At the end of its service life, a halogen bulb only delivers around 70% of its original light output. As light is a significant safety factor and car drivers are often not even aware of this subtle ageing process, it would make sense in principle to regularly replace dipped beam headlight bulbs as a precautionary measure.
Optimum lighting and driving half-blind
In practice, of course, this is quite unrealistic. The ageing process occurs at different rates, depending on the vehicle voltage, nobody wants to keep a log of how far they have driven with their lights on and, ultimately, nobody wants to replace parts that may still be in perfect working order.
However, when one bulb finally fails, this should be a clear indicator to replace the other one as well. High-quality bulbs have a very similar service life and you can assume that the filament of the second bulb will also have severely deteriorated. Replacing bulbs in pairs restores the lighting to optimum condition and also prevents you from driving half-blind again just a few weeks later.
Only get your hands dirty once
This argument is probably not quite so important for professionals or enthusiasts who spend almost every weekend tinkering with their cars anyway. But for many other people, replacing both bulbs at once saves a lot of time and energy and avoids having to deal with the other side again after just a short while. And if you can’t replace the bulbs yourself, it only takes one visit to the workshop.
By law, it must be possible for drivers to replace bulbs themselves
It’s been a while since replacing the bulb on a car could be called "child's-play": bonnet up, a clear view of the headlight and lots of room, even for large hands to hold the screwdriver. As technology has advanced, the space within the engine compartment has been filled, making it more and more difficult to replace the bulbs. This development reached its peak around 2000, when changing headlight bulbs on cars like the Golf IV or the Astra G was almost impossible without a visit to the workshop.
The EU then intervened and introduced legislation specifying that, from August 2006, all newly developed vehicles were required to enable drivers to replace bulbs themselves using the instruction manual and the vehicle tool kit provided. Xenon is still the exception, as vehicle manufacturers generally advise having these bulbs replaced in the workshop.
The situation has improved somewhat since then and if space is too tight, many vehicles allow you to simply loosen the headlight and pull it forward. As is often recommended, have a look at it in dry weather and when there is lots of light, before doing it for real under less favourable conditions.
And one final tip: Replacing the bulbs restores optimum vehicle lighting, but a quick wash of the headlights in between can often work wonders.