In the Series folder. There are .srs files for each series of cars. In the .srs file (which you can open in Notepad) there are entries that looks like this:
// Safety car threshold multipliers, in other words an adjustment for the sensitivity of the game to calling
// full-course cautions. These can be different for road vs. oval courses. Lower numbers result in more
// full-course cautions; default=1.0, possible values are 0.01 (very sensitive) and up. Values more than
// 5.0 will probably result in no full-course cautions being called by the game (note that there are now
// multiplayer admin commands to call cautions).
SafetyCarRoadMult=4.90
SafetyCarOvalMult=2.90
// Whether full-course cautions may be called for a spin more than 90 degrees where the driver keeps going.
// These can be different for road vs. oval courses. Higher values will result in more full-course cautions
// due to these temporary spins; default=0.0, possible values are 0.0-2.0.
SpinRoadSeverity=0.25
SpinOvalSeverity=1.75
You can adjust the values to change when caution flags and the safety car might be engaged. Just experiment to get the desired sensitivity that you want for a given series. Remember to back-up your original .srs files somewhere.