Roads with national speed limits often have no speed signs.
GPS speeds are often incorrect , my local dual carriageway ( 70mph) shows as 50mph for around 5miles.
Local country road has a 20mph speed limit over a bridge (nsl 60mph)
But not shown on gps while same road in opposite direction incorrectly shows 20mph for entire road ( around 1 1/2 miles)
In the main, mine is accurate.
There will always be exceptions.
Any system should be set up to be, on the whole, accurate and effective.
"Should" being the operative word.............................