Going to Greater Lengths

Using the ruler on Google earth, it's apparent that Dubai is to have the world's longest runway at 18,700ft....once underway extensions are finished. It can be viewed via this link. As you can see from the image, 30L/12R is being lengthened. The work looks to be about 80 per cent complete.
This makes it the world longest active civil runway. Why would Dubai need it? It may be something to do with the summer heat. It can hit 50°C and even at sea-level, generate a density altitude that must have an effect upon payload for take off. Pilots will be paying rapt attention to maximum tire rotation speeds and vying NOT to be the first one to overrun it on landing. That would be hard to explain away. Nevertheless, an A380 at maximum takeoff weight on a very hot day in Dubai will need quite a lot of that bitumen. 777's will also benefit and it will definitely be helpful to Delta's 7600 mile service to Atlanta Georgia.

However the effective usable length might actually be subject to a safety strategy. Sources say that the work in progress is to stagger the two runways so that traffic landing/departing from 30R/12L will not have to cross the other runway on the north side. When completed the new 30L/12R will "operationally" finish about half way down the other runway. Hopefully the rest of the bitumen will be left in situ to accommodate the overrunners.

Even though the runway at Area 51 is 25,300 ft, it is located on a Dept of Defense base on a remote tract of land (including a dry lakebed) in southern Nevada, located at 37.239° N 115.816° W. That is the world's longest runway (and it is actually all paved). However, it is far from active. There are "do not use" X's painted over sections of the runway and cracks all over the pavement.

If dry lake beds are to be counted, then 17/35 at Edwards is 40,000ft, 15/33 is 33,000 ft, 05/23 is 27,500ft, 18/36 is 24,000 ft, 07/25, and 02/20 and 11/29 are 21,000ft.

But for comparison purposes:
Vostochny (Russia) 16404 ft
Gavia (Bravil) 16295 ft
Denver Colorado >16,000ft
Bangda, China: 5500m / 18044ft
Upington (South Africa) 16076 ft
Harare (Zimbabwae) 15502 ft
Kinshasha Ndjili (Congo) 15420 ft
Mafikeng (South Africa) 15158 ft
Hawange National Park (Zimbabwe) 15091 ft
Edwards AFB (USA) 15013 ft
source - http://googleglobetrotting.com/info.php/mid/10179