One of the results of our big headlight survey (Stray light – Headlight survey) is that some drivers wait too long to switch from their daytime running lights to their low beams. I don’t want to bore anyone here by citing regulations and laws. What is more important to me is to clearly state that: daytime running light is not adequate for driving at twilight or full dark, even in the city, and the reason is safety.
Although daytime running lights are weaker than low beams, they can dazzle oncoming drivers more. Why is that? To sufficiently catch the attention of oncoming drivers, the distribution of the light emitted by daytime running lights is similar in optical terms to the high beams. That’s no problem during the day. The eye can handle it thanks to the ambient brightness. But it is a very different matter in the dark: the daytime running lights have a dazzling appearance because of the strong contrast between them and the dim surroundings. For this reason alone, it’s important to switch on your low beams at the right time, and that goes for driving in tunnels, too.
Another reason lies in how daytime running lights are wired. Most car manufacturers have only the daytime running lights turn on during the day, not the rear lights. Although that’s legal, I think it’s a bad decision. Red lights in the back signal clearly that a car is in traffic, not parked. In the dark (or in a tunnel!), the rear lights are all the more important. But on most cars, they only go on with the headlamps.
And then we have the very poor visibility. Daytime running lights simply are not designed to light up the road, but rather for better visibility. Only the low beams can illuminate the road, particularly the right-hand side.