DDI credited with safety, savings; Fillmore project starts in Aug.

       A diverging diamond interchange (DDI) design emerged relatively late in the planning process over the past 20 years for a Fillmore/I-25 replacement.

A construction scene on Fillmore Street during last year’s Chestnut/Fillmore project. The interchange is in the background.
Westside Pioneer file photo

       When that finally happened about three years ago, it was mainly the cost savings the newer, unorthodox design offered that changed the minds at the Colorado Department of Transporta-tion (CDOT), explained Maureen Paz de Araujo of HDR, an engineering consultant for both Colorado Springs and CDOT.
       The current Fillmore schedule calls for hiring a contractor in June or July, with work expected to start in August and continue until October 2015. During the project, the current bridge will stay open, project manager Don Garcia said.
       Until the new design was accepted, the project had slim prospects of starting this soon. A new Filllmore/I-25 had been part of the environmental assessment that led to the COSMIX bridge-and-widening project of 2005- 2007. It had a more traditional single-point urban interchange (SPUI) design then, with a cost estimate of about $40 million, and COSMIX couldn't afford it. By 2008, allowing for inflation, the estimate had grown to more than $80 million. No funding was in sight, and local scenarios showed the interchange project not getting going until at least 2021.
       Then in 2009, the first DDI in the United States was built over I-44 in Springfield, Missouri. Traffic engineers around the country took notice because not only did the design make left turns safer, its cost was about a third of the more traditional alternative.
       Since then, 34 DDI's have been built in America, with one of the newest at US 6 and US 50 in Grand Junction, according to divergingdiamond.com, which describes itself as the “Official Website of the DDI - “A Diamond Interchange With a Twist.”
       CDOT did not jump immediately on the DDI bandwagon for Fillmore. But when long-range studies factored in traffic to the year 2035 instead of 2025 (as the previous study had), they showed significantly higher traffic numbers. As a result, “we tried to find something cheaper and better,” Araujo summarized.

A CDOT graphic at an open house shows the design for the diverging diamond interchange where Fillmore Street crosses above I-25, as well as a list of its advantages. Following arrows drawn in the lanes, it can be seen how westbound and eastbound traffic cross each other at either end of the interchange (aided by a stoplight, not shown). The result is that vehicles can flow on and off the interstate in either direction -- making left or right turns -- without needing to cross in front of oncoming traffic. A note about the aerial photo used for this design rendering: The photo must have been shot before 2012. Since then, the gas stations have been removed from the old Chestnut/Fillmore intersection next to the interchange, the houses have been removed from the end of Parker Street nearest to Fillmore (allowing a cul-de-sac to be built short of the Waffle House) and the Chestnut realignment shown in the graphic has been built.
Courtesy of HDR Consulting and Colorado Department of Transportation (CDOT)

       It helped that HDR has been a consultant on both Fillmore projects - to the city on the recently completed Fillmore/Chestnut work funded by the Pikes Peak Rural Transportation Authority (RTA) and to CDOT on the interchange. So the company has been involved in the process, including the coordination between two entities, for several years.
       When asked who had actually proposed the idea for Fillmore, Araujo grinned and said, “Me.”
       Where did the idea originally come from? Actually two places. In France in the 1970s, such a design was built at road crossings in three communities. But the idea did not pop up in the U.S. until 2000 when Gilbert Chlewicki, a graduate student at the Universiy of Maryland (who was unaware of the French projects at the time) drew up a very similar design as part of work for his master's degree in transportation engineering. And it was Chlewicki, now a civil engineer, who gave the design its name.
       The cost savings result from fewer lanes being needed, reducing the structural size and the time for construction, explains divergingdiamond.com. On Fillmore, for example, there will be five lanes in all. Three will be on the westbound side, with one becoming a dedicated southbound I-25 left-turn lane. Backups for that turn have been a particular issue on the current interchange, Araujo said.
       In addition, there is a DDI factor that Araujo described as “monetizing the reduction in delay” - which essentially means that cars don't wait as long in traffic. According to a Popular Science article on the Springfield DDI, “the Federal Highway Administration estimates that the diverging diamond configuration enables 600 left turns onto the freeway per hour per lane - double that of an ordinary interchange, where drivers cross oncoming traffic.”
       At Fillmore, the project might have been even cheaper if the original idea of laying a new DDI “deck” on top of the old structure could have been culminated. But Araujo said that further study showed concern that the existing, half-century-old span would not last beyond 15 more years. As a result, final plans call for the interchange to have two separated bridges, one for each direction of traffic, plus a pedestrian/bike path on the eastbound side.
       A few drawbacks have been identified with DDI interchanges, including driver unfamiliarity affecting merging maneuvers, exiting traffic unable to immediately re-enter the freeway in the same direction (which also prevents emergency management from using the exit and entrance ramps to bypass a freeway crash, and although there is a separate pedestrian/bike path, it crosses traffic lanes in several places.

Westside Pioneer article

Note: This article is condensed from a longer version that first appeared on the Westside Pioneer’s online site at westsidepioneer.com. It is archived under 2014 in the Transportation category, Fillmore/I-25 subcategory.