Generate random images at different intervals

i'm creating a game like Traffic Racer, the player control a car, and he must avoid the others drivers on the road. The game is in 2D, and there is 3 ways on the road.

I want to generate random cars at different intervals, on the first, the second or the third way, randomly, and the cars can't be 3 at the same time, because the player would be blocked and and couldn't avoir the cars.

In the didMoveToView, I generate the nodes of the three cars :

    car1.anchorPoint = CGPointMake(0.5, 0)
    car1.position = CGPointMake(self.size.width * 1/6, self.size.height - 100)
    car1.size.width = 50
    car1.size.height = 97
    self.addChild(car1)

    car2.anchorPoint = CGPointMake(0.5, 0)
    car2.position = CGPointMake(self.size.width * 3/6, self.size.height - 100)
    car2.size.width = 50
    car2.size.height = 97
    self.addChild(car2)

    car3.anchorPoint = CGPointMake(0.5, 0)
    car3.position = CGPointMake(self.size.width * 5/6, self.size.height - 100)
    car3.size.width = 50
    car3.size.height = 97
    self.addChild(car3)

I've 3 timers for the 3 cars movement :

    car1MovementTimer = NSTimer.scheduledTimerWithTimeInterval(0.01, target: self, selector: Selector("car1Movement"), userInfo: nil, repeats: true)

    car2MovementTimer = NSTimer.scheduledTimerWithTimeInterval(0.01, target: self, selector: Selector("car2Movement"), userInfo: nil, repeats: true)

    car3MovementTimer = NSTimer.scheduledTimerWithTimeInterval(0.01, target: self, selector: Selector("car3Movement"), userInfo: nil, repeats: true)

and 3 functions to move the three cars :

func car1Movement() {

    car1.position.y = car1.position.y - car1Speed

}

func car2Movement() {

    car2.position.y = car2.position.y - car2Speed

}

func car3Movement() {

    car3.position.y = car3.position.y - car3Speed

}

In the update function, I've conditions to detect if the cars are out of the screen :

if car1.position.y < 0 - car1.size.height {

        var randomYPositionCar1 = CGFloat(arc4random_uniform(100))

        car1.position.y = self.size.height + randomYPositionCar1

    }

    if car2.position.y < 0 - car2.size.height {

        var randomYPositionCar2 = CGFloat(arc4random_uniform(100))

        car2.position.y = self.size.height + randomYPositionCar2

    }

    if car3.position.y < 0 - car3.size.height {

        var randomYPositionCar3 = CGFloat(arc4random_uniform(100))

        car3.position.y = self.size.height + randomYPositionCar3

    }

I don't think using the right method, can you help me ?

Thanks !