Carrate tag in the roads config.txt file. Extract from the CCG:
"carrate
[FONT=Arial,Arial][FONT=Arial,Arial]Defines traffic density on road[/FONT][/FONT]
[FONT=Arial,Arial][FONT=Arial,Arial][/FONT][/FONT]
[FONT=Arial,Arial][FONT=Arial,Arial](minimum seconds between each car generated).
0 = No traffic. Number must be greater than 3."
[/FONT][/FONT]