Why are red lights placed on construction sites?

Question

as in whenever a new site is under construction a red light is placed over it... why is it so?

Answer 1

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

Answer 2

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).

Answer 3

Coz red means - stop/caution/hazard..

In other words, don't go there..