I-90’s predecessor was US 10 for most of its route. The eastern section between Billings and the Wyoming border was preceded by US 87.
Construction on the highway began in the mid-1950s, initially opening the west access road to Butte in 1957, encompassing a few miles of current I-90 and I-115. It was not considered part of the Interstate Highways network at the time, and was presumably upgraded later when the Butte bypass proper opened in 1965. The first longer stretch opened in 1960 between Drummond and Goldcreek in the west of the state. Fragmented sections of I-90 were opened during the 1960s, it took a relatively long time before longer stretches were accessible, in 1971 the section from Butte to Bozeman was completed, the first through section of I-90 between two larger towns. The fragmented sections of I-90 gradually became interconnected throughout the 1970s. It’s not clear exactly when the last missing links opened, but at least between the late 1970s and early 1980s, I-90 was passable through most of Montana. The last stretch of I-90 built was the easternmost portion of the route to the Wyoming border, which opened in 1986.
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The section between Livingston and Springdale was constructed as a super two in the 1960s. This section was widened between 1985 and 1987 to 2×2 lanes over a length of 30 kilometers. Construction costs at the time were $12 million and the project was completed on May 13, 1987. Prior to 2000, a portion of I-90 over Lookout Pass in the Idaho state border area had no lane separation. This was a narrow road with four lanes and only markings between the directions. A jersey barrier was installed here in the summer of 2000.
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Opening history
The years are an indication and derived from the official state highway maps.
| from | nasty | length | date |
| Exit 219 I-15 (South) | Exit 222 Butte (I-115) | 5 km | 1957 |
| Exit 154 Drummond | Exit 166 Goldcreek | 19 km | 1960 |
| Exit 509 Crow Agency | Exit 514 Garryowen | 8 km | 1960 |
| Exit 61 Tarkyo | Exit 65 | 6 km | 1960 |
| Exit 343 | Exit 350 | 11 km | 1961 |
| Exit 434 Laurel | Exit 443 Billings (west) | 14 km | 1961 |
| Exit 497 Hardin | Exit 509 Crow Agency | 19 km | 1961 |
| exit 43 | Exit 47 Superior | 6 km | 1962 |
| Exit 179 | Exit 187 Deer Lodge | 13 km | 1962 |
| Exit 330 Livingston | Exit 343 | 23 km | 1962 |
| Exit 313 | Exit 316 Chestnut | 5 km | 1964 |
| Exit 392 Reed Point | Exit 396 | 6 km | 1964 |
| Exit 65 | Exit 75 Alberton | 16 km | 1965 |
| Exit 109 Milltown | Exit 113 Turah | 6 km | 1965 |
| Exit 208 Anaconda | Exit 211 | 5 km | 1965 |
| Exit 222 (I-115) | Exit 228 Butte (east) | 10 km | 1965 |
| Exit 278 Three Forks | Exit 288 Manhattan | 16 km | 1965 |
| Exit 153 Drummond | Exit 154 Drummond | 2 km | 1966 |
| Exit 228 Butte (east) | Exit 249 Whitehall | 34 km | 1966 |
| Exit 288 Manhattan | Exit 305 Bozeman | 27 km | 1966 |
| Exit 47 Superior | Exit 61 Tarkyo | 21 km | 1967 |
| Exit 96 Wye | Exit 109 Milltown | 21 km | 1967 |
| Exit 211 | Exit 219 I-15 (South) | 13 km | 1967 |
| Exit 426 Park City | Exit 434 Laurel | 13 km | 1967 |
| Exit 443 Billings (west) | Exit 452 Billings (east) | 14 km | 1967 |
| Exit 274 US 287 | Exit 278 Three Forks | 6 km | 1969 |
| Exit 452 Billings (east) | Exit 456 I-94 | 6 km | 1969 |
| Exit 138 Bearmouth | Exit 154 Drummond | 26 km | 1970 |
| Exit 249 Whitehall | Exit 256 Cardwell | 11 km | 1970 |
| Exit 256 Cardwell | Exit 278 Three Forks | 35 km | 1971 |
| Exit 305 Bozeman | Exit 313 | 13 km | 1971 |
| Exit 408 Columbus | Exit 426 Park City | 29 km | 1971 |
| Exit 484 | Exit 497 Hardin | 21 km | 1971 |
| Exit 0 Idaho state line | Exit 10 Saltese | 16 km | circa 1972-1974 |
| Exit 75 Alberton | Exit 96 Wye | 34 km | circa 1972-1974 |
| Exit 113 Turah | Exit 138 Bearmouth | 40 km | circa 1972-1974 |
| Exit 166 Goldcreek | Exit 174 Garrison | 13 km | circa 1972-1974 |
| Exit 377 Greycliff | Exit 392 Reed Point | 24 km | circa 1972-1974 |
| Exit 396 | Exit 408 Columbus | 19 km | circa 1972-1974 |
| Exit 456 I-94 | Exit 484 | 45 km | circa 1972-1974 |
| Exit 10 Saltese | Exit 18 The Borgia | 13 km | 1977 |
| Exit 18 The Borgia | exit 43 | 40 km | about 1979-1985 |
| Exit 174 Garrison | Exit 179 | 8 km | about 1979-1985 |
| Exit 187 Deer Lodge | Exit 208 Anaconda | 34 km | about 1979-1985 |
| Exit 316 Chestnut | Exit 330 Livingston | 23 km | about 1979-1985 |
| Exit 350 | Exit 377 Greycliff | 43 km | about 1979-1985 |
| Exit 514 Garryowen | Exit 530 Lodge Grass | 26 km | about 1979-1985 |
| Exit 530 Lodge Grass | Milepost 554 Wyoming state line | 39 km | 1986 |
Traffic intensities
Every day, 7,000 vehicles pass the Idaho border, rising to 9,000 vehicles west of Missoula. The passage through Missoula is somewhat busier with 21,000 vehicles per day. This drops back to 9,000 vehicles per day until Garrison, where US 12 turns to Helena. The north-south section from Garrison to Anaconda processes 8,000 vehicles per day. The double-numbered I-15 on the west side of Butte has 18,000 vehicles per day, with 13,000 to 17,000 vehicles by Butte itself. East of Butte, 11,000 vehicles cross the Homestake Pass, which remains stable until Bozeman. This rises to 20,000 vehicles through Bozeman and 15,000 vehicles between Bozeman and Livingston.
Further east, traffic volumes drop to about 10,000 vehicles between Livingston and Billings. The passage through Billings is the busiest section of I-90 in Montana with up to 41,000 vehicles per day. This drops to 15,000 vehicles on the east side of Billings where I-94 branches off and just 4,000 to 7,000 vehicles from Billings to the Montana border. The Wyoming border stretch is the quietest, with only 3,900 vehicles per day.
