Cost savings for newer type of interchange design is allowing Fillmore project to be built sooner than expected
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 Transportation (CDOT), explained Maureen Paz de Araujo of HDR, an engineering consultant for both Colorado Springs and CDOT.
She was among several representatives of CDOT and its consultant firms to present exhibits (including a simulated DDI traffic video) and answer questions at a two- hour public open house in the county's Citizen Service Center March 5.
Don Garcia, project manager for CDOT, said that about 70 people came to hear about the upcoming $13 million project. His impression was that most were OK with the DDI plan. A few people were concerned about its elimination of left turns in and out of Sinton Road going north. But he thinks they were satisfied that they can use Ellston Street to get the same result without going too far out of their way. Still under consideration is a previous request -- allowing left turns to access Sinton Road south of Fillmore (and just east of the interchange), where two businesses operate.
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.
Garcia said that the design is almost final now, but the project can't move forward until an intergovernmental agreement (IGA) is signed between CDOT and the city. The IGA is needed because the project is being partially funded through CDOT's Responsible Acceleration of Maintenance and Partnerships (RAMP) grant program. About 200 RAMP grants exist around the state and even though Fillmore's was the first to have its IGA “ready to go,” the state may not actually process the paperwork on its side till June, and after that it will still require mayoral and City Council sign-off, Garcia said.
Until recently, the project had slim prospects of starting even 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.
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 United States 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. This will allow a construction benefit, in that the current interchange (which will be between the two new bridges) can continue to stay open during construction.
Below is a Wikipedia list with pros and cons for DDI (edited to be as specific as possible to Fillmore/I- 25):
- Provides for two-phase signals with short cycle lengths, significantly reducing delay.
- Reduced horizontal curvature reduces risk of off-road crashes.
- Increases the capacity of turning movements to and from the ramps.
- Reduces the number of lanes on the crossroad, minimizing impacts to existing right-of-way.
- Substantially reduces the number of conflict points, thus theoretically improving safety.
- Theoretically improves pedestrian safety.
- Increases the capacity of an existing overpass or underpass, by removing the need for turn lanes.
- Drivers may not be familiar with configuration, particularly with regards to merging maneuvers along the left side of the roadway or the crossover flow of traffic.
- Free-flowing traffic in both directions on the non-freeway road is impossible, as the signals cannot be green at both intersections for both directions simultaneously.
- Exiting traffic cannot re-enter the freeway in the same direction without first leaving the interchange on the crossroad.
- Emergency management cannot use the exit and entrance ramps to allow freeway traffic to bypass a crash at the bridge.
- Although there is a separate pedestrian/bike path, it crosses vehicular traffic in several places.
Westside Pioneer article