Passport applications are taking longer to process than average, and the U.S. Department of State said the increased wait time could last through 2018.

This wait time leads back to 2007, when the Western Hemisphere Travel Initiative (WHTI) was enacted. Because the passports issued that year will soon expire, it has caused an increase in demand for renewals.

The WHTI requires travelers traveling into the United States from countries in the Western Hemisphere to show a passport. Those countries include Canada, Mexico, Central and South America, the Caribbean, and Bermuda.

“You can renew a passport conveniently by mail at any time if you meet certain criteria,” the department explained. “If you are applying for the first time or are under 16 years old, you must appear in person at a passport acceptance facility, including designated U.S. post offices, clerks of court, and public libraries.”

Passport acceptance facilities offer routine and expedited service. Routine service usually takes around 4-6 weeks and expedited service could take around 2-3 weeks.

“Submit your passport application well ahead of your planned travel dates in order to avoid possible delays,” the department suggests.

The Department of State issued more than 15.5 million passports in fiscal year 2015, and expects that demand to continue through 2018.

For more information on how or where to apply, please visit Travel.State.Gov/ApplyEarly or call the National Passport Information Center toll-free at 1-877-487-2778/1-888-874-7793 (TTY/TTD).