One of the main disappointments of some people is that the maps are NOT PERFECTLY ACCURATE or they find their favorite street in a subdivision is missing.   Forget it.  None of the currently available maps are perfectly accurate.  And they do not have to be perfectly accurate for the user to receive quite satisfactory results.  For instance,  roads and highways are being built,  moved,  and renamed and renumbered all the time.  Many municipalities are lax in getting their changed map data to the proper agencies and are in arrears for YEARS.   Generally speaking,  maps appear to be from 2 to 4 (or even more) years old.  Some roads (even Interstate Highways) can be out of position by hundreds of feet in spots.  (So many roads,  so few map makers.)  As a result,  you cannot pick up a single printed or electronic map that is completely accurate.  Some ARE better than others.  NavTech and MetroGuide II maps are among the best.  BUT!  Don't expect perfection!  You won't get it.

NOTE:  The above comments apply to the REALLY GOOD quality maps by NavTech and Etak (note - I think he's referring to TeleAtlas who purchased Etak) and a few others as used in Car Navigation systems.   The "not so good" quality maps (such as Garmin R&R,  Lowrance Streets, and Magellan MapSend Streets,  the WorldMap offerings and basemaps in handheld units)  can have much larger errors.  I have seen roads displaced by almost half a mile in a few spots.  Sometimes the shoreline on basemaps is displaced so you seem to be driving in the water.  Forget it!  Things are improving every day,  but perfection is quite a ways off"

Map source, "snap to road" and naming
"I want to mention a couple of points that you may or may not know about navigable digital maps.

Tiger data is the starting point for NavTech and TravRoute's digital maps. The positional accuracy is in the neighborhood of 6-10 meters. It used to be worse.
To display a vehicle Icon on a road segment the developer must make the decision to snap it to the road line. What do you do when the vehicle is not on the road, say in a large parking lot? In addition the combined interaction of the maps positional accuracy and GPS error can often fall outside the developers criteria for snapping to a road line.

Road segments typically have 3 name fields. One of the fields is supposed to be the locally preferred name but, displaying this field is not always the best choice. The I40/CORONADO HWY example you used is a case where the developer is displaying 2 of the field names. You will probably find that as you drive I40 the secondary name changes with each new municipality. Unfortunately, the three name fields and their level of importance is the only information the developer has to draw from. And, to further exacerbate the issue sometimes there are no names."- posted by SteveT on the PocketPCPassion message board
 

Data Collection
By PanEuropean in the sci.geo.satellite-nav newsgroup

"Database Accuracy:  Garmin, unfortunately, is at the mercy of the people (usually governments) who provide the _vector_ map data.  I know from extensive riding experience that the map data for Switzerland, Germany, Belgium and Luxembourg is absolutely fantastic - every single street, path, and cart track in those countries are listed, with great accuracy.  The only problems that exist in those (highly developed) countries are 'typographical' - meaning, by example, I have found a few places where
pedestrian walkway was coded to be a road suitable for vehicles, etc.  Get down south, into Spain, Portugal and Italy, and the quality of the map data falls way down.  Heck, in Spain, big 4 lane autopistas that have been open for over a year are not on the database, compared to Switzerland, where little roundabouts in residential neighborhoods that were built in spring 2002 ARE in the database.  In Lisbon, all the streets in the city are listed - but the fact that half of them are one way is not listed.  There's no way that NavTech surveyed these Swiss residential neighborhoods in the 2 months between roundabout construction and release of the European CD's that
support auto routing - that information must have been supplied to NavTech by the Swiss Federal Mapping Office, most likely in advance of the actual construction of the roundabout.

AFAIK, Garmin buys the cartographic data from NavTech, and NavTech starts their data collection process off by buying _vector_ street and road data from the federal governments of the various European countries.  NavTech then sends their vehicles out to drive the main roads and collect additional data, such as signage, speed limits, weight restrictions, etc. to refine the information even further.  I don't think NavTech has got around to doing a lot of their own data collection work in the Southern European countries yet.  There will be an update released for CN Europe this fall, that will help some and will correct the gross errors, but really high quality, complete coverage for the Southern European countries will depend, for the most part, on the federal governments of these countries compiling accurate and complete _vector_ data describing their roadways.  Don't bet on this happening overnight - 5 years is my personal guess, and only if they get EC funding to do it."