We all know the nightmare: You’ve just left an interstate rest stop, late afternoon, and the overpass is already at a standstill. You continue to wait as an 18-wheeler awkwardly cuts across oncoming traffic to get onto the exit ramp. Honks and shouts follow. Twenty minutes later, you see an opening in the turn lane, but another driver has snuck up from behind and snagged the spot.
You wonder, is it always this bad?
Throughout the U.S., traffic volume is outgrowing road capacity. Pileups, delays and road rage are increasingly unavoidable. The problem gets critical at highway interchanges where left-turning vehicles constantly interrupt the flow. Unfortunately, with widespread budget cuts, most states don’t have the money to rebuild outdated infrastructure.
In the face of these challenges, some transportation officials are discovering a new strategy. Diverging Diamond Interchanges – an increasingly popular design for high-volume junctures – significantly improve traffic flow by changing the road layout. The solution, sometimes called a Double Crossover Diamond (DCD), doesn’t require new bridges or ramps. This cuts time and costs, relative to other models, by over 70 percent.
The concept is considered a Context Sensitive Solution (CSS), part of a changing design philosophy that attempts to meet traffic goals by retrofitting — rather than rebuilding — problematic infrastructure.
Currently, highway interchanges are home to 40 percent of accidents and hours of daily delays. The Federal Highway Administration (FHWA) believes highway traffic volume could increase by 30 percent by 2030, piling many more vehicles onto the overtaxed structures. This leaves cash-strapped transportation departments with a new challenge: Revise the hotspots without building more lanes.
DDIs do this by eliminating left-hand “T-bone” turns — a major source of jams and accidents. On a traditional “diamond interchange,” traffic lanes on overpass (or underpass) are parallel and intersect with connecting ramps. Exiting drivers must turn left.
With a DDI, traffic lanes cross over each other at both ends of the overpass and switch sides in the middle, forming a “diamond.” Vehicles approaching the far crossover point – from the left hand side of the overpass — can veer onto the exit ramp before heading back to the right hand side of the road. It’s a straight shot from the bridge to the freeway.
Although DDIs have been used in Europe since the 1970s, they’re new to the U.S. — the first was built in Springfield, Missouri less than >four years ago. When Time Magazine ran a story about DDIs in 2011, the concept was considered radical. Won’t drivers get confused traveling on the left? Who will monitor? How will pedestrians safely cross?
At the time, there were five operating DDIs in the country. Now there are 18. Utah and Missouri, the pioneering states, have several DDIs and are planning more. Meanwhile, other states including Pennsylvania, Delaware, Indiana and North Carolina are building their first. Officials from nearly every state have been exploring the option.
DDIs first gained the interest of U.S. engineers in 2003 when Gilbert Chlewicki, a graduate student at University of Maryland, presented the concept at the Urban Street Symposium. Chlewicki, who now works for the Maryland-based engineering firm Wallace Montgomery, continues to advocate nationally for DDIs.
Wyoming Department of Transportation (WYDOT) is one of the newest members to the DDI bandwagon. When the agency first built the interchange at College Road and Interstate-25 in Cheyenne, there was a lot less traffic. Over the years, the juncture steadily gained three truck stops and an increasing volume of tractor trailers. Then a nearby business park opened and congestion spiraled out of control.
The agency searched for solutions. They thought about adding signals to the ramps to manage the onslaught, but realized that method wouldn’t effect delays. They considered building a single-point urban interchange, but reconstruction was too expensive. Once they visited Utah’s I-25 corridor, which hosts multiple DDIs, the decision was clear.
“It worked very well,” says Jeff Mellor, a WYDOT geometrics engineer. “The movements from the ramps to the College Drive crossroads had created a lot of delay. We selected DDI to reduce the left-hand turns for the trucks going to and from the truck stops.”
The project, which is set to reopen this fall, is expected to cost WYDOT just six months and $1.5 million. To put this in perspective, traditional interchange remodeling – including roundabouts, cloverleaves and single-point interchanges – can cost between $10 and $25 million and take years to complete. Furthermore, because DDIs don’t disrupt the original structure, drivers may continue to use the College Road Interchange throughout the redesign.
In principal, DDIs are effective because they reduce “points of conflict” where traffic confronts other streams of traffic or pedestrians. The typical Diamond Interchange hosts 26 of these points because each stream of traffic crosses at every artery. A DDI reduces this number to 14: Crossover points serve traffic in two directions and require two stoplights. Four-way interchanges, by comparison, host up to eight traffic paths.
Likewise, spots requiring right-of-way “clearances” are reduced from six to two. This drastically cuts the number of potential stops for the driver.
“The introduction of the signals for the diverging diamond doesn’t really change the character of the roadway,” says Ray Moravec, a transportation planner for Wallace Montgomery. “It helps you minimize the amount of right-of-way required and still have the operational benefit.”
How well do these principals hold up?
It’s too early to get a complete picture – most U. S. DDIs haven’t been evaluated. That said, the initial evaluation of the first U.S. DDI by Missouri Department Of Transportation (MODOT) yielded promising results: 46 percent fewer crashes including a slight reduction in rear-end crashes; 80 percent decrease in average number of daily delays; 40 percent decrease in drivers’ overall wait time.
That doesn’t mean that DDIs are a blanket solution for all junctures. Still, a 2012 Modeling and Simulation in Engineering report by Hindawi — which used ProccessModel, an advanced simulation software — found DDIs reduce wait times in most cases.
The prevailing concern among travel officials, drivers and engineers has been the potential for confusion at the crossover points, where drivers are required to switch to the left hand side. These fears have not been substantiated. A 2009 TechBreif analysis by FHWA, which observed 1,041 runs through a simulated DDI, did not find a single incident where crossover points were mishandled.
The visual confusion extrapolated from an aerial view, Moravec points out, is not what the driver experiences. Lanes cross over each other at 90 degrees, making it difficult for traveler to continue to the right – the angle is too abrupt. “Just because it’s different doesn’t mean that it won’t be understood or accepted by the public,” he says. “When you’rem driving, you don’t even notice you’re going through a diverging diamond.”
In fact, large trucks and tractor trailers – which have chronic difficulty with right-angle exit turns on diamond interchanges – can negotiate curving DDI roadways more easily.
Some states, moreover, are finding paybacks on DDIs aren’t limited to left-hand turns. WYDOT, for example, is creating a “safer” pedestrian zone at College Road and I-25. Previously, the interchange had a sidewalk on one side of the overpass that intersected with ramps. Once the DDI is complete, pedestrians can cross diagonally through the crossover points and travel through the median. In some DDIs, pedestrian paths and bike lanes wind through the busy interchanges.
“Again, it’s a different way of approaching it because you now have pedestrians in the median,” says Moravec. “It is actually safer because you have fewer pedestrian crossings in locations that are not controlled by signals.”
Furthermore, WYDOT says their new DDI can be easily expanded in 20 years to meet volume projections. The layout, officials say, leaves room to squeeze in an extra lane on both sides.
Not everyone, however, is convinced of the added benefits. Because DDI traffic flows continuously to exit ramps, pedestrians can’t safely walk along the borders, where sidewalks are typically placed. Many question if the median, sandwiched between busy lanes, is really the best place for people. If a DDI is expanded, new lanes will cut into the median, making the overpass virtually inaccessible to anyone except drivers.
Additionally, several traffic options are lost when overpass lanes no longer intersect with ramps: Oversized vehicles can’t cross over the bridge using the ramp; traffic that piles up from an accident can’t be cleared to the shoulder; drivers who’ve taken the wrong exit must travel through the entire diamond before turning around.
Nonetheless, for roadways impaled with multiple arteries expected to carry increasing traffic, DDIs have favorable cost-benefit. Thanks to modern simulation software, and an increasing number of examples, transportation officials can finally see how.
“Federal Highway and others are really trying to get a toolbox together on these various innovative interchange designs,” says Moravec. “Diverging Diamonds fits in the library of options.”
Top Image Source: Garrett on Flickr