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Instead, you learn a set of rules that applies to all the numbers, and then the highway numbers themselves tell you how they run: One- and two-digit highway numbers less than 100 identify major routes (e.g., 1, 5, 70, 93) that cross state lines. Even numbers are east-west routes, odd numbers are north-south. Even numbers increase as they move from south to north; odd numbers increase as they move from west to east. Route numbers that are multiples of 5 are major arteries that extend over long distances. For example, I-5 is the westernmost major artery carrying north-south traffic between Canada and Mexico; I-95 is the easternmost major artery carrying north-south traffic between Canada and Florida. I-10 is the southernmost major artery carrying west-east traffic from California to Florida, and I-90 is the northernmost, carrying west-east traffic from Washington State to New York State. Three-digit numbers identify loops, or auxiliary, supplementary routes in or around a city. If the first digit is even, it is a route through or around a city that breaks off of and eventually rejoins the main route. If the first digit is odd, it is a spur into or out of a city and does not rejoin the main route (if you're afraid of getting lost, the auxiliary highways with an even-numbered first digit are thus always a safer bet). Generally, the second and third digits refer to the principal interstate served by the three-digit route. For example, if you are in Northern California and you find yourself on something called I-580, you can deduce the following: