Option to base ETA on average speed or speed limit, whichever is less
Currently, all ETAs are based on the average speed of users. For users who choose to drive the posted speed limit, this tends to give them an inaccurate, shorter drive time which climbs as the drive progresses.
There are many places where the average speed data shows that drivers travel 10+ mph over the limit, which is not acceptable for some users.
An option, to base speed on average or speed limit (whichever is lower) would allow these users to get an accurate ETA while preserving the benefits of traffic slow-down avoidance. Users who don't care what the speed limit is can leave this option off and use Waze as normal.
For segments which do not have listed speed limits, Waze could use the average speed of users since there is no other data available.
I came here to add this idea as well, after driving home for Christmas and discovering that the only way the arrival time estimate could be correct is if most users are driving 15-20mph above the posted speed limit -- which at least explains why there seem to be an unusually large number of roadside memorials along that particular road. It's incredibly irritating to watch your arrival time skyrocket because you're only driving 10mph over the limit, and this one issue is why I won't allow my teenage driver to use Waze. The desire to "keep up" with the ETA is too great for a driver stricken with both the inexperience and the competitiveness of youth.
Bill Walton commented
This has been an ongoing problem for me. I really think Wazers must be atypical, if their average speed is as much above the limit as it seems to be.
I think there is no harm in giving people the option; this feature could be placed in the 'navigation preferences' settings.
As an example, if I want to get to Point B, I enter the address in Waze. I find I am running late, based on the EAT. So I speed up, say 9 over. But because the average is 15 over, I'm arriving LATER than original EAT. Doesn't provide me actionable information. Especially since I do not know what the average speed is along the route.