There is debate on which series is more demanding. Some point out that champions that retired from F1 have won CART championships, e.g. Emerson Fittipaldi and Nigel Mansell, and that drivers who did not excel in F1 have continued their careers in IndyCar, with varying levels of success, while some successful IndyCar drivers have tried but failed to get a seat in even a low level Formula One team (others, however, had success, including Mario Andretti and Jacques Villeneuve, who became Formula One champions, and Juan Pablo Montoya who won several F1 races). Conversely, some point to the different track designs of IndyCar (see below) as presenting a bigger challenge to the drivers.