On the A55 around Colwyn Bay, there is a stretch of road with a 70mph speed limit sign.
Why haven’t they simply used a national Speed Limit sign instead?
maybe because its an A-road? 
Its a Dual Carriageway.
Why would A road / B road that make any difference?
National Speed Limits are:
D/C Car 70, D/C Truck 50, D/C 7.5 Tonner 60
S/C Car 60, S/C Truck 40, S/C 7.5 tonner 50.
The road is a dual carriageway so where the sign says 70, all limits are exactly the same as if the sign was a National Speed Limit sign.
I can’t fathom out why they have used the figure 70.
Mentioned here:
LandyLad:
Its a Dual Carriageway.
Why would A road / B road that make any difference?
I dunno it was just an idea 
After reading the link though, you’d of thought they’d of just used the national speed limit sign like you said 
Hi there MRPJ.
Thanks for the Wiki Link.
Having read the link, I wonder if the result is that the limit for ALL vehicles on that stretch of road is 70 - INCLUDING HGVs and Coaches. As it is a Special Road, do all the speed limit regs. NOT apply here?
Or … - what■■?
Thanks anyway
I’ve often wondered this too!
There’s a myth that because the M6 Toll was built privately there’s no speed limit; it isn’t true and the limit’s 70 like any other motorway. Also worth noting is that the maximum speed limits for the category of vehicle are absolute. Some HGV drivers (and instructors) think that a 50 limit sign as on a S/C road applies to every vehicle including theirs which is again incorrect.