Can't up-vote this one enough; worked at a startup for a while tackling all sorts of traffic related stuff with proprietary IP, industry veterans & more. Suffice to say that there are tons of papers out there which clearly say taking N% of the number of cars off the road reduces congestion by Y% - a very much larger impact than the number of cars taken off.

This is because traffic, in general, is really caused by a small number of cars. It seems like it's everywhere but the root cause (there was a paper even on HN that talked about it) is that people can't follow each other at an even speed. This uneven speed then results in more braking, more backup, etc. A few cars in the lead who don't accelerate properly can have a disproportionately negative impact on traffic flow and average speed.