I had the same question so I asked on another forum:
Many thanks to Yurbuh Tuggly at N A V I T O T A L
Speedcam warning distances are dependent on speed of the vehicle at the time.
They can be adjusted in sys.txt as the following:
warn_distances=130:450,110:350,90:300,60:200,40:10 0 ;110:1000,80:800,60:500,40:300
The first figure is the speed in KPH:the second figure is the distance in meters
These are the defaults.
You can add any, and as many speed and distance increments as you like--be sure they are separated by a colon, and a comma between pairs.
Havent tried yet..