Unfortunately this is quite common, both on streetview and in real live. Local kids with nothing better to do swing one of the pair around. it is usually quite easy to work out which is correct.
The other favorite is locals removing the National Speed Limit sign when they don’t like drivers speeding up leaving a restricted section. I have seen this quite a bit, but they usually leave the pole with the lower speed on the other side. 
With the Update last night I have spotted a tick box for Average Speed Cameras so I am just ticking the A13 so I can see what it will look like when the map updates. 
Done the A13 Canning Town to Dagenham and Tower Bridge for the Average Speed Limits. Will be able to report back how it works when the map updates. 
Do we need to start adding nodes to break up segments in line with the camera positions?
Only for the beginning and end of the signed average speed zone, regardless of where cameras are actually located, and not if within 200m of existing nodes.
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With the A13 Average Speed Camera zone they check every car on and off at the cameras at the Junctions so I have gone with where the speed limits change. Only at one place did I split the segments. 
I’ve hit a snag with the speed limits. I’m trying to adjust some on the B3349 through Barkham, Wokingham and while they’re in my drive area and not locked too high, I still get a permission error on some of the segments. (I’ve corrected the ones it let me amend.)
Within the past week the speed limit through Barkham on the B3349 has been reduced from 40mph to 30.
Permalinks:
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You’ve hit one of Waze’s little unhelpful features. 
All these segments were auto-locked. Waze reviews the auto-lock level, which is based on traffic volume, occasionally. One of the things which prompts a review is the segment being edited. So when you try to edit a segment, the first thing Waze does is recalculate the auto-lock level. For all these segments, it decided the auto-lock level should be 3, and then decided that you as a level 2 editor weren’t allowed to edit them. Since no edit actually took place, the recorded auto-lock level was not updated, leaving you with no idea why the edits failed. Genius design! 
Being a higher-level editor (just means I’ve been around longer ), this is no impediment to me and I’ve reset the speed limit for these segments. You’ll see that the auto-lock level is now recorded as 3. Any further posts about edits you’re not authorised to do would better go in the UK Editor Requests forum.
Thanks for your efforts,
Ian
The guidelines from this thread have been published to Wazeopedia - thanks to atrophicshiner for writing it up. Any questions, comments or corrections, please post them here.
Stats Update: 100% of Motorways and Highways are mapped with speed limits, 90% of Primary Streets and 70% of regular streets. Good work everyone!