Will this fast food chain become more popular than McDonald’s?

There’s a fast food chain that’s sneaking its way to the top of the food chain. Sonic Drive-In — known for its Sonic burgers, hot dogs, mozzarella sticks, breakfast burritos, slushes and milkshakes — is gaining popularity and quickly proving to be a major competition for more popular fast food chains like McDonald’s and Burger King.

But it’s not like Sonic is new on the scene. The carhop chain has been in business for nearly 60 years, with 3,500 drive-in locations, serving more than 3 million customers a day. And it’s the fourth largest burger chain in the U.S., behind McDonald’s, Burger King and Wendy’s, with $4 billion in sales last year.

That’s a lot of burgers and slushes. So why is Sonic always being left out of the conversation when the fast food industry talks about its popular chains? Because it’s unlike any other chain.

Most fast food chains offer two options for ordering food — walk-in and drive-thru. Sonic allows customers to walk-in or pull up and park at the drive-in, and carhops (either on roller-skates or on foot) serve the food directly to the cars.

Sonic’s drive-in model has a big advantage over the competition, Sonic’s chief development/strategy officer John Budd told Business Insider

“A lot of times if you pull up to a drive-thru and you’re trying to contemplate the menu, we find that consumers feel stressed out by a line building up behind them,” Budd said. “There’s probably a psychological term for that that I’m not aware of. [At Sonic], you can come in and take your time.”

Allowing customers to take their time while ordering is a lot less stressful, but also allows Sonic to offer a larger variety of menu items — including breakfast all day. According to Sonic, there are over 1.3 million drink combinations available and the entire menu is customizable.

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“The Sonic brand has always been about personalization and customization,” Budd said. “Given the breadth of our menu and the five dayparts we serve, we definitely feel like we have something to [offer] everybody.”

The drive-in model has also given Sonic the chance to tap into new technology, with digital screens that allow the chain to directly market to customers in their cars.

The chain currently has drive-ins in 45 states and growing. So yeah, it’s no wonder Sonic is gaining popularity. It’s only surprising that it took this long.