TxDOT Builds a Rare Diverging Diamond

Explosive Growth in Austin Triggers Novel Traffic Solution

By Bob Austin, P.E.

The MoPac Expressway intersections at Slaughter Lane and La Crosse Avenue in southwest Austin were originally constructed in 1992. Since that time, Austin’s population has nearly doubled, according to the U.S. Census. Average daily traffic on the MoPac Expressway, which was 21,000 in 2010, is projected to reach 66,700 by 2030.

As a result, the intersections at Slaughter Lane and La Crosse Avenue became congested over the years, creating operational problems, causing travel delays, and affecting access, safety and mobility. In 2013, the Central Texas Regional Mobility Authority and the Texas Department of Transportation (TxDOT) initiated an environmental study to analyze and determine options for reducing travel delay and enhancing safety at these intersections.

“During the morning and evening peak hours, motorists had to wait through multiple traffic signal cycles at these two intersections,” said Lucas Short, P.E., TxDOT’s project manager.

Following the study and public review, TxDOT decided to construct underpasses at Slaughter Lane and La Crosse Avenue and extend the MoPac main lanes under these two intersections. The MoPac main lanes, two in each direction and approximately two miles long, would be constructed inside the existing MoPac lanes. The existing Mopac lanes would be reconstructed and serve as ramps to carry main lane traffic to Slaughter Lane and La Crosse Avenue.

The grade separation at La Crosse Avenue and MoPac was configured as a standard diamond interchange while the grade separation at Slaughter Lane and MoPac was configured as a diverging diamond interchange (DDI) – the first in the city of Austin and the second in the Austin area. Other improvements include constructing an additional 10-foot-wide shared-use path on the west side of MoPac from Slaughter Lane to La Crosse Avenue, retaining walls at the bridges, a storm sewer system, traffic signals, new pavement, and striping and signing throughout the project limits.

“The improvements will reduce delays for northbound and southbound traffic in the MoPac Expressway as commuters don’t have to stop at a signalized intersection to travel through the area,” said Short. “Also, constructing the DDI at Slaughter Lane will allow for more efficient left turn movements.”

To implement these improvements, TxDOT selected Lockwood, Andrews & Newnam, Inc. (LAN), a national planning, engineering and program management firm, as the lead design engineer. Kimley-Horn and Associates, Inc. was selected as the traffic control plan and temporary signals sub-consultant, Cobb Fendley and Associates, Inc. as the illumination sub-consultant, and IDCUS, Inc. as the drainage, signing and striping sub-consultant. In August 2017, the project went to letting and Webber was awarded the contract as the low bid.

Diverging Diamond Interchange

A DDI is an innovative design solution that addresses congestion by allowing vehicles to travel more quickly through an intersection. Though a relative rarity in the United States, it is gaining popularity because of its efficiency and safety. The first DDI was constructed a decade ago in Springfield, Missouri. Since then, 88 have been built. DDIs are particularly effective at locations that have a high volume of left-turn traffic. In a DDI, traffic is temporarily shifted to the left side of the road to increase traffic flow by allowing through-traffic and intersecting traffic turning left to proceed through the intersection simultaneously. Consequently, this configuration can accommodate 40 percent more left-turning vehicles than a standard diamond interchange with the same number of lanes.

Additionally, a DDI enhances safety by reducing potential crash points at intersections. Compared to a conventional diamond interchange, the DDI reduces vehicle-to-vehicle conflict points by nearly 50 percent, according to the Federal Highway Administration. Other DDI benefits include additional green time at traffic signals to allow more vehicles to pass through the intersection, increased safety for pedestrians and bicyclists due to additional sidewalks, and low-cost construction.

“Our traffic analysis showed that in the morning peak hours, the predominant movement is east-bound Slaughter Lane to north-bound MoPac,” said Short. “In the afternoon rush hour, the predominant movement is south-bound MoPac to east-bound Slaughter Lane. We considered a variety of intersection types for Slaughter Lane, including a standard diamond intersection, single-point urban intersection, grade-separated roundabout, and the DDI. Ultimately, we chose the DDI because it can handle substantial left-turn volumes.”

Environmental Protections

The MoPAC Intersections project is located over the Barton Springs segment of the Edwards Aquifer Recharge Zone. The Barton Springs segment of the Edwards Aquifer supplies water to between 50,000 and 60,000 people, provides habitat for two endangered salamander species, discharges at the Barton Springs complex, and is one of the most studied karst aquifer areas in Texas (the word karst describes an area of irregular limestone in which erosion has produced fissures, sinkholes, underground streams and caverns).

To protect the Edwards Aquifer, TxDOT and the project team implemented rigorous environmental measures that met, or even exceeded, the Texas Commission on Environmental Quality (TCEQ) compliance requirements. The measures included four categories of environmental controls: environmental compliance management, karst void mitigation, temporary construction controls, and permanent post-construction controls.

• Environmental Compliance Management – TxDOT hired an independent environmental compliance manager to monitor construction and ensure that Best Management Practices (BMPs) for environmental protection are implemented and function as designed. This is in addition to construction management and environmental compliance personnel provided by the contractor. This level of environmental compliance oversight is atypical for TxDOT projects and exceeds TCEQ requirements.
• Karst Void Mitigation – Construction plans and specifications provide construction details and notes directing the contractor on how to proceed if a karst feature is encountered. In addition to specific construction details, the contractor is directed to notify the TxDOT project manager, environmental compliance manager, TCEQ and the City of Austin.
• Temporary Construction Controls – The project incorporates erosion and sedimentation controls such as silt fences for perimeter control, rock berms for concentrated flow and construction exits to control sediment onto roadways.
• Permanent Water Quality Controls – The project includes a suite of storm water quality measures that, in combination, will exceed the 80 percent Total Suspended Solids (TSS) removal requirement of the TCEQ Edwards Aquifer Protection Program rules.  These measures include vegetated filter strips, permeable friction course, and retrofit of the existing vertical sand filters. In addition, the project includes seven batch detention ponds functioning primarily as hazardous material traps, but also providing extended detention to remove the Total Suspended Solids.

Accelerated Construction

Due to the high traffic volumes in the two intersections, TxDOT wanted to minimize the impact to the traveling public during construction. To this end, the project team employed a number of strategies to expedite construction. One such strategy was implementing Accelerated Bridge Construction (ABC) techniques such as precast bent/abutment caps and post-installed column aesthetic features.

“On conventional bridges, you excavate to the top of the foundation, which in many cases are the drilled shafts,” said Mario Rosino, Webber’s project manager. “Then you build the columns up, build the cap in place, set beams and then pour the deck. On Slaughter Lane, we built the bridges essentially at-grade. We built the drilled shafts and the columns through the existing pavement under nightly lane closures. Then we pushed traffic into a new detour section and excavated down to sub-grade, exposing the drilled shafts and then set the pre-cast caps in place.”

By using pre-cast elements, Webber was able to find savings in concrete cure time, thereby reducing the total schedule. After completing the bridge, the contractor dressed-up the columns with Class K concrete so they had a smooth finish with the appropriate form liner. Also, by using a new detour section, Webber was able to build the bridges in Slaughter Lane in one phase instead of two, which accelerated the construction process.

To expedite the construction, TxDOT also employed a disincentive/incentive strategy. As part of this strategy, TxDOT set the contractor three milestones with penalties for exceeding the timeline and bonuses for early completion. The first milestone, which had to be substantially completed in 150 working days, required opening the two new bridges for Slaughter Lane, opening one MoPac main lane in each direction, and closing a single frontage road lane in each direction. This milestone also required opening the DDI in temporary configuration. TxDOT set the disincentive/incentive for late/early completion at $20,000 per day.  The second milestone, which required substantial completion in 70 working days with a disincentive/incentive of $15,000 per day, involved opening Slaughter Lane and all the frontage roads within 300 feet of Slaughter Lane in the final DDI configuration. The third and final milestone required opening La Crosse Avenue bridge and a single MoPac main lane in each direction in 70 working days, with a disincentive/incentive of $10,000 per day.

“The goal is to motivate the contractor to complete the project ahead of schedule,” said Ralph Condra, P.E., LAN’s project manager for construction engineering and inspection services. “TxDOT employs this strategy for critical projects where traffic inconvenience and delays need to be held to a minimum.”

Thanks to these innovative design and construction strategies, TxDOT and the project team have kept the project on schedule and under budget. The construction of the $53.5 million project began in January 2018 and is expected to be completed in winter 2020.

“The Slaughter Lane and La Crosse Avenue intersections are the only remaining at-grade intersections in the entire MoPac corridor,” said Short. “This project will help us remove a critical bottleneck and go a long way toward reducing congestion in southwest Austin.”

Bob Austin, P.E., is a vice president at Lockwood, Andrews & Newnam, Inc. (LAN), a national planning, engineering and program management firm. He can be reached at RDAustin@lan-inc.com.