Since You Asked: Mile markers follow different rules for state, U.S. roadways
It appears that mileage markers (indicating distance from the Oregon/California state line) increment up as we travel from south to north on Interstate 5. Why do mileage markers on Highway 101 on the Oregon Coast increment down when traveling from south to north and vice versa for I-5? — Bob W., by email
Well, Bob, since it's the holiday season, we'll answer your question even though we answered a similar question in 2008. Consider it our gift to you — or maybe it's your gift to us, since we don't have to research it again. In any case, back in the day we got an answer from none other than Gary Leaming, a spokesman for the Oregon Department of Transportation then and now. Here's the answer:
... Oregon highways, such as Highway 99 and Highway 101, have mileage markers that run north to south, growing larger the closer they are to the California border. But Interstate 5's markers run the opposite way, starting with Mile Marker 1 just this side of our southern border.
It wasn't always so. When the interstate was first built, the numbers ran north to south, just like the numbers on the other highways in the state. But on April 20, 1971, the then-Oregon State Highway Department sent out a memo noting that the numbering would be reversed, "due to a recent directive from the Federal Highway Administration."
Leaming tells us federal and state highway officials made the decision so that all states would follow the same south-to-north measurements on the interstate system. For interstates that cross the country, the mileage increases from west to east. There's never been a directive from the feds to follow the same pattern with state highways, so they vary from place to place.
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