Early routes

The general location of the I-15 corridor through Nevada can be traced as far back as the early 1900s. Regular automobile travel through southern Nevada was established by 1914 along the Arrowhead Trail, a road connecting Los Angeles and Salt Lake City.^[5] In 1919, the Nevada Legislature amended its newly adopted highway law to add SR 6, which was described as "Commencing at the Arizona line near Mesquite and running southwesterly over what is now known as the Arrow Head [sic] trail through Las Vegas to Jean, Nevada."^[6] Two years later, the route was revised to pass through Las Vegas and head "via Jean or Goodsprings to a junction with the California state highway system."^[7]

US 91 was later proposed as part of the original 1925 U.S. Highway System plan. As originally proposed, US 91 would have followed SR 6 southwest from Mesquite to downtown Las Vegas, where it would then turn southeast along SR 5 to California via Searchlight.^{[citation needed]} The revised 1926 plan had proposed US 91 follow SR 6 through Las Vegas and Jean to the California state line.^{[citation needed]} Nevada's 1927 official highway map reflects the routing of the final 1926 plan; however, a few maps from the era indicate the original proposal.^[8][9]

US 91 / SR 6 began at Primm and followed the Los Angeles Highway toward Las Vegas. Within the city limits, it ran along Fifth Street (now Las Vegas Boulevard) through downtown and into North Las Vegas, where it departed from 5th at Main Street. Exiting the Las Vegas area to the northeast, US 91 became the Salt Lake Highway and then headed toward Apex and Glendale. The highway then curved east-northeast before following present-day SR 170 through Riverside and Bunkerville into Mesquite. US 91 was routed through that city on Mesquite Boulevard, Sandhill Boulevard and Fairview Avenue before crossing the Arizona state line. By 1929, the alignment of US 91 / SR 6 was graded throughout much of the state, a distance of 129.5 miles (208.4 km).^[10]

The route eventually gained two other highway overlaps. US 466 was christened in 1934, and was routed concurrently along US 91 from California to downtown Las Vegas before heading southeast to Hoover Dam (the route was deleted in 1971).^[11] Also, in 1936, US 93 was extended from its 1932 southern terminus at Glendale, over US 91 to downtown Las Vegas on its way to Hoover Dam.^[12]

Interstate development

A new alignment of US 91 was completed in 1955. This bypassed both Bunkerville and Mesquite to the northwest, along what would become the future path of I-15.^[13] The original alignment through Bunkerville is now SR 170.

The passage of the Federal Aid Highway Act of 1956 quickly brought upgrades to the US 91 corridor. The first section of the newly designated I-15 opened by 1960. This stretch extended from the California state line to just north of Sloan.^[14] In 1963, the new freeway reached the south end of the Las Vegas Valley at Blue Diamond Road, and more than 20 miles (32 km) south of Glendale was also finished.^[15] By 1967, I-15 had been constructed along the entire length of US 91 except through parts of Las Vegas and near Mesquite.^[16]

The final sections of I-15 to be completed were in North Las Vegas and near the Arizona state line.^[17] Both of these sections were completed in 1974. The US 91 designation, which had remained on its original alignment during the construction of the Interstate, was made redundant by the new freeway and was removed in 1974.^[18]

Between Las Vegas and the California state line, the Nevada Department of Transportation (NDOT) added call boxes at one-mile (1.6 km) intervals in the mid-2000s, for motorists with vehicle problems and without cell phone service. This was done as part of a larger project that expanded this portion of the freeway to three travel lanes in each direction to coordinate with a similar effort in California.

In August 2011, I-15 in southern Las Vegas was substantially expanded. This involved adding between 30 and 85% more capacity and involved the use of the Big Stan drill rig.^[19]