Third-party parking sensors

So... you've just bought your brand new BMW 7-series and are understandably proud of this new shiny vehicular behemoth. Or vehemoth, if you will. You gingerly take it out onto the road for the first time, making sure to leave a wide enough gap either side of the wing mirrors when passing other cars, whilst you attempt to get used to the sheer size of your new personal transportation solution. You arrive at the pub and attempt to parallel park in a space with mere inches to spare to the front and rear. This causes you no problems however, much to the dismay of the slack-jawed pub regulars who marvel at your ability to navigate such a large item into such a small space. Little do they know that you have specified ultra-sonic parking sensors as an optional extra, thus allowing you to accurately determine how close you are to the other vehicles as you steer yourself into place.

You go into the pub, buy yourself an orange juice, grab a seat by the window and prepare to bask in the reflected light from the deep gloss paint of your new acquisition. But horrors! The owner of the hitherto-barely-noticed rusty 4wd pickup truck parked in front of your car returns from the supermarket. He jumps into the pickup, starts the engine and attempts to extricate himself from the space (which is now considerably smaller due to the presence of a large, shiny BMW behind his truck). He backs up slowly, but (perhaps distracted by the thoughts of dinner, cold beer and his wife all waiting at home for him) fails to accurately judge the distance and... and...

What I propose is parking sensors that work in "passive" mode. If they sense something large looming closer, they beep the horn briefly, and perhaps flash the indicators. This would warn whoever was backing into you to back off, as it were. If they came any closer then perhaps two beeps on the horn or two flashes of the indicators could, umm, indicate that situation. In this way, bumper dents and smashed headlamps may be prevented, plus all drivers would eventually become better able to judge the length of their cars, even if they didn't have sensors fitted themselves.

Thank you, I'm new here.