[Someone]

"Do not mix" is too expensive for smaller roads, and may not even be possible without tearing down buildings.

Also "mix everything" is a viable option, too, if people behave themselves. The "woonerf" (http://en.m.wikipedia.org/wiki/Woonerf) is a very good solution for densely built neighborhoods (even in the USA, it seems: http://ithacavoice.com/2015/04/ithaca-housing-project-includ....

In a similar fashion, I see little wrong with cyclists driving on sidewalks. Problems only start when they drive faster than pedestrians in a busy street. In other words: if they do not acknowledge that they are guests on the sidewalk.

That is fairly typical behaviour in the Netherlands. People will cycle through almost empty pedestrian-only zones at 10-15 km/hour, slow down to walking speed when coming close to pedestrians, and start walking, bike in hand, if things get really busy. Legal? Not everywhere, but this rarely gets you a fine.