On SOME vehicles, the daytime running lights use less power than do the normal headlights. On Volvos, this is not the case and they use the same brightness. However, the taillights are not lit when the daytime running lights are on. I had mine disabled, because I was afraid that I would drive off after dark, see the headlights were on, and forget that the taillights were not. Seems like an unsafe "safety" feature to me.