Usability of GeoData: Sat Nav Example

In my review of the Google maps public transport route calculator recently the major problem I found was that overland trains, a major and quick form of transport in London, were left out.  This is an issue of the usability of data and I recently came across another transport example.

I happened to drive across central London recently as I had to pick up a bike.  If you don't know London that well, this really is a last resort transport solution as our streets are complex and crowded.  I was lent an Android HTC phone to navigate with as it had a Google Maps Navigtion app.  Apart from driving in London being terrifying if you're not used to it (as I'm not) the app worked well.  Some positive notes:

• Voice prompts allowing you to keep your eyes on the road
• Free rather than over $100 for TomTom on the iPhone.  Thanks Google!
• Dynamic Rerouting when I went wrong, it rerouted me which is very helpful though its been possible on all the sat navs I've used to date.
• One Way Streets: It had data on which roads were one way and which weren't, a common problem in London.  It incorporated this into its journey calculation flawlessly.  

Some things I didn't like:

• Voice Prompts on demand:  I'd prefer to have voice prompts on demand, there were times when it told me information I didn't need and other times where I was wondering where to go that I wanted to make it give me an update.  Maybe it could talk when you touch the screen?  This would mean your eyes could remain on the road.
• Restricted Turns Data: It didn't have a complete data base of the restricted turns in London (it may have none of this information).  Its common in my home city that even though a street is not one way you are not allowed to turn right or left at a junction.  

The second of these is the more serious.  On the way out I encountered the problem and got away with it.  However, on the return journey I got lost and into some tiny medieval streets in London city.  I totally lost my bearings, the app found me a route but it wanted me to turn left at a junction where it was forbidden.  I drove away trying to find my way around the problem but being completely lost, I ended up coming in a big circle and found myself being instructed to turn left at the same restricted junction for the second time.  I was tired and confused and I have never sworn so loudly at an inanimate object in my life as that poor HTC phone when I got to that junction for the second time.

Conclusion:  Its common that usability issues with web maps come from the interface or symbolisation, what I hope I've shown here is that if the data isn't complete, they can come from the data too.