I-45 follows the route of former US 75, which today begins in Dallas. In 1943, it was decided to build the first section of I-45 between Galveston and Houston, and on September 30, 1948, the first section was opened between downtown Houston and Telephone Road. The highway was then extended southeast to Galveston in the 1950s and 1960s. The last section opened to traffic in 1976. Between 1982 and 1988, the section between Downtown Houston and I-610 was modernized and widened, and the section was extended to the Sam Houston Beltway between 1991 and 1997.
According to topschoolsintheusa, the section of the North Freeway through Houston was opened to traffic between 1959 and 1963, and was largely widened and modernized to current extension standards between 1985 and 1998. In 2003 the last part for The Woodlands was widened. Between Houston and Dallas, Interstate 45 largely directly follows the old route of US 75, which was made grade-separated around 1960.
In 2015, the section between Conroe and New Waverly was widened to 2×3 to 2×4 lanes. This means that the I-45 has at least 2×3 lanes to well north of Houston.
Opening history
Gulf Freeway
| From | Unpleasant | Length | Date |
| Dowling Street | Telephone Road | 4.5 km | 30-09-1948 |
| Telephone Road | Griggs Road | 1.2 km | 00-02-1951 |
| Griggs Road | Reveille Street | 1.6 km | 00-07-1951 |
| Reveille Street | Broadway Street | 2.0 km | 02-08-1952 |
| Broadway Street | Alameda Genoa Road | 8.0 km | 00-06-1959 |
| Alameda Genoa Road | Dixie Farm Road | 6.4 km | 00-10-1964 |
| Dixie Farm Road | League City | 11.5 km | 00-12-1970 |
| League City | FM 1764 | 13.8 km | 00-00-1976 |
North Freeway
| From | Unpleasant | Length | Date |
| Cross Timbers Road | Little York Road | 5.3 km | 00-12-1959 |
| I-610 | Cross Timbers Road | 1,8 km | 00-04-1961 |
| FM 525 | FM 1960 | 9.9 km | 00-12-1961 |
| Memorial Drive | I-610 | 5.8 km | 24-07-1962 |
| Little York Road | FM 525 | 6.9 km | 00-02-1963 |
| FM 1960 | Sawdust Road | 12.1 km | 00-03-1963 |
Houston – Dallas
The long rural section between Houston and Dallas was built mainly in the 1960s. The first section completed was in 1959 between Ennis and Dallas, followed by the Huntsville bypass which also opened circa 1959. In 1960 there was a fairly long section from Richland (SH 14) to Ennis, except for the Corsicana bypass itself, which opened circa 1964. The middle part was opened in phases between 1965 and 1971. With the opening of the last section between Buffalo and Streetman on October 13, 1971, I-45 was continuous between Houston and Dallas.
| From | Unpleasant | Length | Date |
| Huntsville (South) | Huntsville (north) | 10 km | ~1959 |
| Richland | Corsica (South) | 15 km | ~1960 |
| Corsica (north) | Ennis | 23 km | ~1960 |
| New Weaverly | Huntsville (South) | 21 km | <1962 |
| Corsica (South) | Corsica (north) | 11 km | ~1964 |
| Conroe | New Weaverly | 24 km | ~1964 |
| street man | Richland | 10 km | ~1964 |
| Huntsville (north) | Madisonville (North) | 44 km | ~1965 |
| Madisonville | Centerville | 29 km | ~1968 |
| Centerville | buffalo | 25 km | ~1969 |
| buffalo | street man | 55 km | 13-10-1971 |
Dallas
The first sections of I-45 opened outside of Dallas at the time, with the first section opening between Ennis and Ferris in 1959, followed by a section further up to I-20 a year later. This connected to what is now SH 310, and was the old route into Dallas. This section was bypassed in the 1970s by the current I-45, which was completed in 1976. After that, the old part was renumbered as SH 310 and partly US 175 (Wright Freeway).
The Wright Freeway was later demolished and US 175 reconnects with a new interchange south of I-45, which opened on February 21, 2020.
| From | Unpleasant | Length | Date |
| Ennis | ferris | 23.3 km | 20-10-1959 |
| ferris | I-20 | 14.4 km | 00-00-1960 |
| Overton Road | Lamar Street | 4.6 km | 00-11-1972 |
| Grand Avenue | Spur 366 | 3.6 km | 23-08-1973 |
| Central Expressway | Overton Road | 8.5 km | 00-00-1975 |
| Lamar Street | Grand Avenue | 1,8 km | 25-02-1976 |
Future
I-45 at Huntsville.
Houston
For more information, see Southwest Freeway, East Freeway, Gulf Freeway & North Freeway.
It is planned to adapt I-45 around Downtown Houston. The section along the south side of downtown that runs on an overpass is to be demolished, after which I-45 traffic will be directed along the east and north sides of Houston. These parts ( Southwest Freeway and East Freeway ) will be drastically widened for this purpose. It is also planned to provide the North Freeway with managed lanes.
The draft EIS was established in April 2017. The final EIS followed in September 2020.
Widening Houston – Dallas
It is planned to eventually widen I-45 in its entirety to 2×3 lanes between Houston and Dallas. Between November 2015 and December 2018, the remaining portion in Navarro County (from FM 246 at Streetman to I-45 Business on the north side of Corsicana) was widened 35 kilometers to 2×3 lanes for $212 million. Since early 2017, I-45 in southern Walker County has been widened to 2×3 lanes from the end of the 2×3 portion from Houston to SH 19 in Huntsville. Between 2020 and 2025, an additional 15 miles north of SH 75 at Huntsville has been widened to 2×3 lanes.
In 2020, work began on the plan for the remaining 150 kilometers of I-45 between Huntsville and Streetman. The cost of widening this section of I-45 in its entirety was estimated at $2.4 billion at the time, divided into 15 different segments and 8 different contracts. The existing central reservation of I-45 will then be used by realizing a cross section of 2×3 lanes with left and right emergency lanes, with a concrete barrier between the driving directions. Thus, the total space requirement of I-45 does not increase.
Dallas
US 175 (CF Hawn Freeway) extends south from Dallas to I-45, building a new interchange south of its Lamar Street junction. Thereafter, the northern section of US 175 (SM Wright Freeway) will be demolished for 2.7 kilometers and replaced by a 2×3 lane urban arterial. To accommodate traffic from the Wright Freeway, I-45 will be widened to 2×4 lanes for 2.5 kilometers. Every day, 85,000 vehicles travel on the existing Wright Freeway and 70,000 vehicles on the parallel portion of I-45.
Traffic intensities
The data below concerns intensities after the relevant exit.
| Exit | Location | 2007 | 2012 | 2016 |
| 1 | Galveston | 65,000 | 57,000 | 68,000 |
| 15 | Texas City | 82,000 | 88,000 | 98,000 |
| 23 | League City | 133,000 | 142,000 | 146,000 |
| 29 | Houston | 206,000 | 183,000 | 158,000 |
| 32 | Houston | 220,000 | 197,000 | 210,000 |
| 39 | Houston | 291,000 | 245,000 | 259,000 |
| 40 | Houston | 247,000 | 194,000 | 235,000 |
| 44 | Houston | 249,000 | 213,000 | 231,000 |
| 48 | Houston | 260,000 | 210,000 | 232,000 |
| 52 | Houston | 326,000 | 266,000 | 280,000 |
| 60 | Houston | 307,000 | 312,000 | 314,000 |
| 70 | The Woodlands | 217,000 | 232,000 | 233,000 |
| 87 | Conroe | 94,000 | 119,000 | 154,000 |
| 118 | Huntsville | 32,000 | 41,000 | 60,000 |
| 142 | Madisonville | 32,000 | 31,000 | 34,000 |
| 178 | buffalo | 28,000 | 28,000 | 33,000 |
| 197 | Fairfield | 30,000 | 26,000 | 32,000 |
| 232 | Corsicana | 34,000 | 37,000 | 40,000 |
| 251 | Ennis | 41,000 | 46,000 | 40,000 |
| 275 | Dallas | 67,000 | 64,000 | 87,000 |
| 285 | Dallas | 141,000 | 136,000 | 150,000 |
Lane Configuration
The northern starting point of I-45 in Dallas.
| From | Unpleasant | Lanes | Comments |
| exit 0 | exit 32 | 2×3 | |
| exit 32 | exit 46 | 4+1+4 | Houston |
| exit 46 | exit 47 | 2×3 | Houston |
| exit 47 | exit 48 | 2×4 | Houston |
| exit 48 | Exit 60 | 4+1+4 | Houston |
| Exit 60 | Exit 68 | 5+1+5 | Houston |
| Exit 68 | Exit 72 | 2×4 | Houston |
| Exit 72 | Exit 79 | 2×5 | Houston |
| Exit 79 | Exit 89 | 2×4 | |
| Exit 89 | Exit 98 | 2×3 | |
| Exit 98 | Exit 247 | 2×2 | |
| Exit 247 | Exit 283 | 2×3 | Dallas |
| Exit 283 | Exit 284 | 2×5 | Dallas |
| Exit 284 | Exit 285 | 2×3 | Dallas |
| Exit 285 | Exit 286 | 2×4 | Dallas |
