 Rather than number the west coast highway US 91, and lose four available north–south numbers (93, 95, 97, and 99) which, under the numbering plan, are supposed to be west of US 91, or assign the primary west coast highway a "lesser" number (one not ending in 1), AASHTO made an exception to its two-digit rule.Thus, US 101 is treated as a primary, two-digit route with a "first digit" of 10, rather than a spur of US 1, which is located along the east coast, on the opposite side of the U.S.Thus, US 101, not US 99, is the westernmost north-south route in the U.S. Highway System.