If "maximum speed sign" you mean road speed limit, that data is not in speedcam, it's in the map (.fbl). If you're see it wrong, it's the map that has it wrong.
Make sure you select metric instead of english speed in Settings->Region->Units and Formats
It is for historical speed profile and has nothing to do with your problem.
You're mixing road speed with speed of speedcam. You can tell proggy to alarm when you're max speed is certain percentage above road speed. Check Settings->Warnings->Speed Limit
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