Question of the Day - 09 June 2017

Because of the name "Interstate Highway System," it’s a common misconception that interstate highways have to cross state lines. They don’t.

For example, Hawaii has three designated interstate highways (H-1, H-2, and H-3), when for Hawaii to truly have “interstates,” it would need a 2,400-mile-long bridge to connect it to San Francisco. (The interstate system was conceived as a defense network across the country; indeed, the Interstate Highway System was authorized in 1956 by the National Interstate and Defense Highways Act. Thus, all of Hawaii’s interstates connect Pearl Harbor to the other military installations around Honolulu.)

Alaska, as well, has four interstate-designated highways, though only two sections of them have been upgraded to freeway status, both around Anchorage. But none of Alaska’s “interstates” are signed as such; like Hawaii, they’re called A-1 through A-4.

Within the continental U.S., primary or “main line” interstates are designated with one or two numbers, such as I-5 that runs through California, Oregon, and Washington and I-15 that runs from California through Las Vegas and on to Utah, Idaho, and Montana. (However, even these don't necessarily have to run between at least two states; for example, I-2 doesn't leave Texas and I-96 runs straight across Michigan.)

Auxiliary interstates are given three-digit route numbers, such as I-215 and I-515 in Las Vegas, with the second two digits based on the “parent” freeway and the first digit designating whether it returns "home" (in which case it’s an even number) or doesn’t (odd number).

For example, I-215 (even) is a circumferential loop that returns to I-15, while I-515 (odd) is a spur freeway that branches off of I-15 in downtown and connects to US 93 in around eight miles.

To further complicate matters in Las Vegas, only the east side of the Beltway qualifies to be official “interstates” (215 and 515). West of I-15, the Beltway is called Clark County (CC) 215, since it doesn’t conform (yet) to the standards of the Federal Highway Administration, which sets the regulations for the Interstate Highway System. CC 215 is called an “interim expressway”; once it’s completely upgraded to interstate status, CC 215 will almost certainly be designated I-215.

Interestingly, the 50-mile Beltway was planned, built, and paid for entirely by Clark County. It’s the first time in the history of the interstate system that a county built an interstate highway with no state or federal funding.