Road trip - Get the highways right

In an otherwise well-written and engaging story today about a Louisiana family who used a school bus to flee to South Carolina's Midlands was this sentence:

They found their way back to Interstate 90, backtracked to Larayette, then turned northeast to Baton Rouge and Mississippi.

Oops. If they backtracked to Interstate 90, they were almost at the Canadian border. They took U.S. 90, one of the few main roads in the New Orleans area that affords a way out. It's the kind of error that, even given the nature of the times, can make readers question whether we can get our facts straight.

Given the importance of highways and highway numbers and names in the aftermath of Katrina, it's worth reviewing the Interstate numbering system:

-- Even numbered highways run east-west and are numbered from south to north, with I-10 at the Gulf Coast and I-90 near Canada.
-- Odd numbers run north-south and are numbered from west to east, with I-5 in California, etc., and I-95 along the East Coast.
-- Circular bypasses generally have three numbers beginning with an even number.
-- "Spurs" into cities generally have three numbers beginning with an odd number.

Interstate highways are very specific highways built to certain standards, and not every federal highway is an interstate. Most are not (although parts of U.S. 90, for instance, are a freeway).

Instead, they are U.S. (or as once known, "federal shield") highways. Many are just two lanes wide and meander along local streets. And the numbering system is not as easily disentangled. Generally, odd numbers are for north-south routes and even numbers for east-west. But, for instance, while U.S. 20 and U.S. 6 both begin on Cape Cod, U.S. 20 follows the northern tier to the Oregon Trail, while U.S. 6 cuts across country to Long Beach, Calif. (Generaly, U.S. highways ending in 0 or 5 were to conform to a grid. Others could have more diagonal routes.) Many of these U.S. highways have been decommissioned in some states (such as U.S. 40 in California and the legendary U.S. "Route" 66) and their signs removed. (Here's a good site for some more background.)