Car help

programming
1

I just ran into a problem with my car.
I want it to not be able to turn when its not moving.
hers the code:

onClipEvent (load) {
speed = 0
slide = 0
}
onClipEvent (enterFrame) {
 //This code will advance the car forward.
 if (Key.isDown(Key.UP)) {
 speed += 2;
 this.lights.gotoAndStop(1);
 }
 // This will make the car go backwards
 if (Key.isDown(Key.DOWN)) {
 speed -= 2;
 this.lights.gotoAndPlay(2);
 }
 //The car will start to slow down after the speed of 25
 if (Math.abs(speed)>15) {
 speed *= .6;
 }
 // This will change the angle of the car
 if (Key.isDown(Key.LEFT)) {
 _rotation -= 12;
 }
 if (Key.isDown(Key.RIGHT)) {
 _rotation += 12;
 }
 // This will make the car move
 speed *= .98;
 x = Math.sin(_rotation*(Math.PI/180))*speed;
 y = Math.cos(_rotation*(Math.PI/180))*speed*-1;
 if (!_root.land.hitTest(_x+x, _y+y, true)) {
 _x += x;
 _y += y;
 } else {
 speed *= -.5;
 }
 
}

Hope you all can help:)