On Tue, 28 Sep 2010 22:59:17 +0100, "Martin Rich"
wrote:

Because it's
common in America for one stretch of road to form part of more than one
highway, you'll sometimes see a road which carries more than one number and
more than one direction (I'm not sure if I'm explaining it very well;
perhaps an American could put it better)

Not an American, but I have some experience of their roads

Generally speaking, odd numbered roads are N-S; even numbered E-W. So
I-5 goes from the Canadian border to the Mexican border and 1-90 goes
from Boston in the East to Seattle in the West, etc. etc. In some
places, roads merge then separate but keep their numbering before and
after.

As an example, east of Reno, Nevada, Interstate 80 and Highway 95
share the same road: I-80 East/West is the same piece of road as 95
North/South.