Sections between downtown Fort Worth and Grapevine are freeway, including a small segment near Hurst that coincides with Interstate 820.

It is very heavily traveled south and west of McKinney and has become an urban highway due to the rapid growth of the Dallas–Fort Worth metroplex.

However, in recent years with an increase in traffic, the 19 mile section between the Collin–Fannin county line and its junction with SH 56 in Bonham has been upgraded to a "Super Two" highway system.

In addition, a two-mile stretch between SH 56 and US 82 within Bonham city limits, was upgraded to a four-lane highway system with a continuous left-hand turn lane.

Long term planning calls for upgrading the 19 mile stretch, from the Collin–Fannin county line to SH 56 in Bonham, into a four-lane divided highway system.

Construction began in 2009 on the extension of 121 south from Fort Worth to Cleburne, as a toll road called Chisholm Trail Parkway in Tarrant and Johnson Counties.

[3] The northbound freeway lanes are currently under construction, which also include new, separate exits and entrances for Freeport Parkway and Grapevine Mills Boulevard/Sandy Lake Road.

In early 2007, the Spanish firm Cintra agreed to a $2.8 billion, 50-year deal where they would lease the highway from TxDOT, finish building and maintain it, and collect the tolls.

In addition, general opposition to the toll plan organized, pointing out that the segment through Denton County had already been paid for by taxpayer money.

[6] The Authority announced their proposed bid in early May 2007, which would provide approximately $3.3 billion in road funding, but is based on traffic counts that differ from those used by Cintra.

The final section between McKinney and Trenton was not completed until November 20, 1963, including an extension around the western side of Bonham.

The CTP's frontage roads in southwest Fort Worth are signed as SH 121. Business State Highway 121-D (Bus.

North of I-35E, the highway runs just south of the city's central business district and turns in a northeast direction.

The highway continues to run east and meets FM 544 at an intersection before ending at an interchange with SH 121/Sam Rayburn Tollway.