Provable privacy based on 'anonymized' published data sets that resists third-party information and linkage is really hard -- arguably impossible in general. When people claim 'anonymized' data, I tend to not be fully convinced.
I completely agree with you on the difficulty of truly anonymizing data.
I do wonder if there any holes in this idea though: What if they released the data the number of cars per hour per segment of road (a segment meaning a continuous piece of road between two intersections) rounded to the nearest multiple of 10?
As someone else said, Google shares something similar with cities via the Waze Connected Citizen Program[0][1]. Waze publishes a list of cities that participate here[2].
I think Google can't share the traffic data it shows users as it's probably an aggregation of multiple sources, and they probably don't have the rights give away that information. The Waze CCP is an opt-in service on Waze where people can share their information and input with cities.
Because that data is viewed as proprietary and useful for ads. In fact it's the reason Apple ended up making their own maps! They didn't want to share that info with Google but it was the only way Google was gonna allow for turn by turn directions in the app.
Google makes driving time estimates in advance available to consumers through Google Maps. I use it all the time when planning events around rush hour.