For my world creator spreadsheet, I created a little formula to determine the travel code for a system:
Atm 10,11: +1
Atm 12+: +2
Pop: 2,3: -1
Pop: 1: -2
Govt: 0,7,10: +1 + Faction Modifier + Law level Modifier
Govt: 6,11+: +Faction Modifier + Law level Modifier
Govt: Other: No Faction or Law level modifiers
Faction: +1 per Faction of "Notable" size or higher, to a max of +2
Law level: 0: +1+Govt Modifier
Law level: 8: +0.5
Law level: 9: +1
Starport A: -1
Starport B: -0.5
Naval base: -1
TAS Hostel: -2
Imperial base: -2
Pirate base: +1
2D6-4 + DMs
Total>8: Amber
Total>11: Red
This seemed to produce a reasonable spread of Ambers (c. 10%) and very few Reds (less than 1%), but is probably a bit unwieldy to use outside of a spreadsheet.
(and apologies for duplication, as I have posted this in another thread somewhere too...)