1 million
Interstate 15 passes through many jurisdictions and portions of the road have local names. According to Wikipedia, some of the other names are: Escondido Freeway, Avocado Highway, Temecula Valley Freeway, Ontario Freeway, Barstow Freeway, CHP Officer Larry L. Wetterling and San Bernardino County Sheriff's Lieutenant Alfred E. Stewart Memorial Highway, or Mojave Freeway.
For the record, there are places on the USA interstate that charge money to use, especially those in the older Northeast part of the country. The New York State Thruway and the New Jersey Turnpike charge money to travel on them. Many bridges and tunnels on the interstate do charge a fee to drive on. The USA interstate is not always a freeway. There are places that do charge tolls to use.
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As of 2012, the US interstate highway system includes almost 50,000 miles.
47,856 miles
The distance from the Interstate Highway at the Kentucky border and Interstate Highway 75 a the Michigan border is 211.5 miles. Like most of the Interstate Highway System, the exact distance is subject to change as it evolves and road construction continues to improve both traffic flow and road safety.
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As of 2006, there were 46,876 miles of roadway in the Dwight D. Eisenhower National System of Interstate and Defense Highways.