botach wrote:
Question is - are rear fog lights mandatory on HGV, and their use ??
As to their use. I rarely use them, for two reasons. The first being that they distract from a following drivers view of the brake lights, and if I need to brake in those sort of conditions, I want anyone following to know about it. Secondly, the passage through fog of a large vehicle displaces so much air that it creates a visibly clearer area to the rear of the vehicle. It was particularly noticeable on that particular Thursday night/Friday morning, the number of car drivers that would overtake into undisturbed air, pull back in and Brake.

The higher driving position also tends to aid visibility as fog density can vary according to height above road surface.
High intensity rear lamps are a product of the 1960's, when we
really had pea-soupers and city smogs. The Clean Air Acts of that era brought about a move away from the domestic burning of fossil fuels and quite frankly, I can only remember one occasion in the last 20 years where I encountered fog in visibility of less then 100 metres. That being the point at which rear fogs
should be utilised.
What was the speed of the vehicles you were encountering? I would assume it was in the range of the mid 50's - on the limiter. On a damp road surface. Which infers a visibility of 100 metres or more. Any less than that, and I wouldn't have been comfortable at such a speed.
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Considering the conditions - visibility poor when normal truck lights were pinpricks and fogs stood out , i'd say lights should have been on.
You haven't mentioned at what speed you were travelling to approach these such poorly illuminated vehicles. I assume >56mph, which then begs the question, "Was your speed appropriate for the conditions?"