now, northbound I-35 is reduced to two lanes between Highway 97 and Highway 8 and ramps at Highway 97 and Broadway Avenue are open. Southbound I-35 is open to three lanes but will periodically shift to two open lanes near Highway 8 until late August. Between early August through fall, southbound I-35 will be reduced to two lanes between Highway 8 and Highway 97, with ramps at Highway 97 and Broadway Avenue continuing to remain open.
- Owner: Minnesota Department of Transportation
- Construction Method: Design-build
- Cost: $60 million
- Location: Minnesota (I-35E/I-35W and I-35 from Lino Lakes to Forest Lake)
- Project Length: 6 miles
- Construction Timeline: October 2017 to October 2019
- Lead General Contractor: Shafer Contracting Co.
- Lead Design Firm: TKDA
- Bridge Subcontractor: Lunda Construction Co.
- Geotech Design/Quality Management: American Engineering Testing, Inc.
- Design-Build Verification: HNTB
Summary of Work
- Resurface existing pavement
- Replace the Highway 97 bridge over Interstate 35 with a diverging diamond interchange for better traffic flow and improved safety
- Update crosswalks at Highway 97 to meet modern accessibility standards
- Replace the flyover ramp from westbound Highway 8 to southbound Interstate 35 with a loop ramp
- Drainage improvements