4 Reasons For Subway's Explosive Growth

The sandwich chain has more than 40,000 locations, compared with McDonald's 35,000.

And while McDonald's is struggling with lagging sales, Subway is plotting an ambitious expansion to 100,000 restaurants by 2030.

Here are a few reasons Subway has become ubiquitous.

Easy access. Subway restaurants are efficient to build, and require less square-footage than the average McDonald's. As a result, Subway can easily open a restaurant nearly anywhere. Unusual locations include a laundromat, church, and car dealership.

Healthy perception. "When customers want something fresher or lighter than burgers and fries, Subway is an easy choice, and its range of vegetables and cold cuts lend to a broader brand perception of Subway being better for you," Sam Oches, editor of QSR Magazine, told Business Insider.

Ability to customize. Customers today are all about making meals to their specifications. McDonald's has responded to this trend by testing tablets that allow customers to customize a burger. Subway has been all about customization from the beginning, adding to its popularity.

Simple franchising model. Subway stores are easy and economical to open. As a result, franchisees are lining, up, Oches said.

Despite having more locations, Subway won't overtake McDonald's anytime soon when it comes to sales.

"McDonald's, despite having nearly half as many units as Subway, does almost three times as much as Subway in U.S. sales.," Oches said.