The French interconnection

Round Rock tabbed as first city in state to build Franco-designed highway interchange system

Traffic at the University Boulevard overpass of I-35 is among the worst in Williamson County, according to a Texas Department of Transportation study of the state’s most congested roadways.

In order to alleviate the congestion, TxDOT and the Round Rock Transportation Department are planning the construction of an unorthodox highway interchange model they believe will speed up the flow of vehicles while also saving the state millions of dollars in construction.

Originally conceived and built in France two decades ago, the roadway design is referred to as a “diverging diamond interchange” (DDI), and one of its most identifiable features is a crossover that directs drivers to the left side of the road. TxDOT announced in November its plans to install a DDI at the University Boulevard overpass of I-35 in Round Rock.

“The bottom line of this thing is the improvement,” said Chris Bishop, TxDOT-Austin District public information officer. “From what my engineering guys are telling me ... it can be as much as a 75 percent improvement in delay time. This could be the key that unties the knot of Round Rock traffic.”

Bishop said that construction on the project should begin in 2014, barring any environmental review delays.

Identifying the problem

The basic concept of installing the DDI in Round Rock is it allows traffic to flow onto and off of University Boulevard more efficiently and at a higher volume. TxDOT identified the University Boulevard overpass of I-35 as an ideal location for the state’s first DDI because of the rising traffic congestion in the area. TxDOT has even tagged University Boulevard/FM 1431 as one of the state’s 100 most-congested roadways, Bishop said.

“We’ve been looking at this with Round Rock and Williamson County for about 18 months,” Bishop said. “The goal here is to deal with the rapid growth, and this is an alternative that allows more traffic to flow because it simplifies the signal times that are required—it cuts the delay. … Based on the [vehicle] traffic patterns, this is the way it should go.”

According to TxDOT engineering studies, the design of the interchange is especially helpful with moving the flow of left-turn traffic—a major source of the backup at the intersection.

“The big benefit to [DDIs] is by switching traffic to the opposite direction, you free up [left turns],” Round Rock Transportation Director Gary Hudder said. “When you have very crowded intersections like [University Boulevard] is, the big conflicting movement that causes most of the delays is the left turns, because those have to be cycled into a signal.

“When you remove that … there is no further contact on the left turns, and it flushes out a tremendous amount of traffic in a hurry.”

Pattern of success

The first domestic DDI opened in June 2009 in Springfield, Mo., and the idea has since caught on with several states’ highway departments.

“They have worked very well,” said Bob Brendel, Missouri Department of Transportation special assignments coordinator. “Not to say there wasn’t a little skepticism when they were first proposed. But as we have built some and put them in operation … the public has accepted them very well, they understand them now, and there is probably a cry to build more.”

The success of the DDI in reducing traffic congestion in Missouri has caught the attention of other state highway transportation departments.

In the past several years, DDIs have been installed at major highway intersections in Georgia, Kentucky, Maryland, Nevada, Tennessee, Utah and New York.

“There’s only 15 to 18 [DDIs] in this country, and [Round Rock] is a perfect place to put one,” Bishop said. “When funding comes ... you are talking about a 12 month [construction] turn-around for the whole thing—for a 75 percent improvement in traffic flow.”

Cost benefit

Aside from increasing the flow of traffic along University Boulevard, the other major benefits cited by transportation officials is the relatively low cost of installing a DDI versus more conventional enhancements such as bridge and road widening.

“We think it is a great design,” Hudder said. “We think it will offer a lot of help with an area that is very congested. And obviously TxDOT doesn’t have tens of millions [of dollars] to throw at alternatives.

“The two alternatives are: don’t do anything—which obviously isn’t going to work. … The other is to completely restructure the overpass … and TxDOT is never going to come to the table with that.”

Bishop estimated the cost of expanding the University Boulevard overpass at more than $40 million, versus approximately $4 million to install the DDI, which does not require road widening.

“The best solution for the least amount of money … is the [DDI],” said Chad Wood, Round Rock city traffic engineer. “The conventional intersection improvement would be to widen the bridge twice as wide as it is now and have three or four lanes in each direction so that you have all these lanes for left turns. But that isn’t going to give you as much improvement as the DDI, and it is going to cost a lot more. It will cost 10 times more and probably not give you even 80 percent of the improvement.”

Bishop said TxDOT plans to host public meetings this winter to introduce the DDI concept to the public.

“When TxDOT takes on a project with this degree of enthusiasm and effort, they intend to get it constructed,” Hudder said. “They are very enthusiastic to get it done; that tells us they either know where the money is, or they will find it.”