Interstate 15 (I-15) is an Interstate Highway in the U.S. state of Nevada that begins in Primm, continues through Las Vegas and it crosses the border with Arizona in Mesquite. The freeway runs entirely in Clark County. Many motorists use I-15 to visit Las Vegas, as it is the only primary Interstate Highway in the city. The highway was built along the corridor of the older U.S. Route 91 (US 91) and Arrowhead Trail, eventually replacing both of these designations.