In the 1960s, road racing gained popularity in North America, and Formula One-style design ideas changed IndyCars, which until then had all been classic-styled front-engined roadsters. When North America's road racing championship, Can-Am Challenge, collapsed in the 1970s, the IndyCars were ready to fill the void. IndyCar was a combination road- and oval-racing championship from this time until the Split. Compared to F1 cars, IndyCars were partly specialized for oval-racing: they were larger and had other safety features, and were designed to run at the higher speeds necessary for oval racing. Because IndyCars were usually "customer" cars that the teams purchased from constructors, and because of rules to contain costs, they were considerably less expensive than F1 cars, each model of which was designed by the team that used it. After the Split in the 1990s, CART maintained the old formula while the IRL drifted toward the "spec" design that has been the only IndyCar model since 2003 (which changed in 2012, with specialized aero kits available from 2015 to 2017).