GPS on phones is extremely inaccurate. GPS on cars could be better, but that's not clear.
The car has a lot more information than a phone though - and much less of it is extrapolated.
Odometer - which is federally monitored to be accurate - rather than accelerometer value x time x fudge factor.
Speedometer vs. GPS change x time elapsed x fudge factor.
Cars with ultrasonic parking systems or the newer side blind spot monitors can directly detect cars close by, and likely can tell if the shadows are the same car (as opposed to a sequence of cars).
Waze works because of its 10,000 unpaid mappers constantly correcting the crap routes generated by GPS - but it isn't ever going to get better because the quality of incoming data is just not going to improve. For example, which lanes in a given road are better vs. the 'average'?
There's also things like: are you in a car pool lane or not (and whether the car in question is eligible).
New fancy cars actually can tell how many people are in them...
Only those people who don't actually deal with real world data directly think that which comes out of the cell phone sensors are accurate.