I have noticed that on some cars with daytime running lights (DRLs), the light goes off whenever the indicator on the same side lights up. But there are other cars where the DRL continues to stay on while the indicator is flashing. Why is this so?
Daytime running lights are not yet mandatory across the globe and not in Singapore either. However, many car manufacturers are incorporating DRLs as standard features, partly in anticipation of an international mandate for all vehicles.
There are significant benefits in having lights on a moving vehicle during the day, as they enhance visibility to other road users. This is the same argument for switching on headlights in bad weather during the day - it is not for you to see the road ahead, but for others to immediately notice your presence.
Due to the natural ambient light during the day, the current generation of LED DRLs has a particularly high level of luminescence. Hence, when an indicator is switched on, the white or yellow DRL could, to the human eye, optically overwhelm the amber flashing signal.
To avoid this adverse condition, the DRL is totally turned off, so the amber flashing light appears as the only light displayed in the headlamp unit. This is the current format in lighting technology.
When DRLs first appeared on cars a few years ago, they were either halogen or regular incandescent filament types instead of today's LEDs, so the amber indicators were made of higher-wattage lamps to appear sufficiently brighter. With these headlamp systems, you may see the DRL and indicator flashing alongside each other.
Aftermarket DRLs are a different matter altogether. Firstly, they are not always compliant with regulations and, secondly, they may not have been designed with sufficient consideration for relative brightness.