By Kevin Wilcox
A highway expansion project in Austin includes a diverging diamond intersection to improve safety and traffic flow at a critical juncture.
A rare diverging diamond intersection design has been selected to alleviate bottlenecks at the crossing of Loop 1 on the west side of Austin, known locally as the MoPac Expressway, and Slaughter Lane. Lockwood, Andrews and Newnam
June 7, 2016—The Texas Department of Transportation (TxDOT) and its consultants are completing the engineering design of a 2.069 mi, $53-million project that will alleviate congestion on Loop 1 on the west side of Austin—known locally as the MoPac Expressway, for the Missouri Pacific Railroad—a critical north-south artery in the state's capital. To accomplish this, the department plans to use both a traditional diamond intersection at one point (on La Crosse Avenue) and an uncommon diverging diamond intersection (DDI) at another (Slaughter Lane).
The DDI is still a relative rarity in the United States, though it is gaining popularity because of its efficiency and safety. The first DDI was constructed in 2009, and in the following six years, approximately 59 others were completed. In a DDI, motorists are diverted to the left side of a bridge as it passes over the main lanes and diverted back to the right after the intersection (see the green arrow in the illustration below). One benefit of this arrangement is that the southbound frontage road traffic (shown by the orange arrow at the top left of the illustration below) will not need to stop when the traffic following the green arrow has a green light. Also, the green arrow traffic crossing the expressway will have a signal-free left turn.
"DDIs reduce the number of conflict points, so the number of accidents and the severity of accidents go down as well," explains Terry McCoy, P.E., the district engineer for TxDOT's Austin District, who notes that the first DDI built in the Austin district, at the intersection of Interstate 35 and RM 1431, reduced delays by 50 percent while handling a 50 percent increase in traffic.
The project on the MoPac Expressway is critical because the population of Austin has doubled since the expressway was completed in 1992—to an estimated 931,830 residents in 2015. And average daily traffic on the expressway, which was 21,000 in 2010, is projected to reach 66,700 by 2030.
Motorists will be diverted to the left side of a bridge as it passes over the main lanes and diverted back to the right after the intersection, shown by the green arrows, above. Southbound frontage road traffic, shown by the orange arrow at top left, will not need to stop for traffic from the green-arrow lanes. Lockwood, Andrews and Newnam
"This project has been on the radar for quite some time," says McCoy. "We are faced with two signalized intersections on a major expressway. The motivation here is to grade-separate those intersections to alleviate congestion and address mobility."
As congestion has grown on the expressway, morning and evening rush-hour commuters often wait through several traffic signal cycles to make left turns. Having a DDI with only two primary signal phases will enable the green light for left turns to last longer than it would in a conventional diamond on a four-phase cycle, says Robert Austin, P.E., an associate with Lockwood, Andrews and Newnam (LAN), headquartered in Houston. TxDOT hired LAN for the engineering of the design/bid/build project.
The plan also includes the addition of four main lanes—two in each direction—along a picturesque portion of the expressway that passes by the Circle C Ranch, a planned community, and under the diamond intersection La Crosse Avenue. La Cross Avenue and Slaughter Lane will both cross over the expressway via a prestressed concrete girder bridge comprising two 100 ft spans.
TxDOT considered a variety of intersection types for Slaughter Lane, Austin says, but chose the DDI configuration because of the high incidence of left turns there. A DDI can accommodate 40 percent more left-turning vehicles on the same number of lanes as a conventional diamond, he notes.
The engineering team utilized 3-D software to better visualize the details of the DDI, checking bridge clearances over the main lanes and sight distances at intersections. The software enabled the engineers to virtually fly through the intersection and also to send animated traffic flow through it to ensure that it works as expected.
"We, as a district, have implemented several different innovative intersections. A diverging diamond makes sense in some situations," McCoy says. "But we are also using continuous flow intersections in a few different places around the district. The traffic and growth has motivated us to take the blinders off—to not work in that traditional box, but to look for answers that make people's lives much better."
The project was subjected to a rigorous value engineering (VE) study that uncovered approximately $5 million in savings—roughly 10 percent of the project budget. The VE study team, which includes TxDOT Mopac project managers Lucas Short, P.E. and Adrian Martinez, P.E., identified extensive savings. Some of these include reusing the base under the existing expressway lanes and mixing it with new base, as well as using such accelerated bridge construction methods as precast pier caps and collars.
Austin says that one of the biggest challenges of the project so far has been moving the many facets forward simultaneously on a tight schedule. "This project had everything. It had roadway design, bridges, retaining walls, illumination, traffic signals, storm sewers, signing, striping, and temporary and permanent water quality features," Austin says. "Everything you can think of, it had it in there—and on an accelerated schedule. We had weekly meetings to make sure every design team was progressing, and that nobody was waiting on anyone else."
TxDOT is hoping to let the project in fiscal year 2017. The construction team will have a planned 30-month schedule.
"As a district, as a region, and as a community, we look at our entire system to find where we may have some shortcomings and where we can make major improvements," McCoy says. "MoPac acts as a check valve for people being able to get into and out of the city in the morning and afternoon, and in addition to that, people being able to get into and out of neighborhoods. It is an important part of our network, and this is a great opportunity to make some improvements."