why are red lights placed on construction sites?

You see this a lot in London - lights are put at the top of massive cranes.

I always thought it was to let aircraft know, just incase they were flying a bit low (the London Air Ambulance doesn't fly too high and I suppose could hit an un-lit crane).

#1 Powerful Tactical Flashlight : http://flashlight.uzaev.com/?BvCJ

Coz red means - stop/caution/hazard..

In other words, don't go there..

Danger area, caution is needed while you are around this area................