Thanks for your input, I guess "Headlights on interior" then heavily depends on other settings. Under p140 + 1.2.4 (also on previous versions) when everything in the lighting page is set to default, just reducing "Headlights on interior" unfortunately does almost nothing even when set to very low levels or at 0%. It might have to do with default brightness at 300% and other settings that mess this multiplier.
In the following screenshots with everything else on the Lighting FX set as default, the first is default "Headlights on interior" 100%, then 10% which looks the same. I believe this was also the reason for
@Jempy 's comment.
images
(for these screenshots "Limit some things if there are a lot of cars" on the general settings was disabled, so all car lights from behind show right at session start)
Edit:
I hope you convey this to x4fab, as it's important both for track and car config making to achieve realistic looks under a sensible default value that'll be left untouched for most users. 300% is very exaggerated, making common cars look like they have Le Mans spec headlights.