Odd-numbered highways move north and south, while those with
even numbers move east and west. Highways with one- or two-digit numbers are
through routes, often long ones used for distance driving. Three-digit routes
that begin with an even number are usually beltways around a city. Three-digit
routes that begin with an odd number are spur routes in a city or town. (The
Book of Answers, by Barbara Berliner)
