These aren't like the big multi-lane ones, these are supposed to be traffic calming devices and have been installed on narrow single family home streets with car parking on each side. They fill up the intersection, forcing cars to (usually) slow down somewhat to make their way around them. Of course, it is legal to go either way around them and the ones I most encounter are on a street that parallels an arterial that has several traffic lights on it, so despite being the designated bike route, there are always some cars speeding down it trying to avoid the lights and going around the circles whichever direction they please!