Answer
The car can move faster on banked road than on flat roads. On flat road, speed is limited, because if the car will have large speed, large centripetal force will be required, otherwise the car will slip.
Work Step by Step
The car is under normal, and gravitational forces.
$Nsin\theta$ is centripetal force.
Weight is $mg$ and is balanced by $Ncos\theta$.
Friction will not act here if roads are smooth.
But that is not the same case as flat roads. Friction force provides necessary centripetal force and so friction acts on the tires. Otherwise the car will slip on the road due to unbalanced forces in radially inward direction.
Due to this, the car can move faster on banked road than on flat roads. On flat road, speed is limited, because if the car will have large speed, large centripetal force will be required, otherwise the car will slip.
