[xenon]

Yes but if you're pulling up behind a car at traffic lights you don't need them and the constant beeping whilst you sat out the light would drive you insane. Conversely to what you say, I've never come across front sensor where you don't need to turn them on or select reverse. Certainly all the BMWs I've had, RRS, Porsche and Mercedes work like this. Which cars do they come on all the time?