How to smoothly move a RigidBody a fixed distance on Keydown

I'm trying to build a 3 lane runner with similar mechanics to Subway Surfer.

Using c# what is the technique I could use to move the player smoothly from one lane to the next?
I have a RigidBody attached to my player so I am of the understanding that I need to use AddForce().

Currently I am doing something like this.

Code (CSharp):

1.     void Update() {
2.         if (Input.GetKey("left"))
3.             rigidbody.AddForce(15, 0, 0);
4.        
5.         if (Input.GetKey("right"))
6.             rigidbody.AddForce(-15, 0, 0);
7.        
8.     }

In psuedo code I'm trying to do :

IF key pressed, THEN move player 5 units along the X axis.

Any suggestions?
Thanks

 

Letting the physic drive your character is a bad idea.

You should look at; http://docs.unity3d.com/ScriptReference/Rigidbody.MovePosition.html

 

Thanks LightStriker, MovePosition proved to be a much better approach.
And to answer my opening question about movement. I found the iTween library to be useful.

 

Hi, this video tutorial will answer your question.