I long thought that a distance sensor would be a nice addition to Mindstorms robots, but ultrasonic ones were bulky and power hungry (I've since found Robot Electronics Ultrasonic Range Finder that can perhaps be used), and simple infrared method like the one I used in my radar car detects obstacles but don't give true distance measurement. The solution came from Andreas Peter (thanks Andreas for showing me these devices !) who interfaced a Sharp GP2D02 to RCX.