If we assume the hill is perfectly circular, then we can assume that the cars motion about the hill is centripetal, and therefore the acceleration of the car can be given by v^2/r. In order for the car to not fly off the hill, the centripetal acceleration of the car cannot exceed the acceleration due to gravity, 9.81 m/s^2, therefore for maximum acceleration g=v^2/r, and rearranging this sqrt(g*r)=v. v=sqrt(9.81*30) m/s
