When the infrastructure is eligible to go up by one (based on the years), as outlined in this archived feature request:
https://www.airwaysim.com/forum/index.php/topic,78252.msg463282.html#msg463282gameYear maxInitialTraffic maxTraffic maxInitialInfra maxInfra
1950 2 3 3 4
1960 2 4 3 5
1970 3 5 3 6
1980 3 6 4 7
1990 4 8 4 8
2000 4 10 4 9
2010 5 10 5 10
at first, only 1 airport, with the highest traffic (not traffic level, the actual traffic) would go up. Next period readjustment period (next year), 2 would go up a level, next year 3 would go up etc.
There should also be traffic level requirements for increases. For example:
Infrastructure level 2 needs traffic level 4
Infrastructure level 3 needs traffic level 6
Infrastructure level 4 needs traffic level 8
Infrastructure level 5 needs traffic level 10
After level 5, it would just go by year of availability of new infrastructure level, with slow speed of spread
So looking at GW3, even though we don't have the traffic level, but let's assume the order is:
1. LAX
2. ORD
3. LHR
4. AMS
5. ATL
6. JFK
All these airports are Traffic Level 9, I believe. So let's say
Jan 1 2004: none would go up in infrastructure since non meet Traffic level 10 requirement and probably all would go up in Traffic Level 10
Jan 1 2005: LAX -> 5
Jan 1 2006: LAX -> 6, ORD+LHR -> 5
Jan 1 2007: LAX -> 7, ORD+LHR -> 6, AMS+ATL+JFK -> 5
This would simulate the real world where certain infrastructure items are introduced at small number of airports and then later they are adopted at other airports.
Also, it would be useful to show another statistics for airports: Total Traffic (pax + cargo).
Maybe the traffic requirements could be relaxed to make more airports eligible:
Infrastructure level 2 needs traffic level 2
Infrastructure level 3 needs traffic level 4
Infrastructure level 4 needs traffic level 6
Infrastructure level 5 needs traffic level 8
Infrastructure level 6 needs traffic level 10