Many have repeated this. I have always doubted it.
Upon further research, this is not the case AT ALL. I just knew there was no way the over regulated United States of America would have NO regulations.... on... well
anything they could regulate. 'Merica!
US REGULATION FMVSS STANDARD 108:
Each DRL optically combined with a turn signal lamp must be automatically deactivated as a DRL when the turn signal lamp or hazard warning lamp is activated, and automatically reactivated as a DRL when the turn signal lamp or hazard warning lamp is deactivated.
So, it’s pretty straightforward for vehicles with combined blinkers and DRLs. If a car’s turn signal and DRL are separate, though, that doesn’t mean it can necessarily keep its DRL on while signaling. It all depends on how close the two light sources are. In the “Spacing To Turn Signal Lamps” section, FMVSS 108 states:
Each DRL not optically combined with a turn signal lamp must be located on the vehicle so that the distance from its lighted edge to the optical center of the nearest turn signal lamp is not less than 100 mm...
There are a few exceptions to this 100 mm rule for non-optically combined signals and DRLs, the first of which is:
(a) The luminous intensity of the DRL is not more than 2,600 cd at any location in the beam and the turn signal lamp meets 2.5 times the base front turn signal photometric requirements
In other words, if the turn signal is very bright and the daytime running lamp is fairly dim, then the DRL won’t obscure the turn signal and it’s fine if both lights are close to one another and simultaneously on.
Text from:
https://jalopnik.com/heres-why-modern-cars-turn-off-one-of-their-lights-when-1792394567
Actual reg:
https://www.gpo.gov/fdsys/pkg/CFR-2004-title49-vol5/xml/CFR-2004-title49-vol5-sec571-108.xml
That explains why my Mazda doesn't wink out the DRL when turning.