Idaho Historic Highways System
 Interstate Highway History
 U.S. Highway History
 State Highway History
 Sampson Trails and Early State Roads
 National Historic Routes and Trails
 Scenic Byways
 Idaho Highways

Related Pages
 Idaho's Route Markers
 Idaho Home Page


Idaho's Interstate system was started with the Interstate 15 being the first interstate highway to be opened in the state. The interstate system was completed in Idaho in 1974. Currently Idaho's interstate system consists of 612 miles of Primary Interstate highways and 154 miles of Business Loops. And one primary interstate, Interstate 86, is located entirely within the state.


Historic Idaho Interstate Routes
Old # New # Replaced US
I-15 I-15 US 91, US 191
I-80N I-84 US 30S, US 30
I-15W I-86 US 30
I-90 I-90 US 10
I-180N I-184 N/A

Idaho's Interstate system was started with the Interstate 15 being the first interstate highway to be opened in the state. When the interstate system was first completed in Idaho split routes were allowed in the interstate system. Which allowed interstates 80N, 180N and 15W to be designated. Later Highway officials, such as AASHTO, required that there be no split interstate highway routes, (this happened in the 1970's) and all interstates that had split designations were given new route numbers (see table). Only 2 Split designations remain in the interstate today as an exception I-35E and I-35W, both in St. Paul/Minneapolis, MN and Dallas/Ft. worth, TX.

To see examples of historical interstate markers used see the Route Markers page.


Idaho's U.S. Routes were part of the first transcontinenntal interstate highway system built in the U.S. When first laid out the AASHO, (now AASHTO) allowed for split U.S routes which created many split routes, such as US-30S and US-30N. Now split US routes are discouraged. but several states are still holding on to the split U.S. Route designations since they are so well established in the highway system. In some places even decommissioned US routes that have since become state highways have retained their former split numbers like in Washintton and Oregon with their routes 99E and 99w for example or 9A in New York.

For more history about Idaho's U.S. highways including a route table and more information on specific routed see the Historic US highways page or try Idaho Highways or for national US Highway History see


Idaho's state highway system has been in existence since the first numbered signed routes were posted in Idaho back in the late 1920's and 30's. Several of these routes have been renumbered over the years and rerouted onto better roads as well. Some state highways have gone out of existence entirely due to the US highway and Interstate systems To see a table of the old highway route designations and what replaced them as well as specific history on some of the more important or notable routes see the Historic State highways page.

For more history about Idaho's highways you can also visit Idaho Highways


One of Idaho's very first highway systems was a network of trails known as Sampson trails. Sampson trails were proposed by and named after Charles Sampson. Charles Sampson was a Boise businessman who also was a proponent of good road networks. His system of roads used a single letter designation "A, B, C, D, ..." and the Trails were marked marked with a bold black letter on an orange paint background. These routes were replaces by the more modern numbered state highway system and fell out of use in the 1930's. More information and a table of the different Sampson Routes on the Sampson trails page.

For more history about Idaho's Sampson trails visit Sampson Trails or Sampson Trails @wikipedia.


Idaho has several routes of historic importance. such as the Oregon Trail which travelled near where US 30 and US 30S were routed. Other historical trails include the Nez Perce Trail and the Lewis and Clark Trail. the trails of the earliest explorers and migrants to the northwest. Other routes of significant importance are the old national trails roads system. marked by signs along each route. One of these routes was the Yellowstone Trail which in Idaho travels mostly along what is now I-90. The National Park to Park highway also passed through portions of the state as it completed it's loop of many national parks throughout the west. Another Highway was the Evergreen National Highway which traveled from BC to El Paso, TX.


There are many scenic Byways in idaho. The state has one of the most organized scenic byway systems in the country. byways are well signed and state and national maps have been issued to highlight the many routes of historic and scenic value in Idaho. More information on the scenic byways in Idaho can be found at

For more history about Idaho's Byways you may also visit America's Byways or National Scenic Byways @wikipedia.