> Any Interstate Highway that is in the 200's
> circles a city.  All the north south running ones
> are odd numbered.  All the east west running ones
> are even numbered.  Finally, any one in the 400's
> is a shot straight shot to a city.  That's all
> the rules I know.

300's are dead-end spurs off a main interstate, usually ending in
the middle of some dufus city. :)

the north-south ones also end in odd numbers (35, 47, 53)
the east-west ones end in even numbers (22,84, 20)

north-south numbers start in the west (lowest numbers in
california, highest numbers in maine)
east-west numbers start in the south (lowest numbers in florida,
highest numbers in montana/north-dakota)

the "interstate" highways in Hawaii are just numbered H1, H2, H3,
and H4.