SH 121, also known as the Sam Rayburn Tollway (SRT), is a multi-level interchange in Collin County. Segment 4 was a $221 million project covering SRT from south of Hardin Boulevard to south of SH 5, and US 75 from north of Ridgeview Drive to south of El Dorado Parkway.

This project, which crossed the City of McKinney, City of Allen and the Town of Fairview, included the new construction of a six-lane tollway on SH 121 and reconstruction of US 75 to eight lanes and a four-level interchange with direct connecting ramps at SH 121 and US 75. All electronic toll collection and ITS systems were also provided as part of this segment. This was the largest project undertaken by the NTTA to date.

This was a complex earthwork, paving and bridge project, located in the middle of an already-congested major interchange. Intricate phasing was required to construct the project. Maintenance of the design and construction schedule drove the decision by the owner to utilize only prestressed concrete beam spans, rather than the usual long-curved structural steel spans normally seen on major freeway interchanges. The restricted span lengths of the prestressed concrete beams resulted in several unusually-large concrete straddle bents over travel lanes, many of which required post-tensioning to meet design requirements.

VRX inspectors performed a major role in the inspection of all aspects of this project, including grading, drainage, MSE walls, concrete paving and bridges. VRX also provided training for the project bridge inspectors.  Bridge construction was very complex, especially at the SH 121 and US 75 interchange, requiring extensive traffic control monitoring to assure safety of workers and the traveling public. VRX staff also performed constructability review.