What’s it all about approaching the overhead gantries near the Brynglas tunnel the other night and there’s a 20 flashing up ffs nearly had Johnny Foreigner enter the back of my trailer as he was on the limiter, yes there’s work going on but this Is 04.30 am and motorway was dead.
20 on a deserted motorway feels like you’ve ground to halt.
Could understand it if theres an accident and the road is completely closed, but only with a countdown on speed.
For an open road its more likely to cause a ruddy accident, but that requires whoever is controlling those signs actually thinking and we all see the idiotics they often do. Maybe they are bored at night, whereever they are based?
Smart motorway red circle/flashing red spots or orange?
20’s generally only set, when there has been a report of a vehicle travelling contraflow in the carriageway, or seen to enter a motorway exit slip the wrong way, to try and reduce a head on impact speed.
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Traffic Rat:
20’s generally only set, when there has been a report of a vehicle travelling contraflow in the carriageway, or seen to enter a motorway exit slip the wrong way, to try and reduce a head on impact speed.Sent from my PLK-L01 using Tapatalk
Last time I saw the limit set to 20 it was because horses were loose. Someone in a car ignored the signs and hit a horse. Made hell of a mess.
I’ve seen the 20 limit there before when they’ve had one of the tunnels shut.
The tunnel usually is shut overnight as they’re doing something roadworks wise.
Once it reopens, the signs remain set until all the road signs have been taken down. (And the workers have gone home and are tucked up in bed).
It’s not uncommon to travel along that section of the m4 at 430-530 am with 40 signs on the gantry and some people flying past you because the road is actually open again.
20 is super slow on a motorway. The worst is when they can’t even get the whole gantry to read the same speeds, ranging from 20 in lane 1 to 30 and 60 in the other two. 
M4 - Brynglas tunnels - it’s every night ATM - one side or the other, sometimes both. Red X’s closing lanes before they’ve even arrived with cones to shut it off. Whoever is putting the signs on, or ordering it, wants bloody shooting. 20 is dangerous - easy to be shunted from behind.
Roadworks on M621 heading into Leeds are set at 30mph with average speed cameras and it does feel like you’re standing still.
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gardun:
M4 - Brynglas tunnels - it’s every night ATM - one side or the other, sometimes both. Red X’s closing lanes before they’ve even arrived with cones to shut it off. Whoever is putting the signs on, or ordering it, wants bloody shooting. 20 is dangerous - easy to be shunted from behind.
Too right it’s dangerous as I mentioned this foreigner was so close to me it was unnerving, most of the gantries were lit up at 40 but the one with the cameras on was 20 for some strange reason !!
These “extreme” speed limits on motorway gantries - do no good at all.
If you follow them to the letter - there is a risk of road users slowing down too quickly, and risking being rear-ended as described by OP.
If you ignore them - then you either get flashed by the downward-looking cameras OR they don’t do the deed, and you’ll ignore them forever as a result!
The lowest speed limit on a motorway - needs to be something reasonable, like 40mph.
If there is a need for a lower speed than this, then it will already be evident by 3 or 4 lanes of halted-walking pace traffic.
Overnight on M42 several years ago,the gantries frequently showed 20 mph limit for several miles and then stopped for reasons unknown.If they were manually operated the guy is a muppet,if automated they don’t work.
They often show a limit of 30 or 40 which was presumably left over from the rush hour.
If not used correctly they are going to be seen as nothing at all to do with safety.
Gidders:
Overnight on M42 several years ago,the gantries frequently showed 20 mph limit for several miles and then stopped for reasons unknown.If they were manually operated the guy is a muppet,if automated they don’t work.
They often show a limit of 30 or 40 which was presumably left over from the rush hour.
If not used correctly they are going to be seen as nothing at all to do with safety.
Of course, as time goes on - more and more of us are just going to ignore them altogether. Or at least those ones that don’t have the downward looking flash cameras on them. 
Had the same thing at the M1 southbound where it meets the M25. Overhead gantry sign went from 60 straight down to 20, had to brake pretty hard, glad it was the middle of the night. Next gantry was back up to 60. If I was a cynic…
Winseer:
Gidders:
Overnight on M42 several years ago,the gantries frequently showed 20 mph limit for several miles and then stopped for reasons unknown.If they were manually operated the guy is a muppet,if automated they don’t work.
They often show a limit of 30 or 40 which was presumably left over from the rush hour.
If not used correctly they are going to be seen as nothing at all to do with safety.Of course, as time goes on - more and more of us are just going to ignore them altogether. Or at least those ones that don’t have the downward looking flash cameras on them.
A good argument for stricter enforcement.
Twenty Means Twenty !
It is not unknown for the speed limits set for the overnight roadworks to be displayed at 11.00am with lane closures also being shown. One time when I went to Swansea the A465 was ok for 70mph but the M4 was 40mph, signs also stated the obvious “Poor driving conditions”.
The variable speed limits are supposed to be monitored and a 20 has to be physically chosen rather than other variable speed signs which can be set by triggers, camera or speed monitors. Down to the person in the box somewhere.