{"id":490,"date":"2013-06-03T15:30:56","date_gmt":"2013-06-03T15:30:56","guid":{"rendered":"http:\/\/www.trafficsafetystore.com\/blog\/?p=490"},"modified":"2013-06-03T15:30:56","modified_gmt":"2013-06-03T15:30:56","slug":"diverging-diamond-interchanges","status":"publish","type":"post","link":"https:\/\/www.trafficsafetystore.com\/blog\/diverging-diamond-interchanges\/","title":{"rendered":"Road Work on a Budget: How Divergent Diamond Interchanges are Revolutionizing our Roadways"},"content":{"rendered":"

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.<\/p>\n

You wonder, is it always this bad?<\/p>\n

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 Unfortunately, with widespread budget cuts, most states don’t have the money to rebuild outdated infrastructure.<\/p>\n

In the face of these challenges, some transportation officials are discovering a new strategy.\u00a0 Diverging Diamond Interchanges<\/a> \u2013 an increasingly popular design for high-volume junctures \u2013 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.<\/p>\n

The concept is considered a Context Sensitive Solution (CSS)<\/a>, part of a changing design philosophy that attempts to meet traffic goals by retrofitting — rather than rebuilding — problematic infrastructure.<\/p>\n

Currently, highway interchanges are home to 40 percent of accidents and hours of daily delays. The Federal Highway Administration (FHWA)<\/a> 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.<\/p>\n

DDIs do this by eliminating left-hand “T-bone” turns — a major source of jams and accidents. On a traditional \u201cdiamond interchange,\u201d traffic lanes on overpass (or underpass) are parallel and intersect with connecting ramps. Exiting drivers must turn left.<\/p>\n

With a DDI, traffic lanes cross over each other at both ends of the overpass and switch sides in the middle, forming a \u201cdiamond.\u201d Vehicles approaching the far crossover point \u2013 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.<\/p>\n

\"DDI-flow\"<\/p>\n

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\u00a0>four years ago. When Time Magazine ran a story about DDIs in 2011, the concept was considered radical. Won\u2019t drivers get confused traveling on the left? Who will monitor? How will pedestrians safely cross?<\/p>\n

At the time, there were five operating DDIs in the country. Now there are 18. Utah and Missouri, the pioneering states, have several DDIs\u00a0and 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.<\/p>\n

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<\/a>, continues to advocate nationally for DDIs.<\/p>\n

Wyoming Department of Transportation (WYDOT)<\/a> 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.<\/p>\n

The agency searched for solutions. They thought about adding signals to the ramps to manage the onslaught, but realized that method wouldn\u2019t effect delays. They considered building a single-point urban interchange, but reconstruction was too expensive. Once they visited Utah\u2019s I-25 corridor, which hosts multiple DDIs, the decision was clear.<\/p>\n

\u201cIt worked very well,\u201d says Jeff Mellor, a WYDOT geometrics engineer. \u201cThe 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.\u201d<\/p>\n

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 \u2013 including roundabouts, cloverleaves and single-point interchanges \u2013 can cost between $10 and $25 million and take years to complete. Furthermore, because DDIs don\u2019t disrupt the original structure, drivers may continue to use the College Road Interchange throughout the redesign.<\/p>\n

In principal, DDIs are effective because they reduce \u201cpoints of conflict\u201d 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.<\/p>\n

Likewise, spots requiring right-of-way \u201cclearances\u201d are reduced from six to two. This drastically cuts the number of potential stops for the driver.<\/p>\n\n