High intensity strobes are now common on aircraft for anti-collision purposes but ground obstructions, both permanent and temporary, are still lit only by steady (and low intensity) red lights. That may be okay in a dark countryside but in a built-up area with lots of light pollution something more is needed so that obstacles like this crane really stand out. I know that this would only add to the light pollution but I see no other alternative.
The problem with this in my view, and with active transponders bolted to cranes and any similar suggestion, is this: they don't fail safe.
So what happens when the extra hole in the cheese lines up, and the battery fails, or the xenon tube blows, or the wind takes the antenna off the transponder, or whatever?
Result: The accident still happens.
All you have actually done is provide a way to absolve the pilot of blame (which I accept for many on PPRuNE is the desired goal of all accident investigations) but you've not necessarily done much to improve aviation safety.
Any safety feature you design needs to answer a fundamental question: what happens when it breaks? Take as an example three-aspect railway signals:
If the Green signal fails, the signal head automatically reverts to Yellow (a safer aspect.)
If the Yellow signal fails, the signal head automatically reverts to Red (a safer aspect)
If the Red signal fails, the signal in the rear automatically reverts to Red. Also, the standing instruction to drivers is also to treat any unlit signal head as a Red.
When the signal is anything but green, or ought to be, the Automatic Warning System reset electromagnet is de-energised, causing the permanent magnet just next to it to alert the driver to a restrictive signal (and will stop the train if he doesn't confirm it) - so even an unlit signal head will sound a horn.
If the Automatic Warning System electromagnet/electronics fails, the permanent magnet alerts the driver to a restrictive signal (whether it is or not) - the safest outcome.
If the permanent magnet fails, the laws of physics have evidently changed and all bets are off.
That is a good example of safety critical systems design. When a component fails, it fails to a safer outcome (albeit less efficient to railway operations - people who whine about the amount of disruption caused by signal failures and the like ought to remember that that's because their safety is being prioritised, but that's a different moan.)
Now consider what happens with your proposed solution:
If the flashing light/transponder fails: Pilots who have been told that flashing lights/transponders will be bolted onto everything assume the lack of said light/transponder means no obstacle, and chances of collision are increased.
This is a
bad example of safety critical systems design...