From Wazeopedia
==Problems with average road speeds==
If the average road speed is not correct, then the route will not be optimal. The following are reasons why using the average road speed can be the wrong value to use.
===Time of day variations===
Consider a road that most wazers drive at 5pm when the average speed is 12 mph. You choose your route at 10am when the road is clear and the average speed is 60mph. It is the best road to take, but Waze chooses a different route because it has no information about the speed at 10am and assumes the speeds is 12mph all day.
More Waze users would help fix this.
===Turn delays===
Consider a road of length 1 mile where you drive straight ahead and go through the intersection all at 100mph taking 36 seconds. Or at the end of the road you can sit and wait 5 minutes to make a turn. In the first case your average speed is 100mph. In the second case your average speed is just under 11mph.