The number of Americans with passports has hit a record high.

According to Census and State Department data, 21.4 million passports were issued in 2016, which is the most ever. That means 42 percent of Americans hold a passport, a growth of 15 percent since 2007. In 1990, only four percent of Americans had one.

The U.S. still lags behind some other countries such as the U.K., where 76 percent of people have a passport, and Canada, where 66 percent hold one.

A key driver of the rapid U.S. growth came in 2007 when passports were required for Americans to visit Canada, Mexico, South America, the Caribbean and Bermuda. Previously a passport was not required. The change was a response to the 9/11 terrorist attacks on the United States.

According to U.S. Government data, 35.1 million Americans traveled abroad in 2016. That was up 4.1 percent from 2015.