|Purple Heart Trail|
I-90 highlighted in red
|Maintained by ITD|
|Length||73.888 mi  (118.911 km)|
|West end||I-90 at the Washington state line|
|East end||I-90 at the Montana state line|
Interstate 90 (I-90) is a transcontinental Interstate Highway that runs east–west across the northern United States. Within the state of Idaho, the freeway travels for 74 miles (119 km) from the Washington border near Spokane to Coeur d'Alene and the panhandle region at the north end of the state. After traveling through the Silver Valley along the Coeur d'Alene River in the Bitterroot Range, I-90 crosses into Montana at Lookout Pass.
I-90 was created by the federal government in 1957, following the general route of U.S. Route 10 (US 10) and the mid-19th century Mullan Road constructed by the U.S. Army. The freeway was constructed in stages between 1960 and 1992.
Interstate 90 enters Idaho at a crossing of the Spokane River in the community of State Line in Kootenai County, approximately 20 miles (32 km) east of Spokane, Washington.  The freeway travels northeast through the Rathdrum Prairie and into the outskirts of Post Falls, passing several retailers and a business park along the Spokane River. I-90 continues east along the river, paralleled to the south by the North Idaho Centennial Trail and a section of the BNSF Railway, and passes near the eponymous Post Falls and the city's downtown, served by a signed business route. The freeway intersects State Highway 41 (SH-41) and continues through the city's eastern residential neighborhoods and passes the nearby town of Huetter. I-90 then reaches Coeur d'Alene, the largest city in northern Idaho, and intersects a business route and US-95, the state's main north–south highway, near the Kootenai Medical Center. 
The freeway travels around the north and east sides of central Coeur d'Alene, intersecting several local streets in the process, and turns southeast towards Fernan Lake and Lake Coeur d'Alene on the Spokane River. I-90 ascends Potlatch Hill and makes several stair-step turns to follow the northern shore of Lake Coeur d'Alene while traversing several forested hills. It passes 240 feet (73 m) over a valley overlooking Bennett Bay on the Veterans Memorial Centennial Bridge, a high concrete bridge that was completed in 1987.  The freeway descends from the hills and travels southeasterly along Wolf Lodge Bay, passing Coeur d'Alene Parkway State Park and intersecting the northern terminus of the Lake Coeur d'Alene Scenic Byway ( SH-97) at the east end of the lake. I-90 continues east along Cedar Creek into the Coeur d'Alene Mountains, part of a national forest,  and reaches Fourth of July Summit at an elevation of 3,173 feet (967 m). 
From the summit, I-90 continues southeast along Fourth of July Canyon towards the Coeur d'Alene River valley and intersects SH-3 near Rose Lake. The freeway bends northward around a hill and travels east into the river valley, passing Cataldo and the Old Mission State Park, home to the state's oldest standing building.  In Cataldo, it also crosses the river and the Trail of the Coeur d'Alenes, a major rail trail that follows the highway into the Bitterroot Range.  I-90 crosses into Shoshone County, one of only two counties along the highway within Idaho,  and continues along the north sides of Pinehurst and Smelterville, passing the Shoshone County Airport at the latter. The freeway travels southeasterly through Kellogg, passing the Silver Mountain Resort and its ski gondola,  and continues around the towns of Osburn and Silverton along the north bank of the South Fork Coeur d'Alene River at the bottom of the narrow Silver Valley.  I-90 also intersects a set of business routes in Osburn and Silverton that loop back to the freeway.
The freeway turns east in the town of Wallace, also served by its own business route, and travels to the north of downtown on an elevated viaduct cut into a nearby hill.  At the east end of Wallace, I-90 intersects SH-4, which connects to the ghost town of Burke. The highway continues along the south side of the river and Trail of the Coeur d'Alenes as it ascends further into the mountains and meanders around hills and gulches.  The trail ends at the town of Mullan,  named for road-building U.S. Army captain John Mullan and home to a business route,  and I-90 continues east along the Northern Pacific Trail. The freeway travels uphill from the river along a southern ridge and passes a scenic viewpoint commemorating the 1903 Willow Creek avalanche. [ citation needed] I-90 then turns south and leaves Idaho at Lookout Pass, elevated 4,725 feet (1,440 m) above sea level and located adjacent to the Lookout Pass Ski and Recreation Area.   The freeway continues into Montana and travels through the cities of Butte, Bozeman, and Billings. 
I-90 is the only Interstate in northern Idaho and a major east–west route,  forming part of a transcontinental highway. At 74 miles (119 km), it is the second-shortest mainline Interstate in the state, after Interstate 86, and the third-longest.[ citation needed] The entire highway was designated as the Purple Heart Trail by the state legislature in 2008.   The Idaho section of I-90 is maintained by the Idaho Transportation Department (ITD), which conducts an annual survey of traffic on certain highway segments that is expressed in terms of average annual daily traffic (AADT), a measure of traffic volume for any average day of the year. Traffic volumes on I-90 ranged from a minimum of 7,861 near Mullan and 60,729 in Huetter on an average day in 2017. 
Interstate 90 from Coeur d'Alene to Lookout Pass roughly follows the route of the Mullan Road, constructed in 1859–60 by the U.S. Army and civilian volunteers led by captain John Mullan.  Mullan received a series of appropriations from U.S. Congress from 1855 to 1860, totaling $230,000, to build the first engineered road west of the Mississippi River and connect Fort Walla Walla to Fort Benton.   The road also followed the Coeur d'Alene River and its south fork, which was long used by the Coeur d'Alene people to visit hunting grounds in modern-day Montana. 
Other sections of I-90 were built over the Yellowstone Trail and U.S. Route 10,  though some parts of I-90 were cut as new roadway.   The link from Post Falls to the Washington state border was completed in 1972 at a cost of $1.1 million to upgrade the existing road and build a new bridge over the Spokane River. 
The small town of Wallace in the Silver Valley still prides itself on having what was the last stoplight on I-90. Its downtown has many historical buildings, which would have been wiped out by the original planned route of the freeway,   so in 1976, city leaders had the downtown placed on the National Register of Historic Places.    Alternatives discussed in 1963 included tunnels and twin levels.  
As a result, the federal government was forced at great expense to reroute the freeway to the northern edge of downtown and elevate it. Work on that section was resumed in 1984,  and it opened in September 1991.  A bicycle path is routed beneath part of that segment. Before the move to the viaduct, I-90 went from a freeway at the western edge of Wallace, then turned to surface streets and followed the main arterial as U.S. Route 10 through town, which included the last stoplight on I-90 between Seattle and Boston.  Upon reaching the eastern edge of the town it became a limited access divided highway once again. 
|County||Location||mi ||km||Exit ||Destinations||Notes|
|Kootenai||||0.000||0.000||I-90 west – Spokane||Continuation into Washington|
|Bridge over Spokane River|
|Post Falls||1.160||1.867||1||Beck Road (via Pointe Parkway)|
|2.080||3.347||2||Pleasant View Road|
|4.632||7.454||5||I-90 Business Loop east (Spokane Street) – City Center||Exit is just east of northernmost point on I-90|
|5.450||8.771||6||I-90 Business Loop west (Seltice Way / 4th Avenue) – City Center||Westbound exit and eastbound entrance|
|7.128||11.471||7||SH 41 north – Rathdrum, Spirit Lake|
|Coeur d'Alene||11.290||18.169||11||I-90 Business Loop east (Northwest Boulevard) – City Center|
|12.040||19.377||12||US 95 (Lincoln Way) – Sandpoint, Moscow, Silverwood, Canada|
|14.780||23.786||15||I-90 Business Loop west (Sherman Avenue) – City Center|
|||16.882||27.169||17||Mullan Trail Road|
|||22.063||35.507||22||SH 97 south – Harrison, St. Maries|
|||28.380||45.673||28||Fourth of July Pass Recreation Area|
|||34.028||54.763||34||SH 3 south – Rose Lake, St. Maries, Harrison|
|||39.024||62.803||39||Old Mission State Park|
|||47.687||76.745||48||Smelterville, Shoshone County Airport|
|Kellogg||49.715||80.009||49||Bunker Avenue – Silver Mountain|
|50.321||80.984||50||Hill Street||Eastbound exit and westbound entrance|
|50.551||81.354||51||Division Street – Wardner||No westbound entrance|
|Osburn||57.036||91.791||57||I-90 Business Loop east – Osburn|
|||59.553||95.841||60||I-90 Business Loop west – Silverton, Osburn|
|Wallace||60.990||98.154||61||I-90 Business Loop east (The Harry F. Magnuson Way) – Wallace|
|61.948||99.696||62||I-90 Business Loop west (Bank Street / The Harry F. Magnuson Way) / SH 4 east (Burke Road) – Wallace, Burke|
|||66.500||107.021||66||Gold Creek||Eastbound exit and entrance|
|||67.353||108.394||67||Morning District, West Mullan|
|Mullan||68.100||109.596||68||I-90 Business Loop east – Mullan||Eastbound exit and westbound entrance|
|||68.908||110.897||69||I-90 Business Loop west – Mullan, East Mullan|
|||73.888||118.911||I-90 east ( Lookout Pass) – Missoula||Continuation into Montana|
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|
- "Milepoint Log Report" (PDF). Idaho Transportation Department. Archived from the original (PDF) on July 13, 2015. Retrieved July 11, 2015.
- Geranios, Nicholas K. (January 8, 2011). "Idaho town to lottery winner: Please show yourself". San Diego Union-Tribune. Associated Press. Retrieved September 24, 2018.
- Google (September 24, 2018). "Interstate 90" (Map). Google Maps. Google. Retrieved September 24, 2018.
- "Bennett Bay Bridge offers best of form and function". The Transporter. Idaho Transportation Department. February 10, 2017. Retrieved July 15, 2018.
- Prager, Mike (May 16, 2017). "Snow returns to the mountains, but warmer weather will arrive by the weekend". The Spokesman-Review. Retrieved September 30, 2018.
- "Idaho to cut Cataldo Mission park funding". Missoulian. Associated Press. January 15, 2009. Retrieved October 3, 2018.
- Blair Jr., Seabury (September 25, 2007). "72 Miles of Idaho Fun". Kitsap Sun. p. 17. Retrieved October 3, 2018.
- Dukes, Lucy (March 15, 2008). "Pick Of The Litter?". The Coeur d'Alene Press. p. C1.
- Geranios, Nicholas K. (September 16, 2007). "Silver lining — Former Idaho mining town finds new life as swanky ski resort". Missoulian. Associated Press. Retrieved October 3, 2018.
- Prager, Mike (May 3, 2010). "Wallace honoring savior". The Spokesman-Review. p. A8. Retrieved October 3, 2018.
- Malgesini, Tammy (August 18, 2012). "Rich history of Wallace". East Oregonian.
- Geranios, Nicholas K. (June 15, 2003). "Little-Known Road Played Big Role in Conquest of the West". The Los Angeles Times. Associated Press. p. A28. Retrieved October 3, 2018.
- Montana Highway Map (PDF) (Map). Scale not given. Helena, Montana: Montana Department of Transportation. 2016. Retrieved September 30, 2018.
- Idaho Official Highway Map (2007 ed.), Boise, Idaho: Idaho Department of Transportation, 2007
- Russell, Betsy (August 12, 2014). "I-84 now officially Vietnam Veterans Memorial Highway". The Spokesman-Review. Retrieved June 18, 2018.
- "Idaho Statues 40-513C: Designation of Purple Heart Trail". Idaho Legislature. 2008. Retrieved October 1, 2018.
- "ATR & WIM Data: Interstate 90". Idaho Transportation Department. Retrieved September 30, 2018.
- Mccroskeyjeri Mccroskey, Jerry (June 4, 2005). "John Mullan created road with future travelers in mind". The Spokesman-Review. Retrieved September 24, 2018.
- Kramer, Becky (July 4, 2005). "A road to remember". The Spokesman-Review. Retrieved October 1, 2018.
- Schwieterman, Joseph P. (2004). When the Railroad Leaves Town: American Communities in the Age of Rail Line Abandonment. Kirksville, Missouri: Truman State University Press. pp. 124–128. ISBN 1-931112-13-4. OCLC 56968524. Retrieved October 3, 2018 – via Google Books.
- "Idaho eyes twin-level freeway". Spokane Daily Chronicle. Washington. Associated Press. December 18, 1963. p. 6.
- "Idaho seeking funds to buy right of way for freeway route". Spokane Daily Chronicle. Washington. February 1, 1957. p. a3.
- Leeright, Bob (July 31, 1971). "Displaced residents add to cost of highways". Spokane Daily Chronicle. Washington. Associated Press. p. 5.
- Evarts, J. Neal (October 9, 1972). "Freeway work nears completion". Spokane Daily Chronicle. p. 1. Retrieved December 5, 2018 – via Google News Archive.
- "Plans for Interstate 90 explained to Wallace CC". Spokane Daily Chronicle. Washington. January 23, 1970. p. 3.
- "Wallace Chamber seeks I-90 shift". Spokane Daily Chronicle. Washington. January 18, 1973. p. b3.
- "Wallace freeway hearing ordered by federal judge". Spokesman-Review. Spokane, Washington. Associated Press. July 23, 1976. p. 13.
- "I-90 study includes historic site survey". Spokane Daily Chronicle. Washington. January 10, 1980. p. 10.
- Collin, Matthew (January 26, 1981). "Wallace freeway 12 years overdue". Spokesman-Review. Spokane, Washington. p. 6.
- "Freeway alternatives reviewed for Wallace". Spokane Daily Chronicle. Washington. Associated Press. October 25, 1978. p. 5.
- Bond, Dave (July 18, 1984). "Wallace bypass work to start". Spokesman-Review. Spokane, Washington. p. 1.
- Idaho Transportation Department Archived September 26, 2006, at the Wayback Machine.
- "He stalled Interstate 90 right in the middle of town". Eugene Register-Guard. Oregon. Associated Press. February 13, 1981. p. 15F.
- "Lone stoplight on Interstate 90 to pass by after new bypass". The Bulletin. Bend, Oregon. Associated Press. September 8, 1991. p. A9.
- Cannata, Amy (September 10, 2006). "Freeway at 50". Spokesman-Review. Retrieved May 11, 2018.