The DDI concept was born in Europe, and was only recently brought to American roadways. The first American DDI was opened in 2009 in Springfield, Missouri, and on Aug. 23, 2010, the DDI at American Fork Main Street in Utah was opened, making it the first DDI built from the ground up in the United States. Other DDIs will soon be constructed or are being considered in other parts of the United States.

In Utah County, the I-15 interchanges at Timpanogos Highway (Highland/Alpine) and at 500 East in American Fork are both being reconfigured as DDIs. Another DDI is being considered in Salt Lake County at the intersection of Bangerter Highway and S.R. 201.

While different kinds of interchange configurations were considered for the Highland/Alpine interchange, UDOT chose the DDI for this location because it will function more safely and efficiently than other possible options, while preserving the existing I-15 infrastructure. The DDI thus becomes another tool UDOT can use in its constant search to find the best possible solutions to Utah's ever-expanding transportation needs.

The DDI located at the Highland/Alpine interchange is part of the Timpanogos Highway project. This project includes widening the existing road to five lanes from I-15 to S.R. 74, building "Commuter Lanes" from I-15 to Highland Boulevard and constructing segments of multi-use trails to enhance the existing trail system in the area.

The DDI looks different than a traditional freeway interchange because there are cross points where the traffic lanes cross over to the left side of the interchange. This may seem a little confusing at first, but these cross points are actually the reason why the DDI will function so smoothly, because they allow for more open traffic flow between the freeway and the interchange.

The standards of safety and efficiency have been at the forefront of the development of the Timpanogos Highway DDI. From the medians and barriers, the landscaping and aesthetic treatments to the pavement marking and signs, everything about the reconfigured interchange has been designed to ensure that navigation is seamless, intuitive and safe.

1. What is a DDI?
   A DDI, short for Diverging Diamond Interchange, is a European engineering concept in which the two directions of traffic are crossed to the opposite side to provide easier access on and off the freeway. This reduces the number of traffic signal phases needed to move motorists through the interchange to two, as opposed to the usual three or four. DDIs efficiently move high volumes of traffic without increasing the number of lanes in an interchange. UDOT reconstructed the American Fork Main Street interchange into a DDI, and is also planning on reconfiguring the Highland/Alpine Interchange into a DDI.
2. How do I drive through a DDI?
   To learn how to drive the Highland/Alpine DDI, please visit the DDI interactive map.
3. How is the DDI different from other interchanges?
   DDIs temporarily cross traffic lanes to the left side of the road in order to create direct access with the freeway. It limits the number of traffic signal phases required to move motorists through the interchange, ensuring that traffic moves quickly and efficiently. Movements on and off the freeway are free-flowing and have fewer conflict points.
4. How is the DDI at the Highland/Alpine Interchange different from the DDI at American Fork Main Street?
   The primary difference is the DDI at the Highland/Alpine interchange will actually go underneath the freeway, while the American Fork Main Street DDI bridges over I-15. Also, no bridges are being built in association with this work, as opposed to the American Fork Main Street DDI, for which two new parallel bridges were constructed off-site and moved into place. The American Fork Main Street interchange was completely demolished and then rebuilt to accommodate the DDI; the Highland/Alpine DDI is an interchange reconfiguration, which means that crews will work with the existing bridge structures and within the existing footprint of the road to construct the DDI. To help guide traffic through the interchange, there will be appropriate signing, striping, and a 68-inch long median with 42-inch high barrier to ensure that all movements through the DDI are intuitive and safe.
5. Will there be pedestrian and/or cyclist access provided for the Timpanogos Highway DDI?
   No. While the new interchange will greatly improve traffic flow, there is not sufficient room under the existing bridge to provide pedestrian/bicycle access. In the future, when I-15 reconstruction reaches the Highland/Alpine exit, the freeway bridges will be reconstructed and provisions will be made for pedestrians and cyclists.
6. Are other states building DDIs?
   Yes. The first American DDI was opened in June of 2009 in Springfield, Mo., where an existing bridge was reconfigured to create a DDI. Missouri is also planning to build additional DDIs. The Pioneer Crossing DDI at American Fork Main Street is the first American DDI built from the ground up, and is the first DDI built in the western United States. The Timpanogos Highway DDI will be the third DDI in Utah. Several other states plan to construct DDIs including Oregon, Idaho Missouri and Tennessee. DDIs are also being considered in Kentucky, Maryland, Michigan, New York, Ohio and Virginia.
7. Is UDOT building DDIs in other parts of Utah?
   Yes. UDOT is currently constructing two DDIs in Utah County. In addition to the DDI being constructed as part of the Timpanogos Highway project, another is being built at the 500 East exit in American Fork as part of the I-15 CORE reconstruction project in Utah County. Another DDI is being considered in Salt Lake County at the intersection of Bangerter Highway and S.R. 201.
8. Why did UDOT choose a DDI for this area?
   Although different kinds of interchanges were considered, UDOT believes that the DDI is the right choice for the Highland/Alpine interchange because it will function more safely and efficiently than other possible options for this location.
9. When is a DDI the best interchange option?
   A DDI works well in a high traffic area with relatively equal volumes of traffic coming from both directions. It also works best in areas where there are no conflicts with side streets or frontage roads in close proximity to the interchange. A DDI is also a cost-effective option that requires less land to purchase, so it is a good choice when funding is limited.
10. Will a DDI accommodate the growing traffic in northern Utah County?
    Yes. Our engineering studies indicate that the DDI will help alleviate bottlenecks, freeway backups and can accommodate more traffic with fewer lanes well into the future, making it an ideal solution to transportation needs of northern Utah County's growing population.
11. Is a DDI a safe interchange option for this area?
    Safety was a critical factor that was used in analyzing the viability of the DDI. Our studies show that because traffic crosses from the right side of the road to the left side, movements onto the freeway are free-flowing, with less potential for conflict with vehicles traveling in the other direction. Left turns across traffic are also eliminated and potential conflict points are minimized.
12. What are some of the safety features of a DDI?
    A DDI has fewer conflict points, better sight distance at turns and eliminates the left hand turn across opposing traffic when entering or exiting the freeway.
13. How do I navigate the interchange if the traffic light goes out?
    As is the case at any freeway interchange, if there is a power failure with the interchange traffic signals, motorists should stop at the stop bar and treat the situation like a two-way stop. Drivers from each direction should take turns and courteously and cautiously allow each other to pass, first one then the other, until power is restored to the lights.