Not sure how standard the 12,000 feet is. 12,000 feet = 3658m (since I think in metres these days). It's a useful length, with a 747-400 getting off in that length at MTOW and at standard ISA and 3000 feet airport elevation, or at ISA + 17 oC and 1500 feet airport elevation. As Sir GC says, the 747 dominated new airport design for many years, and many airports have been built to that sort of design. My observations over the last few years though have been around three different classes of runway length. These have been designed as (and these are not necessarily representative of existing airports):
- 2600m (8500 ft), allowing widebody aircraft flying shortish (5 hour) flights and narrow bodies most everywhere,
- 3300m (10,800 ft) allowing widebodies flying serious intercontinental distances,
- 4500m (14,750) ft) which seems to be preserve of various sheiks trying to outdo the neighbouring sheik.
barit1 touched on a couple of useful points. Firstly, with modern technology, there is little point indeed in longer runways. The widebody twins can haul a big load off a short runway, and designs for the A330 class aircraft are now for shorter runways. Secondly, and a big problem, is the advent of runway end safety areas (RESAs). These add a current required 90m at each end of the runway, or future 240m at each end. It has substantially changed the overall length needed and for new airports, I now automatically add half a kilometre to any planning length of a runway or airport for the RESAs, and that causes a lot of angst.