I'm not sure exactly but I imagine it's to do with the town name sign surrounded by the red border, i.e. one is entering a municipal area. When you leave a French town/village the name is displayed again but with a diagonal red line through it. I assume that any obligation the driver was under in the municipal area is now removed. Just a guess.

BTW one thing I do enjoy about French road signs in general is their height. In France they are short and at drivers’ eye level rather than high up a pole. Some peculiarities though. On direction signs in a town at a junction one is presented with a destination town (or obscure village) and ‘Autre Routes’. So if the town name means nothing to you, you keep following ‘Other Routes’ until you have circumnavigated the town centre and ended up back where you started. It’s then time to plumb for a name or look for some obscure village en route to your destination.

Tying this thread back to things electrical though, Paul made a comment regarding the diamond as opposed to the triangle for warnings on roads – and it was interesting to see that Ireland has chosen to follow the North American/Australian standard rather than the European one. When one sees danger/warning signs on equipment (including electrical) in the US it is on a triangle and instruction/computer manuals draw attention to potential problems by using a triangle as well – not a diamond.

For interest, South Africa used the European style triangles, circles, etc. with their red borders but inside the background was dark blue with the numbers/symbols/figures etc. in white. In the last 10 years the signs have changed to the standard European convention of black on white in a read border. Compulsion signs/minimum speeds are in white on a solid blue circle.

A sign that cased me great amusement at the time (1982) was in Zimbabwe on the road to Kariba Dam by the Zambian border. It was in two parts – a red triangle with solid yellow centre above a yellow square with the picture of a huge trumpeting elephant on it. We joked about it but then found ourselves weaving around the largest piles of dung that I have ever seen on a highway! [Linked Image]