[scoot]

Except that Google uses an average of the recorded speed of vehicles using a road segment, linked to time of day, and not the speed limit.

[cyberdrunk]

... which, given that most people are speeding, makes their estimates completely unrealistic if you want to actually obey the law.

[lemagedurage]

So they should add a "route options" -> "follow speed limits" checkbox, or track your speed compared to the average driver.

[Nightshaxx]

This. Google has a major advantage in that they have a massive dataset. They can use their data to improve the cost function even in real time to detect traffic.