A geography quiz: When you cross a Potomac River bridge between Virginia and the District of Columbia, are you traveling (a) north/south, or (b) east/west?

Answer: It depends upon which of the bridges you travel, although all provide similar transportation linkages. On the 14th Street crossings, Key Bridge and Chain Bridge, one travels more or less north/south. On the Theodore Roosevelt and Memorial bridges, because of a bend in the river, you're traveling east/west.

It may seem of little consequence. But it is brought to mind by an announcement from the National Park Service that is of crucial importance to thousands of commuters who now use the heavily traveled Memorial Bridge, where rush-hour backups are daily events.

Be warned, the service's National Capital Parks office said: Between April 19 and Sept. 25, two of Memorial Bridge's six lanes will be closed to traffic around the clock so that the 53-year-old span can be repaired and redecked. Much traffic will have to shift to other bridges.

What confused us at first was the statement that two "eastbound" and two "westbound" lanes of Memorial Bridge would remain open throughout the period. We always thought of traffic moving between Virginia to the south and the District to the north.

But a look at a map shows, indeed, that Memorial Bridge is chiefly on an east/west axis: when you're going across it toward Washington, you're going eastbound, and vice versa.

Theodore Roosevelt Bridge is nearly parallel, and calling it an east/west bridge is fully justified by the fact that it carries the traffic of two east/west highways, I-66 and U.S. 50. Key Bridge and the 14th Street bridges run on substantially north/south axes, and both carry the traffic of north-south highways: U.S. 29 on Key, I-395 and U.S. 1 on 14th Street.

We've intentionally ignored the Cabin John Bridge, which runs north/south but carries a highway, I-495, that goes in a circle, and the Woodrow Wilson Bridge, which runs east/west but carries a north/south Maine-to-Florida highway, I-95.