mrpj:
What if the road has streetlights but has a limit over 30mph? The way this reads suggests you wouldn’t need headlights on these roads as well
I was tempted to post about the definition of a ‘Restricted Road’ but, thinking back to the days when trucks ran on dynamos, as opposed to alternators, then it was quite common, even on Motorways, for truck drivers to switch off their headlights when entering a ‘lit’ stretch.
In those days, Emergency vehicles with VHF radios had ‘valve’ sets and had to be plugged into a battery charger for 8 hours out of every 24. How far we have progressed. 
On the subject of ‘dim-dip’ this was invariably achieved by placing a resistor in series with the bulb element. A resistor = heat. Heat, in the wrong place, causes fire.
Heat, whether in the form of a resistance, or as a light source, requires ‘energy’ which, in the case of a vehicle, equates to fuel. Whilst it might only be in the terms of a ‘cupful a fortnight’, it is still an Operating cost.
As for ‘daylight running lights’, as in the case of Volvo cars, I’ve more than once perceived a headlamp flash, to then relaise that the vehicle has hit a bump in the road surface.
Grrrr
