Waze seems to have problems detecting traffic fast enough in zones where it isn't heavily used. In these zones other navigators simply do better in what Waze should be the king.
A solution might be adding an option to allow Waze collecting anonymous info such as user speed, location and direction even when closed, as a background service, to help increase traffic accuracy.
This way everybody with Waze installed can contribute to increase traffic accuracy (instead of just those who have the app actually running).
Since this can be considered a privacy concern for certain people, it could be done so that at the first start the app could ask if the user want to contribute traffic info only when Waze is open or when it is closed too (leaving the possibility to change this behavior every time the user wants to in the Waze option menu).