It a pitty that the GPS can not distingwish between the side of the road you are on. By this I mean sometimes the speed camera is only on one side of the road.. Say entering a tunnel from one direction. But entering from the other side there is NO speed camera , but Garmin Nuvi 660 , because of the software still allerts U 2 the speed camera that does not exist in that direction but exists within the overall area.
I can say, its better to be alerted the not at all. When the camera is present is for a driver to identify if the camera is in front of you or behind. Gps units, unfortunately can not gave precise distance between the nearest camera. Also, keep in mind the location must be accurate as well (POI). Make sure your maps are correctly compiled so it will correctly identify the POI.
I too have noticed this....I can be on a dual carriageway, divided down the middle with bushes and trees making it impossible to see with my own eyes the speed camera on the opposite side of the road, yet i still get an alert due to the system not being able to realise which side of the carriageway I'm on.
Even more confusing. Whilst driving along the A4 under the overhead section of the M4 at the Chiswick end, I got a warning for the camera above me! I suppose expecting the locations to include altitude would be a bit much given that this hardly ever happens. I've even used my sat-nav in the Eurostar to check its speed, only to have cameras on parallel roads come up on the screen. 186mph in a 70 limit - not bad.
Depending on the software, the camera alerts can be set up directionally. The original software on the Mio seemed to have this however the software was updated when I put new maps on it in 2009 and half the time it alerts it is because of a camera that is located in a side road that doesn't apply (obviously some of them do though). I even noted that some point-to-point cameras that were on the other side of the road for oncoming traffic would alert when I am going in the opposite direction. Bit of a pest & hopefully these things get rectified. It sounds rather trivial but annoying alerts are just that...annoying.
If you check your GPS you will find that the accuracy varies greatly depending on conditions at the time and number of satelites it sees..
Sometimes 3 metres and others 20+ meters, so I wouldn't be too worried about an alert from the other side of a road. Better to get an in-accurate alert occasionally then none at all.
Also, the GPS units for cars is nowhere near as accurate as the units used by surveyors, that cost far in excess of your car GPS, and, are stationary as well.
Recon the cheap mobile GPS units we use are good value for the cost.