Innovative DDI Interchange Comes to Ankeny
A new intersection design known as a Diverging Diamond Interchange (DDI), or Double Crossover Diamond Interchange, takes traffic safety and efficiency to a new level.
“In a traditional diamond interchange there are approximately 26 conflict points that can lead to traffic crashes,” explains Wade Greiman, PE, Transportation Business Unit Leader for Snyder & Associates. “DDIs have just 14 conflict points. Fewer conflict points means fewer opportunities for vehicles to collide, resulting in a safer, more efficient flow of traffic.”
In a DDI, traffic is shifted from the right side of the road to the left side of the road at each intersection between the interchange ramps and side road. This means left-turning traffic doesn’t cross paths with oncoming traffic, so the chance for collision is reduced. Once left turn intersections are cleared, traffic is brought back to the right side of the road.
For example, when driving west to east on the new E. 1st Street DDI in Ankeny, you’ll be able to turn right onto the southbound I-35 entrance ramp or proceed through to a signalized intersection that moves you to the left side of the roadway.
If you travel through to the left side of the road, you will be able to make a free left turn onto the northbound I-35 entrance ramp or proceed through to a second signalized intersection that moves you back to the right side of the road as you continue east on E. 1st Street.
Exiting the interstate is also safely accommodated with a DDI.
From the exit ramp, you’ll turn left or right onto the side road, which is E. 1st Street in this case. Due to the design of the DDI, drivers don’t cross traffic from either direction, which increases traffic movement efficiency and intersection safety.
Since the first DDI in the U.S. opened in Springfield, Missouri in June 2009, over 80 more DDIs have opened in the U.S. with many more currently underway. The E. 1st Street DDI is the second of its kind in the State of Iowa following the opening of the I-80/Grand Prairie Parkway DDI near Waukee in 2015.
“Ankeny’s population is growing at a rapid pace. There’s a significant amount of residential and commercial development going on in the community, and many people commute to work in Des Moines or other places within the metro area,” says Greiman. “All of these elements combined, led to an increase of traffic congestion and crashes near the E. 1st Street Interchange. Changing the interchange from a traditional diamond configuration to a DDI allows more vehicles to travel safely through the area with minimal impact to the surrounding area.”
Safety & Operational Benefits of DDIs
As often is the concern with roundabouts, DDIs utilize an unfamiliar traffic pattern that can create apprehension and fear for some drivers. Driving on the left side of the road may feel counter-intuitive and hazardous. In reality, DDIs provide numerous safety, operational, and cost benefits over traditional diamond intersection designs including:
- Reduced collision potential and increased safety by reducing the number of times vehicles cross paths. In particular, left-hand turns through opposing traffic are eliminated.
- Improved sight distance when turning.
- Reduced potential for wrong-way movement onto freeway ramps, an ongoing problem on our nation’s highways.
- Increased left turn capacity without needing additional lanes.
- Improved traffic flow and safety by reducing the number of traffic lights.
- Increased bicyclist and pedestrian safety by routing active transportation users through the median or along the road.
- Reduced congestion, resulting in reduced fuel consumption and air pollution.
- Reduced construction time with little to no additional right-of-way needed to retrofit an existing interchange.
Signage and pavement markings are used throughout DDIs to help guide drivers as they become acclimated. However, Greiman says public outreach is also essential.
“DDIs are unfamiliar for many people, so public engagement is important. Addressing public concerns and questions early on helps garner support. Modeling and animation technology can be particularly useful because it gives drivers an idea of what it will be like to drive through the area once it’s complete.”
A Positive Outlook for the Future of DDIs
It’s been nearly a decade since the first DDI was implemented in Springfield, Missouri, and so far, the results are encouraging.
A 2011 performance evaluation by the Missouri DOT yielded positive feedback from Springfield drivers with 80 percent of people reporting that traffic flow had improved. In its findings, the Missouri DOT shared that total crashes went down by 46 percent in just the first year of operation. Left-turn crashes were eliminated, and right-angle crashes were reduced by 72 percent.
In 2016, Carlos Sun, professor of civil and environmental engineering at the University of Missouri, along with colleagues across the country, published the first in-depth safety analysis comparing DDIs to traditional diamond configurations. After analyzing over 28 years of crash and other site data prior to DDI conversion and 19+ years of data post-DDI installation at seven U.S. locations, the group determined that:
- Diverging Diamond Interchanges reduce the number of traffic crashes by 33 percent
- Crashes resulting in injury and fatality are reduced by approximately 60 percent
While it was the first research of its kind and more is likely necessary, all signs point to DDIs as an efficient, effective, and most importantly—life-saving—intersection design.
As they become more common, the unfamiliar will become second nature. With positive reviews from the Federal Highway Administration as well as communities and agencies throughout the country, DDIs are on track to become a favorable way to route high volume traffic for years to come.
Reflecting on the I-35/E. 1st Street DDI, Greiman is excited to be a part of a project that will transform the safe, efficient movement of traffic for the City of Ankeny, the Iowa Department of Transportation, and the traveling public.
“We continually keep the goal of Zero Fatalities in mind as we design improvements for our nation’s roadways. Currently, vehicles back up on I-35 exit ramps waiting to turn onto E. 1st Street. This DDI will reduce the possibility of that occurring by moving vehicles onto E. 1st Street more efficiently. It will also greatly reduce the likelihood of rear-end collisions at this location,” he shares. “It’s very rewarding to know that what we do today at E. 1st Street and I-35 will serve the traveling public well as Ankeny continues to grow.”