How do I make the car slow down when touching walls?

programming flash
1

[COLOR=#000000][FONT=verdana]Hello,[/FONT][/COLOR]

[COLOR=#000000][FONT=verdana]I’m making a car game and I have made so the car moves when I press the keys. But now I want to car to slow down when it drives into the walls. How do I do that? This is my code:

[/FONT][/COLOR]package AS {	
	import flash.display.MovieClip;
	import flash.events.*;
	
	
	public class Car extends MovieClip {
		
		private var speed:Number = 0;
		private var keys:Array = [false,false,false,false];
		
		public function Car() 
		{
			super();
		}
		
		
		
		public function update(keys:Array):void
		{
			this.keys = keys;
			
			if (keys[0]) {
				speed += 1.5;
			}else if(!keys[0]&&!keys[1]){
				speed = 0;
			}
			speed = speed * 0.98;
				
		 
			if (keys[1])
			{
				speed -= 0.8;
			}
			   
			if (keys[2])
			{
				this.rotation += 8;
			}
			   
			if (keys[3])
			{
				this.rotation -= 8;
			}
			   
			this.x += Math.sin (this.rotation * Math.PI / 180) * speed;
			this.y += Math.cos (this.rotation * Math.PI / 180) * -speed;
		 
			if (Math.abs (speed) > 20)
			{
				speed = 20;
			}
			   
			if (speed < -10)
			{
				speed = -10;
			}
		}
	}
	
}
[COLOR=#000000][FONT=verdana]

[/FONT][/COLOR]package  {	
	import flash.display.MovieClip;
	import flash.events.Event;
	import AS.Car;
	import flash.events.KeyboardEvent;
	
	public class Main extends MovieClip 
	{
		
		var timer:CustomTimer;
		var car:Car;
		private var keys:Array = [false,false,false,false];
		
		public function Main() 
		{
			addEventListener(Event.ENTER_FRAME, update);
			
			trace(stage);
			addEventListener(Event.ADDED_TO_STAGE, init);
			
			
			car = new Car();
			addChild(car);
			
			timer = new CustomTimer();
			addChild(timer);
		}
		
		private function init(e:Event):void
		{
			removeEventListener(Event.ADDED_TO_STAGE, init);
			stage.addEventListener(KeyboardEvent.KEY_DOWN, setKeyDown);
			stage.addEventListener(KeyboardEvent.KEY_UP, setKeyUp);
			
			car.x = stage.width / 2;
			car.y = stage.height / 2;
		}
		
		public function update(e:Event):void
		{
			timer.update();
			car.update(keys);
		}
		
		private function setKeyDown(event:KeyboardEvent):void{
			switch(event.keyCode){
				case 38:
					keys[0] = true;
					break;
				case 40:
					keys[1] = true;
					break;
				case 39:
					keys[2] = true;
					break;
				case 37:
					keys[3] = true;
					break;
			}
		}
		
		private function setKeyUp(event:KeyboardEvent):void{
			switch(event.keyCode){
				case 38:
					keys[0] = false;
					break;
				case 40:
					keys[1] = false;
					break;
				case 39:
					keys[2] = false;
					break;
				case 37:
					keys[3] = false;
					break;
			}
		}
	}
	
}
[COLOR=#000000][FONT=verdana]

Thanks in advance![/FONT][/COLOR]