There is little detailed information about the construction of I-35 between Laredo and San Antonio. The first parts of I-35 in South Texas were the southernmost parts, so I-35 was already ready in 1965 between Laredo and Cotulla. The missing section between Cotulla and southwest of San Antonio was constructed in the second half of the 1960s. Certainly in 1973 the entire route was completed.
Construction of the I-35 in San Antonio began before the creation of the Interstate Highway system, the first section opened in the south of San Antonio in 1951, which was extended in 1957 to the I-410 south of the city. In 1957, the portion along the north side of downtown San Antonio also opened. Between 1961 and 1964, the remainder of I-35 through northeast San Antonio was opened.
San Antonio – Austin
According to Topschoolsintheusa, the section between San Antonio and Austin was one of the first Interstates in Texas to be completed between two larger cities, although Austin was still quite small at the time. By 1960, almost all of I-35 between San Antonio and Austin was completed, except around Kyle, which opened in the first half of the 1960s. Certainly in 1965 the highway between the two cities was ready.
The stack interchange between I-35 and US 290 in Austin.
The Austin opening of I-35 took place on March 29, 1962. In 1975, a double-deck section was constructed with through lanes with no exits at Level 2. Level 1 is where the exits are.
The original interchange between I-35 and US 183 had only two spur connectors, from south to west and vice versa. Three additional direct connectors have been installed, from north to east and vice versa, and from north to west. The project cost $204 million and started on January 24, 2018. The last flyover opened on September 10, 2021.
I-35 at Waco.
The route between Austin and the I-35 split at Hillsboro was largely completed in 1967. The route through Waco started construction in 1958 and was opened in phases until October 1972. The route through Waco was originally planned a bit further east than where the highway was eventually built.
It is unclear when the section in Waco will be widened to 2×3 lanes, this section may already be equipped with 2×3 lanes when it opens. In about 2002 the I-35 between Georgetown and Jarrell was widened to 2×3 lanes, in about 2009 the part from Jarrell to Salado followed and in 2015 the part from Salado to Belton. In the period 2008-2013, the section between Waco and Hillsboro was widened to 2×3 lanes. From 2012, some parts between Temple and Waco were widened, from 2016 this entire part was widened. On July 2, 2014, two extra- dosed bridges opened for the frontage roads over the Brazos River in Waco. The widening between Waco and Hillsboro was completed in July 2015 and in July 2016 the widening between Bruceville-Eddy and Waco was completed.
A separate project is the widening of 9 kilometers of I-35 from Loop 363 on the north side of Temple to north of Troy. This part will be widened from 2×2 to 2×3 lanes. Work started in March 2012 and was completed in August 2018. This part has been considerably widened, with realigned and widened frontage roads and the complete reconstruction of the main carriageways and replacement of all structures. The total road profile became twice as wide.
Between 2013 and 2019, I-35 through Temple was widened from 2×2 to 2×4 lanes. This concerned the part within Loop 363 over a length of 8 kilometers. The highway has been completely reconstructed, all artworks have been replaced. Work started in September 2013 and was completed in May 2019. The project cost $170 million.
The 10-kilometer stretch through Waco has been widened from 2×3 to 2×4 lanes between 2019 and 2022. The large-scale work began in May 2019. The work was estimated to cost $425 million. The project was delivered on August 10, 2022.
See Interstate 35E in Texas
Construction of I-35E through the Dallas area has been split with the construction of two freeways, the Stemmons Freeway from downtown to the north and the RL Thornton Freeway to the south. In 1959, the first section of the Stemmons Freeway opened at Downtown Dallas. Construction of I-35E north of Dallas progressed at a rapid pace, with the highway completed all the way to Denton by 1963. Construction of the RL Thornton Freeway was also progressing rapidly and was completed in 1966. This allowed through traffic to pass through the Dallas area.
The Stemmons Freeway was built directly with 2×5 lanes, it was one of the widest freeways in Texas at the time. In 1976 the part in the northwestern suburbs was widened. As part of the ‘LBJ Express’ project of the connecting I-635, the I-35E elevated toll lanes have also been constructed in the outer shoulder of I-35E, which opened in 2014. Subsequently, the major project ‘I-35Express’ was carried out between 2013 and 2017, in which a toll interchange lane was constructed, whereby a large part of I-35E in the northern suburbs was redeveloped.
Another major reconstruction was the “Horseshoe Interchange” between I-35E and I-30 near Downtown Dallas, which was reconstructed between 2013 and 2017 at a cost of $700 million. After 2000, I-35E was also widened south of Dallas, but not as widely as north of the city, since the south side of Dallas is less heavily suburbanized. The highway has been widened here in phases to 2×3 lanes until the fork at Hillsboro. Most of it was completed in 2018.
See Interstate 35W in Texas
In the Fort Worth area, construction of I-35W is divided into the South Freeway and the North Freeway. Construction of these highways began before the Interstate Highway era, the number being US 81 at the time. The first section of Fort Worth’s South Freeway was opened in 1949, making it one of the first freeways in Texas. By 1955, most of what would become I-35W south of Fort Worth was completed. The North Freeway was built in the 1960s, directly as I-35W. In 1969 the North Freeway was completed as far as Denton. However, the last section of the South Freeway did not open until 1976 between Hillsboro and Burleson.
In 1960, the “Fort Worth Mixmaster,” Texas’s first 4-level stack interchange between I-30 and I-35W, opened in Fort Worth. However, this was a junction with design requirements of the 1950s and the disadvantageous design played a major role in traffic growth from the 1970s. The node was completely replaced by a more modern stack in the late 1990s.
As the growth of the Fort Worth suburbs, like Dallas, has been mainly northward, I-35W has been significantly reconstructed under the “North Tarrant Express” project, named after Tarrant County through which I-35W runs. This mega project included not only I-35W but also I-820 and SH 121/183 in the northeastern suburbs. In the first phase, the I-35W / I-820 interchange has been expanded with additional flyovers, in the second phase the capacity of I-35W itself has been expanded, between Downtown Fort Worth and US 81/287 in the north of the city, which was completed in 2018.
The highway began construction between Denton and the Oklahoma border around 1960 and was completed in 1965.
I-35 had one of the few level crossings in the Interstate Highway system, this crossing having been removed in the 1970s.
I-35 is one of the busiest interurban corridors in the United States, especially the stretch between San Antonio, Austin and the junction with I-35E and I-35W toward DFW. Major portions of I-35 have already been widened to 2×3 lanes.
There are plans to widen I-35 north and further north of Laredo to 2×3 lanes. The widening spans 40 miles of I-35 from Shiloh Road in Laredo to north of State Highway 225.
With the growth of San Antonio ‘s urban area, a widening of I-35 northeast of the city is necessary. It is planned to provide I-35 between I-410 (south) in San Antonio and FM 1103 in Schertz with managed lanes on overpasses, originally planned as 2 toll lanes per direction, but later as toll-free express lanes. The planned length of the viaducts is 24 kilometers from I-410 South to FM 3009. From FM 3009 to FM 1103, the managed lanes run in the central reservation. Also, the nodes with Loop 1604 and I-410 must be adapted. The project area is 32 kilometers long. Construction of the first phase of the project began on May 11, 2022, this should be ready by mid-2027.
I-35 through Austin is one of the most congested roads in Texas. Five points on I-35 in Austin are on the list of the 100 most congestion-prone routes in Texas, and I-35 at Downtown is the second most congestion-prone in Texas. In Austin, the political climate is unfavorable for large-scale highway expansions. However, the problems are so great that relatively small adjustments, such as one express lane per direction, can offer little relief.
In 2015, it was proposed to realize a third lane on the double-deck viaduct north of downtown. They also want to deepen the I-35 along the center. Austin’s traffic problems are serious for a city of this size, and realizing the congestion on I-35 will require a large-scale reconstruction that will cost billions. In 2017, an $8.1 billion plan was launched to expand 53 kilometers of I-35 through the Austin metropolitan area to 12 lanes, some with three levels.  If carried out, this would be one of the largest reconstructions in American history.
However, financing the project was a problem. In February 2020, a breakthrough appeared to be taking place, with $4.3 billion in funding being identified. In April 2020, there was talk of an additional $3.4 billion in funding for the mega project.
Capital Express South
Capital Express South, also referred to as “South10” in the past, spans eight miles of I-35 between SH 45 and SH 71 in south Austin. This route originally had 2×3 lanes with frontage roads and was variably deepened to ground level. On this stretch, express lanes will be created in the form of a 4-kilometer-long viaduct above the central reservation of the existing Interstate 35. The express lanes here will have 2 lanes in each direction, in addition to the 3 to 4 lanes per direction on the main carriageways. In addition, I-35 retains frontage roads. South of Slaughter Lane, the express lanes are in the middle, so there are 5 to 6 lanes in each direction side by side. South of Onion Creek Parkway, there is one more express lane in each direction.
The project originally provided 1 toll lane per direction on this section, but due to anti-toll sentiment in the state of Texas, this was later converted to one or two HOV lanes per direction. These are the first HOV lanes in the Austin area. The draft EIS was adopted in March 2021. The final EIS and FONSI were adopted on 21 December 2021.
Capital Express Central
Capital Express Central, also referred to as “Central7” in the past, spans eight miles of I-35 between SH 71 and US 290 along downtown Austin. This section originally had 2×3 lanes between SH 71 and Martin Luther King Jr. Boulevard, partly above and partly below ground level. This included the passage along downtown Austin. Between Martin Luther King Jr. Boulevard and US 290 had 8 lanes of traffic, divided over a vertical parallel structure with 2×2 lanes at ground level and a viaduct with 2×2 lanes on top. It is planned to build two HOV lanes in each direction on this route.
Exactly how this will be designed has not yet been determined. Originally there were alternatives at ground level, on viaducts or in a trench below ground level. Costs at the time were estimated at $550-650 million for an upgrade to the existing highway and $750-850 million for a sunken solution, but this included only one express lane per direction, while later talk of a larger reconstruction with 2 HOV lanes per direction. The estimated cost was also much higher after that at $4.9 billion. A construction start has been postponed from 2020 to the end of 2025.
Capital Express North
Capital Express North, also formerly “North16,” covers 12 miles of I-35 in north Austin. I-35 here also had 2×3 lanes with frontage roads. The environment here mainly consists of businesses and retail. One HOV lane per direction on ground level will be constructed on this route, so that a cross section of mostly 2×4 lanes is created.
The original plans envisioned one toll lane in each direction for 16 miles, further north to RM 1431 between Round Rock and Georgetown. This has been reduced to just a widening south of SH 45 in Round Rock. The cost is estimated at $400 million. A draft EIS was established for this in April 2021. A final EIS and FONSI were adopted on 20 December 2021.
Denton – Oklahoma
At Denton, the ‘I-35Express’ project empties onto I-35. It is planned to extend the express lanes from I-35E to US 380 in Denton, which also includes a portion of I-35 itself. This part will then be widened to 4+2+2+4 lanes. This eventually narrows to 2×4 lanes to the Loop 288 on the north side of Denton and 2×3 lanes north of it.
It is planned to widen I-35 north of Denton to 2×3 lanes, initially 16 miles from Denton to FM 3002 in Cooke County.   Plans were subsequently announced to also widen I-35 from FM 3002 to the Oklahoma border, eventually even to 2×4 lanes, but initially to 2×3 lanes. The bridge over the Red River also needs to be replaced for this.  A FONSI was issued for this in December 2020. 
The data below concerns intensities after the relevant exit.
|see I-35E and I-35W|
|505||Oklahoma state line||34,000||33,000||44,000|
Main Route I-35
|exit 0||exit 8||2×3||Laredo|
|exit 8||Exit 148||2×2|
|Exit 148||Exit 153||2×3||San Antonio|
|Exit 153||Exit 154||2×5||San Antonio|
|Exit 154||Exit 155||4×2||San Antonio|
|Exit 155||Exit 156||2×4||San Antonio|
|Exit 156||Exit 166||2×3||San Antonio|
|Exit 166||Exit 169||2×5||San Antonio|
|Exit 169||Exit 172||2×4||San Antonio|
|Exit 172||Exit 235||2×3|
|Exit 235||Exit 240||2×4||Austin|
|Exit 240||Exit 299||2×3|
|Exit 299||Exit 304||2×4||Temple|
|Exit 304||Exit 333||2×3|
|Exit 333||Exit 339||2×4||waco|
|Exit 339||Exit 370||2×3|
|see I-35E and I-35W|
|Exit 467||Exit 504||2×2|