Why is my Citroen DS4's speed limit detection not working on all roads?

My Citroen DS4 is equipped with a navigation system that displays the speed limit of the road on which I'm driving, but only on about 5 per cent of roads. Surely it should be possible to achieve this on all roads and to display the figure prominently alongside the digital speed display, in front of the driver.

Asked on 16 June 2012 by JC, Gloucester

Answered by Honest John
There are two types of system. One is GPS satnav and the other, fitted by Ford and GM, is one that uses cameras in the car to spot the speed limit signs and displays them on the satnav screen. You appear to have the former, but incompletely mapped. TomTom displays speed limits from mapping reasonably accurately, but, of course, cannot warn of temporary speed restrictions.
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