How are highway route numbers determined?

Question

Both with interstates and local state routes. Is it completely arbitrary or is there some sort of calculated hidden grid that I'm not seeing? I've always wondered how they get the numbers.

Answer 1

Even numbered highways run east & west while odd numbers run north & south. On an interstate every 5 miles there has to be 1 mile of straight road in case a "plane in trouble" or military plane has to land.