Is there an international regulation for this matter or does it differ from one country to another?
In the United States, the FAA has published Advisory Circular 150/5300-13A, Airport Design , which includes standards and recommendations for airport design, including parallel runway separation.
- "For simultaneous landings and takeoffs using VFR, the minimum separation between centerlines of parallel runways is 700 feet (213 m)."
- For simultaneous IFR operations, "Dual simultaneous precision instrument approaches are normally approved on parallel runway centerline separation of 4,300 feet (1311 m). On a case-by-case basis, the FAA will consider proposals utilizing separations down to a minimum of 3,000 feet (914 m) where a 4,300 foot (1311 m) separation is impractical. This reduction of separation requires special high update radar, monitoring equipment, etc." Note: Simultaneous Offset Instrument Approaches allow as little as 750 ft between runway centerlines.
International regulations do differ. ICAO have their own standards. Here is an example of a document which prescribes airport standards compliant to ICAO Annex 14.
Based on ICAO doc9157 Aerodrome Design Manuel Part1 Runway, the minimum distance between centre lines of 2 parallel( or near-parallel ) runways is
For Visual Meteorological Conditions (VMC) operation:
210m for category 3( runway length=1200m-1800m ) and category 4( runway length>1800m )
150m for category 2( runway length=800m-1200m )
120m for category 1( runway length<800m )
And for Instrument flight Condition (IMC) operation:
1035m for independent parallel approaches ( radar separation minima between aircraft and adjacent centre line is not prescribed)
915m for independent parallel approaches ( radar separation minima between aircraft and adjacent centre line is prescribed)
760m for independent parallel departures or Segregated parallel operations (one exclusive for departure and another for landing)
PS: It can be reduced by 30m for each 150m arrival runways is staggered toward arrival aircraft down to 300m between centre line of two runways; and vice versa (but no maximum)
When two runways are closer than the regulations above, only one can be "operated" at the same time (eg: LGW). Even though only one of the parallel runway would be operated at the same time, the separation of the runway have to follow the "Runway strip rule", which is
75m each side from the centre lines for category 3/4
40m each side for category 2
30m each side for category 1
Therefore minimum distance of two parallel runways is the sum of the strips,ie: minimum distance of two parallel 4E/F runways is 75+75=150m from their centre lines.
The above is global standard which represents the minimum requirement of runway , other aviation authorities such as FAA in US may have stricter standard.
The distance between the runways is not the issue. Some of them are just a hundred feet apart.
The issue is that the closer the runways are, the more separation the planes using those runways must have, and the higher the IFR minima.
For example, below is a picture of Oakland International. The two smaller GA runways (28L and 28R) at the top are close together, but when the field is IFR, only one of them can be used for IFR operations. The runway at the bottom (29) is a commercial runway, but it is far enough from 28L that Oakland can have simultaneous IFR operations on 28L and 29.
This page gives you a pretty good idea of what to expect.