Vehicles - Spawn Factors

[subvod]

I know that the in-game time and the car the character is driving at that time affects the types of cars that spawn all around the map, but what other factors affect the cars spawning? The character himself/which character you're playing as? What mission the character is in? Has anyone compiled a list of the factors?

 

I ask because I have gotten two unmarked cruisers as Trevor, gave Michael one, then haven't seen a single one more even after three all-nighters circling the bus station for each character. Is there some sort of car spawn limiter where if a character already has a rare car, it doesn't spawn at any point after he gets it?

Edited March 12, 2015 by WRXSTiFan