Here is an opportunity to post links to interesting articles that you have found on the internet. May as well fill your time in by building up knowledge 
Highways of the USA and how they are given numbers
http://en.wikipedia.org/wiki/United_States_Numbered_Highways
Quote from this article:
Major north–south routes have numbers ending in "1" while major east–west routes have numbers ending in "0". Three-digit numbered highways are spur routes of each parent highway but are not necessarily connected to their parent route.
Highways of the USA and how they are given numbers
http://en.wikipedia.org/wiki/United_States_Numbered_Highways
Quote from this article:
Major north–south routes have numbers ending in "1" while major east–west routes have numbers ending in "0". Three-digit numbered highways are spur routes of each parent highway but are not necessarily connected to their parent route.
