If you exceed the speed limit on a particular road, say 31 in a 30 zone you will only be notified once, you will not be notifed again until you go under the limit for that zone, down to say 29, this is also true if you have already been warned about say doing 41 in a 40 zone and then you continue into a 30 zone, you must be under the speed limit before you are re-warned about exceeding it
I don't know of a satnav that will give constant warnings, as the only thing that triggers a warning is you crossing the limit, so while you have the 'two red bars' you won't get another warning until they are removed
The warnings I am refering to are the ones generated by the satnav by comparing the road speed (taken from map info.) and your actual speed (measured by the GPS), you can have a different set if warnings generated by speed camera info. (either from Garmin or another speedcam database), this second form of warning is generated when your current location (taken from the GPS) is compared to a set of POI (Point Of Interest) locations taken from a speedcam database, they tend to say something like "Bing - Bong . . .Gatso 30" and are triggered as you approach the location of an actual speed camera in the database, NOT a speed sign