We don't have any option to turn the parking sensors on in the US. FYI, US spec A5/S5's don't get front parking sensors like some other countries do.Just put the parking sensors on, and press the bottom right hand "tab" under the dial. It works at traffic lights or when moving slowly even forward.
It remains on below 5 mph in case you are parallel parking etc. This way the camera does not keep switching views everytime you engage 1st gear. If you hit 5 mph, it assumes that you are not going to switch back into reverse again and goes back into map mode or whatever your last mode was......I think its possible because sometimes after I have been in reverse, the camera stays on for a good 10 secs before it switches back, even when I'm already going in 1rst Gear..