http://www.gps.gov/systems/gps/performance/accuracy/
> Real-world data from the FAA show their high-quality GPS SPS receivers attaining better than 2.168 meter horizontal accuracy, 95% of the time.
The application here (driving) constrains the solution to 2D (i.e. known altitude from the map), and hence can since that variable is known quite accurately it can improve the accuracy of the other variables.
They're aware of what they're being used for and optimise for that. Similarly, because driving is 2D solution instead of 3D, the altitude will be used derived from the map, not the signal.
Which is why GPS systems designed for driving are fundamentally not suitable for use in (say) planes which need a true 3D solution.
The more immediate nice thing about the IIIA launches are that it adds more satellites that can transmit the L2 signal. Currently there are 18 healthy IIR-M and IIF satellites which can transmit L2. I believe you need about 24 minimum for full coverage.
Once we have full L2 and we get civilian dual band L1/L2 receivers it will get rid of one of the biggest sources of inaccuracy which is ionospheric delay. A dual band L1/L2 receiver will be able to measure ionospheric delay directly. This means the delay calculated will be for that specific point instead of the area covered by a WAAS ground station. Plus you won't need a direct view of the WAAS satellites.
It is easy to end up comparing apples to oranges with this sort of stuff.
So if you have buildings, hills or mountains blocking your view you're back to 7-15m accuracy. It gets worse thr furer north you go as your angle to the WAAS sattelites gets closer to he ground.
Plus it's North America only. Not sure if Garmin units support EGNOS which is the system in Europe.