I think the key is SLOWDOWN. It’s self-advertising. If just a tiny number of people start in each city, millions of people will be involuntarily involved at first. Then, the REASON for it can be spread far and wide through the interwebs. The people who are SPREADING the word need not appear to be the instigators. They can appear to be simply disinterested third parties who are reporting on what is happening. Pretty soon, it will be common knowledge why these things are happening, with no one the authorities can blame or arrest. If enough people just don’t feel like going the speed limit, is there much the cops can do about it?
Especially since Rush Hour in most cities ALREADY involves massive bumper-to-bumper jams—it should be a SNAP to increase tens of millions of people’s commute from 90 minutes to three, four, five hours!
If people DO go out and buy a couple of $100 beaters, and crash them into each other during rush hour...