More “Best of the States” in Transportation: New Traffic Interchanges, Road Condition Monitoring and Regional Referenda

There is a new issue of the CSG magazine Capitol Ideas out this month that we call our “Best of the States” issue. The magazine contains a wide variety of innovative ideas states are employing to address various problems and needs across numerous areas. Among the innovative ideas in transportation detailed in the issue are a new type of traffic interchange in Missouri, Georgia’s planned regional transportation referenda and Utah’s road condition monitoring cameras that allow the state to determine when to send a snow plow to a remote area. While we tried to pack as much into the issue as we could, there was inevitably plenty of worthwhile stuff that landed on the proverbial “cutting room floor.” Here is a sampling.

Missouri’s Diverging Diamond Interchange

In 2009, the state of Missouri built its first Diverging Diamond Interchange (DDI) or Double Crossover Diamond, as it is sometimes called, at an intersection in Springfield. The interchange is designed to give cars turning left uninterrupted access to the highway through their own ramps by channeling traffic temporarily to the opposite side of the road. While a relatively new type of interchange in this country, the diverging diamond is fairly common in Europe and has been for 30 years. But Missouri’s experience with the interchange is already making it a popular consideration for transportation officials in other states, according to Don Saiko, project manager at the Missouri Department of Transportation, whom I interviewed for the Capitol Ideas issue. Here is the full text of our conversation:

CSG: In what ways has the state of Missouri benefitted from making use of the diverging diamond interchange?

Saiko: “The diverging diamond interchange (DDI) allowed us to reduce congestion at a much lower cost.  By saving money and still reducing congestion we are able to use the cost savings for additional projects.  Motorists that use this interchange daily no longer have to wait in backed-up traffic; they are able to move through this interchange quickly. Prior to the DDI, motorists often experienced traffic backups of at least one mile during normal everyday peak times and up to 2-3 miles during heavy travel periods such as the Fourth of July Holiday.”

CSG: What advantages generally are there to choosing to build a DDI over other configurations?

Saiko: “The advantages we have seen are: A much lower cost than other interchange types that were considered.  The other alternative we considered was to build an interchange called a single point urban interchange (SPUI).  By constructing the DDI instead, we saved almost 70 percent.  The DDI cost $3.2 million and the SPUI would have cost $10 million. The interchange has been safer by reducing crashes by fifty percent.  Our most severe crashes—left-turn right-angle crashes from the arterial street to the freeway—have  been eliminated.  This was done by moving the traffic to the other side of the bridge and giving left turning vehicles a free left instead of having them cross in front of oncoming traffic. Construction time was quicker because we were able to use the existing bridge and did not have to reconstruct the entire interchange with a larger bridge structure. We were able to build the project at I-44 & MO-13 (in Springfield) in six months instead of 18-24 months for a complete rebuild of the interchange.”

CSG: This type of interchange has already been duplicated in Missouri. I understand you’ve also had significant interest from other states that are looking to duplicate Missouri’s success with the DDI. What has been the level of interest you’ve seen and how is this concept spreading around the country?

Saiko: “There has been widespread interest in the diverging diamond interchange, especially when many state DOTs across the country are facing tough economic times and are seeing a dramatic decrease in funding.  These agencies are looking for ways to improve traffic flow with decreased funding.  The DDI is one way this can be done if it's the right solution.  I have spoken to other state DOTs and private consultants in 35 states and many of them are pursuing the use of the DDI because of its low cost and improved traffic flow.”

CSG: Is the DDI something that is easy to duplicate? Are there any drawbacks? Any lessons learned from Missouri’s implementation of the DDI that you would pass along to other states?

Saiko: “The biggest piece of advice I can offer to other states is to run a traffic model for the interchange to make sure the DDI is the correct solution because it won't work for every interchange. The DDI works well for us because we had heavy left-turn movements and an unbalanced traffic flow.  By unbalanced traffic flow, I'm referring to one direction of traffic is heavier than the other during peak times. If the traffic would have been balanced in both directions and our left turns were not heavy then the DDI would not have been the right choice because there wouldn't have been any reason to move traffic to the opposite side of the road. We would have wanted to have opposing traffic proceed at the same time if there was a heavy through movement and not many left-turning vehicles. The DDI can be easily retrofitted to an existing diamond interchange if it's the right solution and it can be done much quicker if the existing bridge does not need to be replaced. One thing we changed after the DDI was implemented was to add traffic signals for the right turns coming off the freeway.  Initially, this was set up as a yield condition but many motorists would glance to the near side of the bridge instead of looking for traffic on the opposite side.  This caused many close calls so we added traffic signals. The DDI works great if it's the right solution.  The DDI is not the fix-all for every congested interchange.  It is important to model the traffic before deciding the DDI is the right choice.”

Additional Reading

You can read more about the Diverging Diamond/Double Crossover Diamond Interchange in a recent article Saiko co-authored for the Federal Highway Administration’s Public Roads magazine. Missouri’s experience with the interchange is also detailed in a Missouri DOT report from last May.

Traffic Roundabouts

Another type of traffic intersection getting a fair amount of attention of late is the roundabout, which was first developed in the United Kingdom in the 1960s. While the first roundabout appeared in this country only about 20 years ago, more than 2,000 have sprung up since then, many of them just in the last decade. According to the Insurance Institute for Highway Safety, more than half the states have programs to promote the use of roundabouts where roads are being built or re-engineered.

Roundabouts are believed to be safer because they take away the number of “conflict points,” where the paths of two vehicles cross. Vehicles are instead channeled into the same direction at similar speeds and while the intersections keep those vehicles moving, they are forced to slow down due to the intersection’s tight turns.  

Studies also show that replacing conventional intersections (those with traffic lights or stop signs) with roundabouts can help vehicles travel more efficiently (less starting and stopping) and in the process can reduce harmful emissions by more than 30 percent.

Additional Reading:

“European Import Has Cars Spinning. Heads, Too.” The New York Times, November 18, 2010

“Traffic Roundabouts: Coming to An Intersection Near You?” The Wall Street Journal, November 4, 2010

Road Condition Monitoring

Our “Best of the States” issue of Capitol Ideas also includes a blurb about Utah’s installation of low-cost road-condition monitoring cameras that allow state transportation officials to keep an eye on dangerous mountain passes and remote areas and quickly determine whether they need to send a snow plow.

The state of Delaware is taking a similar approach. The Delaware Department of Transportation has 11 weather stations built into roads that monitor surface and subsurface temperatures, as well as the moisture hitting the pavement. The stations allow them to determine when and where to apply rock salt. Along with the department’s 119 traffic cameras around the state, the weather stations help save time and money once spent sending road crews out to check weather conditions.  

Technology will also allow the department this spring to start posting travel time estimates for the busiest roadways on its website and elsewhere.

Additional Reading:

“DelDOT’s ‘war room’ technology helps save time, money throughout the year.” Delaware News Journal, February 2, 2011.

Georgia’s Regional Transportation Referenda

Capitol Ideas also highlights Georgia’s plan to have voters in the state’s 12 regions decide on primary election day next year (August 21st) whether to approve a penny sales tax increase to fund transportation projects in their region. The regional transportation referendum is seen as a potentially innovative idea because unlike with statewide funding initiatives, voters would know their tax dollars are going only to projects they approve of in their area—for example, the rural roads they drive on regularly, rather than big city transit systems they seldom use.  

Stephanie Carter, a special assistant to Georgia Transportation Commissioner Vance Smith, was among the speakers at a forum in January at the Transportation Research Board’s annual meeting in Washington, D.C. During her remarks she touched on how the referenda concept came about and what their passage could mean for the state. Here are some excerpts from her remarks:

“Starting around four years ago, it was actually the business community that started pushing the legislature to find a new funding source for transportation. And one of the reasons was because we were losing businesses to places like Charlotte that did not have the congestion that Atlanta did …”

“In the state of Georgia, we have an excise tax of 7.5 cents on motor fuel and that goes straight to DOT. We also have a 3 percent sales tax on motor fuel that also goes to DOT. That gives us an effective tax rate of 15 and ½ to 16 percent. So we’ve been severely underfunded. In fact, there’s only one state that spends less than Georgia per person on transportation and that’s Tennessee. So based on that knowledge we realized we had to come up with something else …”

“We have 12 districts that are based on their regional commission boundaries and each district can levy a sales tax for a period of 10 years of 1 percent. One of the big things in this (referendum process) though is that counties cannot opt out. And that caused a lot of uproar from a lot of the counties and the cities because they were like “just because all the counties in our region want to pass this, doesn’t mean we should be forced to do it.” But (then-Gov. Sonny Perdue) stood strong on that and said “no, the road doesn’t end at the county line. We’ve got to start looking at this from a regional perspective …””

 “If all 12 (regions) end up passing the 1 percent sales tax in 2012, it will (mean) $1.5 billion in transportation funding in Georgia. That’s doubling our current program. Last year, our motor fuel revenue was $852 million. It’s huge for the state ...”

“The good thing about the sales tax is it’s only 1 percent but if we had raised our gas tax to bring in $1.5 billion statewide, we would have had to have increased our gas tax by 28 cents and that is not politically feasible in a red state—in any state ...”

“It could pass in six of the 12 regions and we’ll have that money there or it could not pass in any of them… They were afraid that a statewide vote might not pass but you could at least pass it maybe in places like Atlanta, Coastal Georgia where the ports are so vital and we’re actually deepening our port (the Port of Savannah) down there in advance of the Panama Canal widening and places like Northwest Georgia that are booming right now. And so if those pass in those regions, that’s when (Georgia DOT) will actually start the implementation of the projects.”