CTRMA 183A Phase III

Extending the tollway to support regional growth

Project Purpose

As the greater Austin area continues to see rapid population growth, the daily traffic volumes for both the tolled and toll-free sections of the US 183 corridor in Williamson County are expected to increase by 183% between now and 2041. To accommodate the projected increase in traffic, the Mobility Authority is extending the 183A tollway from the existing US 183/183A intersection to north of SH 29, a distance of about 6 miles. The new extension, known as 183A Phase III, will be constructed in the median section of US 183, allowing the Mobility Authority to increase traffic capacity with minimal acquisition of right-of-way. This project also extends the shared use path north from Hero Way to the proposed Seward Junction Loop project to serve bicyclists and pedestrians.

Project Approach

Serving as a subconsultant to Rodriguez Transportation Group, KFA provided drainage, water quality, and stormwater engineering services for this project, which is located within the watersheds of Brushy Creek and the South Fork of the San Gabriel River. Our tasks included performing hydrologic and hydraulic (H&H) analysis, designing parallel drainage systems and water quality ponds, identifying temporary drainage needs based on project sequencing, preparing the TCEQ Contributing Zone Permit, coordinating with the shared use path designers to mitigate drainage-related impacts, and providing construction phase services.

Over the course of this project, we solved three primary design challenges:

  1. Obtaining and updating a hydrologic analysis for the San Gabriel River watershed for use as the best available model for the design phase when none was available.
  2. Identifying locations for detention and water quality ponds under bridge structures where possible, minimizing the need for additional right-of-way when many of the original pond locations were unavailable due to construction phasing.
  3. Implementing drainage infrastructure prior to excavation and construction of the main lanes for proper drainage during construction, which included maintenance of traffic flow at all times.

Project Results

The design phase of the project has been completed, and construction of the new 183A tollway extension began in the spring of 2021. It is expected to open to the public in 2025. Once complete, the new road will greatly increase corridor capacity and improve travel times for the public while making use of existing right-of-way, helping to support the Austin area’s continued growth for many years to come.