The idle may or may not drop when driving lights are turned on. In a modern vehicle with an electronically controlled idle, activating high loads at idle such as the driving lights will generally cause a slight momentary drop in RPM that is compensated by the idle control system within milliseconds. Vehicles without an electronically controlled idle may experience a slight change in RPM because of the sudden load but the RPM will stabilize itself.
If your vehicle's idle is dropping significantly or stalling when driving lights are turned on, it is possible that there is an issue with the alternator, battery, or electrical system that is causing the low voltage. In this case, it's best to have your vehicle checked by a qualified mechanic.
