> More precisely, it assumes that you're on the road it's telling you to
> drive down, until you get X number of yards away from that road, at
> which point the location indicator suddenly snaps to the road that
> you're *actually* on.

Indeed, whilst moving the accuracy is not good enough to determine 
whether you're on the motorway or on the slip-road 5 metres to the side 
of it, so it assumes you're following the route (if any) or staying on 
the road (if no route planned).  Once the receiver knows, with a 
specific degree of certainty, that you are on a different road, it will 
snap you there on the map.

> Some of this might be the inaccuracy of the road maps, but even at low
> speed it seems to take a very long time to figure out where you actually
> are, which leads me to think that most of the time it's guessing,
> because the GPS fix isn't very accurate.

Once you're more than 10 metres or so away from the route it should snap 
instantly, if it doesn't then you've either got a low signal (GPS 
antenna positioning?) or a rubbish GPS receiver.

BTW the ones built into the car are far superior as the GPS antenna is 
placed in the best spot and it gets input from the steering angle and 
wheel speed (so it still works underground and from the second you turn 
on the car).  If you've used one you'll never go back to the portable 
add-on ones.

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