That is a good idea.

How does the car know impact is imminent? Or is the bag full all the time, making the car way oversize???

THESE BAGS OPEN ON IMPACT - IMAGINE A CAR FALLING UPSIDE DOWN WITH THEIMPACT BAGS OPEN - IT WOULD HELP LESSEN THE IMPACT OF THE CRASH.IN A PILE-UP - IT IS ALWAYS THE FRONT AND REAR FENDERS WHICH TAKE IMPACT, INA PILE-UP - IMAGINE THE LIFES IT WOULD SAFE.

The airbags inside the car actually react to sensors that sense a 'crash' by the accelerations at (outside)bumpers or doors--they have a couple of milliseconds to process and fill-up while the car is collapsing. Side air bags have been much more difficult to design because of the shorter collapse time. If you wanted to put airbags on the outside of the car, you'd need different sensors, because there are no accelerations until there's a crash. I know this could work with radar sensors coupled with speed and movement sensors, but I'd hate to be the one to try and make it work. Would blow out often. Costs much to replace an airbag.

I gotta hand it to you pepindia, you are a regular idea-factory.

i agree that this would not work, the only possible sensors to use are light or sound based distance measuring sensors which would sense the oncoming vehicle and deploy the bags before impact, but this would be terribly difficult to make accurate or reliable, as they would have to compute the distance and speed of the approaching vehicle and react very quickly, and if they are wrong you just blew a very expensive airbag for nothing, which would probably be quite common, since cars very frequently get very close, or have near misses. and i assume that these airbags would be quite expensive since the small ones used now are so expensive that many cars only a few years old, involved in minor accidents are totalled rather than replace the airbags. seems like the bags are worth more than the car, so what do you save by protecting the car with this system, the only beneficial protection system is the interior airbags, because they save the person which is worth any cost.