At about 10 a.m., waiting times were only 5 minutes at the Ambassador Bridge and Detroit-Windsor Tunnel, the Detroit Free Press reported.

Trucks at the Blue Water Bridge between Sarnia, Ontario, and Port Huron, Mich., were delayed about 5 minutes. Cars were crossing without waiting.

"We're the busiest land-border crossing in the country," Ronald Smith, field director for U.S. Customs and Border Protection, said as he watched the tunnel traffic. "The change that's occurring today should be pretty transparent."

Under new rules, everyone crossing the border by road or rail between the U.S. borders with Canada and Mexico needs identification that proves citizenship, like a passport, or government- issued photo identification plus proof of citizenship. Air travelers were already required to have passports.

Children traveling with their families must have birth certificates.