If you don’t have a post-holiday headache, what follows surely will give you one, thanks to the U.S. government’s now-you-need-it, now-you-don’t approach to travel documents.

If you are planning to travel by air anywhere outside the United States and its possessions or territories, go immediately to www.travel.state.gov and apply for your passport.

With luck, you should have it in four to six weeks.

If, however, you are traveling by land or sea — and that means the weekend car traveler to Rosarito Beach in Baja California, Mexico, or the ferry-ride day-tripper from Seattle to Victoria, B.C. — the rules are different, the timeline for the rules is different and the documents you need can be different.

For now, that land/sea border crossing into Canada will require a certified birth certificate and photo ID, such as a driver’s license.

Thoroughly confused? You have a right to be.

The tangle began last January when the first part of the Western Hemisphere Travel Initiative went into effect. It said air travelers needed a passport for travel that involved Mexico, Canada and parts of the Caribbean. The ensuing avalanche of applications swamped the processing system, creating a backlog at one point of 3 million passport applications and waits of 12 weeks or more for a passport’s return.

Congress realized, in the fullness of its heart, that its constituents weren’t getting their passports and were annoyed about missing their vacations. So in June, it relaxed the “passport or else for air travelers” rule until Sept. 30.

Part 2 of the WHTI — the land/sea phase — was to go into effect in June, but toward the end of 2007, Congress realized the potential for another passport debacle (in an election year). It delayed implementation until June 2009.

To add to the confusion, the U.S. State Department is talking about a new passport card, which should be available in the spring.

This limited-use document, or as I like to call it, Passport Lite, would cost $45, less than half the cost of the regular passport, and could be used for land/sea crossings from Canada, Mexico and parts of the Caribbean.