The government has introduced passport cards designed to speed border crossings by U.S. citizens traveling to Canada, Mexico and the Caribbean, and the State Department is preparing to handle an onslaught of up to 30 million applications as the deadline nears when Americans won't be able enter the country from Mexico or Canada without a passport.

The government approved the wallet-size cards as a cheap and convenient alternative to passport books. They may be used only by citizens returning to the United States through land or seaports from Mexico, Canada, Bermuda and the Caribbean. Air travelers must carry full passports.

The cards emit a radio signal picked up by Customs and Border Protection officers at checkpoints. The signal provides inspectors with a file number linked to passport data on each cardholder.

The cards are handy for frequent border crossers, and cost just $45 for a first-time adult applicant, rather than the $100 fee for a passport book. The cards cost $20 for adults who already have a regular passport.

Brenda Sprague, deputy assistant secretary of State for passport services, said the cards contain information identical to what is found in a standard passport, and are just as secure. About 850,000 cards have been issued nationally.

Once the new system is in place, Sprague predicted, lines at Arizona border checkpoints may be shorter because inspectors can verify passport cards more rapidly than birth certificates and other documentation now accepted.

The State Department said Americans won't be allowed to enter the country from Mexico or Canada without a passport after June 1. Those who are turned away will have to seek emergency documents through a U.S. consular office, a process that may take 24 hours or more.

"It concerns me to a point," Sprague said during an Arizona tour this week. "But I know what we are capable of doing. We have built in so many mechanisms to anticipate the surge in demand."

Sprague expressed confidence that most Americans were aware of the new law, which affects U.S. citizens re-entering the country from border nations and the Caribbean.

She doesn't believe huge numbers of citizens will find themselves stranded at U.S. borders, nor does she anticipate increased logjams at checkpoints.

The State Department issued a record 18.5 million passports in 2007.

Despite the looming deadline, Sprague said the 2009 fiscal year is on track for only 12 million applications.