Whilst I am by and large against reducing speed limits on motorways or even A roads, unless there is a specific rationale on the M4 from Newport to Cardiff it is now 50 and it actually makes the journey less stressful and, it seems to me at least, rather quicker, as it smooths the traffic flow on a quite busy section. It used to be zooming along at quasi legal 70-80 then down to 20 or even a momentary stop, whilst now, despite being busier if anything we all trundle along at 45
to 50.
Something I do find a bit annoying and even contradictory is straight extra-wide trunk roads with 50mph limits. Presumably people had been hooning along at 80 doing mad overtakes. Might have been easier and cheaper not have improved the road in the first place and everyone would have done 50 to 60 perfectly happily and hopefully fairly safely
On the fuel saving argument it seems a bit crazy that large engined 2 or even 3 tonne vehicles are doing maybe 12mpg are deemed perfectly OK in and of themselves yet speed restrictions are being proposed to save fuel whilst my fairly sensible little Honda does 44mpg