Portable unit accuracy is shite? What utter 'dangle berries'. Set a destination and drive down the motorway at 110MPH. It counts down the distance to exit as you pass the appropriate markers: how the hell can it be inaccurate? It consistently displays the map for exactly where you are at junctions. I used mine for years to find the house number on long roads in the dark using just the number and the postcode. Or to find housenames on country roads. Drive through the Blackwall Tunnel, and it loses the signal about 1/4 of the way through. But, it carries on with your speed and picks up the signal the instant you exit the tunnel in enough time to direct to the correct exit. It works. It's that simple. Mine is the basic, small-screen, no bells and whistles European maps unit that I paid about £120 for about 6 years ago.
In the open, they work quite well - still nowhere near a proper unit which has the benefit of speed signals and gyroscopes or magnetic compasses. Its when the signal gets week (underground roads, forests, cities), all the portable units can use is dead reckoning, which is find in a long swiss tunnel at a constant 50mph, but
bleeding hopeless/dangerous somewhat inconvenient on the roundabout under Canary Wharf.
I know, I've been there. Hence mine was discarded, as it was actually more of a hinderence than a help. And I do not regret chucking £150's worth of "working" up to date technology out of the window... ...maybe somebody had a good day, or maybe it smashed, I care not. Even now when I have to use a car without a built in unit, the options I have on the iPhone - TomTom, CoPilot and whatever NavFree is now called (with TomTom being the best of them by some margin) - whilst being feature rich, all suffer the same old portable problems of location accuracy in "difficult" areas (ie, when needed most - as most people over the age of 30 can easily, trivially get to the town/village without assistance.