Well, it’s still not 100% sure if painted signage fulfils the requirements of the Road Traffic regs (e.g. does paint on the road amount to “signage”?), but where carried out it does make it easier to comply with the maintainance requirements as you don’t need to worry about veg clearance of signs.
If you do paint the road surface you’ve got to be really accurate so the markings conform to the Traffic Signs Regs (a white circle with 30 in the middle isn’t enough) so they’re more expensive than signs anyway. Some authorities use paint a lot though, lancs and carmarthenshire IIRC.
So, ultimately its not a matter of cost; to be sure the limit is enforceable the signage has to be spot on, which basically means you need to physically install signs and then make sure they’re maintained during summer months. And a TRO of course saying “30 mph limit between X and Y”. And you’d still need back-lit signs whenever going from a NSL to restricted zone, so you may as well carry on and put signs up pregularly along the whole length
Unless of course the authority isn’t concerned with accurate signage and uses them to encourage drivers to generally slow down.
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