How can I-17 and the other interstates listed above, be an interstate if it doesn’t cross one state’s border into another? According to an article by David Rookhuyzen of the Arizona Department of Transportation (ADOT) that quotes the Federal Highway Administration (FHwA), “the Federal-Aid Highway Act of 1956, which created the system, called for uniformity in construction standards. These standards were developed by the American Association of State Highway Officials (now the American Association of State Highway and Transportation Officials, or AASHTO).” These standards are the “benchmark” that a highway must be built to in order to be deemed an interstate. “Metrics include having controlled access, design speeds of 50 to 70 miles per hour depending on terrain, a minimum of two travel lanes in each direction, 12-foot lane widths, a 10-foot right shoulder, and a 4-foot left shoulder.”
I-17 under construction in 1968. (Photo: Arizona State Library)
Moreover, the criteria for adding another highway to the Interstate Highway System (IHS) is also straightforward – it must meet the interstate standards noted above, “be a logical addition or connection, and be coordinated among all jurisdictions it will pass through.”
That is why I-17 and the other highways previously profiled are designated interstate highways (whether they will ever reach into another state or not). “The key,” according to the FHwA, is “each highway must meet interstate standards, be a logical connection to the interstate system, connect to an existing route or be a Congressionally designated future interstate corridor that eventually will connect on at least one end.”
Because of this definition, there are interstates that are beltways around large metropolitan areas or in places such as Hawaii, Alaska and Puerto Rico that cannot connect to other states.
Description and route
Interstate 17 is a north-south interstate. The southern end of I-17 is within Phoenix (at its intersection with I-10), and it ends in the north at I-40 in Flagstaff.
Along most of its route I-17 is known as the Arizona Veterans Highway. However, in the Phoenix metropolitan area, it is primarily referred to as the Black Canyon Freeway.
One of the most scenic interstate highways, I-17 gains more than one mile in altitude between Phoenix (which is at 1,117 feet above sea level) and Flagstaff, which has an altitude of 7,000 feet. The highway has a number of scenic view exits along its route that overlook northern Arizona’s mountains and valleys.
The intersection of interstates 10 and 17 is considered one of the worst freight bottlenecks in the U.S. according to the Federal Highway Administration. (Photo: Federal Highway Administration)
In 1936, State Route 69 (SR 69) was planned as a state route from Phoenix north to Prescott. The road was completed by 1940. Then in 1954, a new route north to Flagstaff was established as SR 79. By 1961, SR 79 from Phoenix to Flagstaff had been built, but not to interstate standards. I-17 was completed from Phoenix northward to Camp Verde by 1971; however there was a short stretch that had not been completed to interstate standards. Another section from SR 279 (now SR 260) north to SR 179 had also been completed. The largest unfinished section was from SR 179 north to Flagstaff. Although still just a two-lane roadway, it did have full traffic interchanges built at crossroads.
In 1993, proposals to extend I-17 to I-15 in Utah were made. To date, no further action has been taken. However, a number of improvements have been made to I-17 and others are underway.
An existing interchange with Happy Valley Road in Phoenix was converted to a diverging diamond interchange (DDI). This type of interchange allows two directions of traffic to temporarily cross to the left side of the road. A DDI moves high volumes of traffic through an intersection without having to increase the number of lanes and traffic signals. This movement also provides easier access to an interstate. ADOT finished this project in the fall of 2020.
Arizona’s population has grown tremendously over the past 50 years. In 1970, the state’s population was 1.775 million; in 2020 it had grown to more than 7.151 million. Because of increasing weekend traffic on I-17 between the Phoenix metro area and Sedona, Flagstaff, and other high country destinations, ADOT plans to widen a 34-mile section of freeway to alleviate some of the congestion. The current section is only four lanes wide and motorists contend with frequent traffic jams to and from northern Arizona. Construction is scheduled to begin in 2022 and be completed in 2025.