There’s a great explanation for why San Antonio drivers have been sitting in more traffic lately. New figures from the U.S. Census Bureau show San Antonio and New Braunfels ranked among the country’s fastest-growing cities from 2017 to 2018.

Based on the largest number of new residents from July 2017 to July 2018, San Antonio was the second fastest-growing U.S. city with at least 50,000 residents. It added 20,824 people and ended up with a population of 1,532,233, according to a May 23rd report from the Census Bureau.

San Antonio remains the country’s seventh largest city. However, as PhillyVoice.com first noted, San Antonio is about to slip past Philadelphia (with 1,584,138 residents as of July 1, 2018) to claim the No. 6 spot. Given that San Antonio has added roughly 24,690 residents per year since 2014 while Philadelphia has added just over 5,150 residents per year in that same time, San Antonio is likely to make up the current 51,905-resident gap between the two cities by 2021, PhillyVoice.com says.

A key to San Antonio’s growth is the surge of immigrants moving here, mainly from Mexico. A recent report shows immigrants made up 13.5 percent of the city’s population in 2017 and 16 percent of the workforce, with nearly 9 percent of San Antonio’s population growth attributed to immigration.

“Immigrants’ contributions to our economic strength is part of the American success story, and San Antonio is a significant beneficiary of their economic power,” Mayor Ron Nirenberg says in a news release.

Based on percentage growth, New Braunfels was the second-fastest growing city with a population of at least 50,000 from 2017 to 2018. The city’s population shot up 7.2 percent, winding up at 84,612, the Census Bureau says.

On the city’s website, Mayor Barron Casteel says people are attracted to New Braunfels because it offers “the charm of a small community and the convenience of a large urban area.”

“We have beautiful rivers and parks, a vibrant downtown, a growing economy, great neighborhoods, two highly regarded school systems, and citizens who make New Braunfels special by their exceptional volunteerism,” Casteel says.

New Braunfels was among seven Texas cities that cracked the list of the 15 fastest-growing cities (based on percentage) with at least 50,000 residents. Meanwhile, San Antonio was among five Texas cities in the top 15 for adding the most bodies from 2017 to 2018.

Joining New Braunfels on the list of fastest-growing cities in Texas was Frisco (6.1 percent, No. 4 for percentage growth), McKinney (5.4 percent, No. 6), Georgetown (5.2 percent, No. 7), Rowlett (5.1 percent, No. 8), Midland (4.4 percent, No. 13), and Round Rock (4.3 percent, No. 15).

Aside from San Antonio, Texas cities that gained the most residents on a numeric basis were Fort Worth (19,552, No. 3), Austin (12,504, No. 6), Frisco (10,884, No. 10), and McKinney (9,888, No. 13).

“Folks are obviously moving to big Texas metropolises and also to the suburbs, but are these fast-growing suburban areas going to be able to accommodate houses and businesses with the kind of infrastructure needed?” demographer William Frey, a senior fellow at the Brookings Institution, a Washington, D.C.-based think tank, tells U.S. News & World Report. “Growth is generally good, but you have to be ready for it, and Texas continues to need to brace for it.”

As a whole, Texas is growing at a rate of 1,000 people per day, and roughly half of these new Texans are migrants, state demographer Lloyd Potter tells the Texas Tribune. The other half are newborns, Potter says.

“A lot of the domestic migrants that are coming in are taking reasonably high-skilled jobs, and when they move here, they’re also moving to suburban ring counties,” Potter says.