More on Understanding bicycle markings
From StreetsWiki:

Class I
Class I bike lanes are “physically separated from motor vehicle and pedestrian traffic,” providing a buffer against faster, heavier vehicles. This physical separation can come in the form of a tree-lined path, a sidewalk, a concrete buffer, bollards, or a line of traffic cones.
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Class II
Class II bike lanes are demarcated by paint on asphalt. In some cases, the entire lane is painted a distinct color so as to be distinguished easily from the rest of the street. In most cases, the lane is marked by a stripe, often thicker than a standard dotted white line. Some Class II lanes also receive a stencil in the middle of the lane (also refered to as a “sharrow”).
Class III
Class III lanes are bike routes that are represented only by posted route signs.

Brendon commented on my post “Given recent severe crashes, I’d say eliminating markings will not make things safer. In a perfect world where drivers always expected and anticipated all legal modes might be using the road, perhaps so, but we don’t live in a perfect world.”
and Hokan said “The point of these markings isn’t so much actual safety (although the hope is that they don’t make things worse), but perceived safety (comfort). This improved comfort is supposed to encourage more people to ride bikes rather than drive cars.”
I argue instead that Class III bike lanes are a meaningless distinction. All roads where bikes are allowed and not given their own marked lane should be considered Class III. Signing (or marking) something in some places that is legal everywhere is confusion-creating. It will lead motorists to think they can ignore bikes (or worse, that they are illegal) where they are not marked, just as drivers ignore unmarked crosswalks.
I understand the logic of network effects that Brendon suggests, more bikes make roads safer by reminding motorists. However more signs do not do that. While the signs may hypothetically attract bicyclists (I would be interested in real counts before and after signage as a Class III bikeway on and off the bikeway, i.e. are bicyclists actually attracted by such signage, or is it just feel-good politically correct actions on the part of the bicycle bureaucracy), but the signs are more visual clutter distracting from important information about the environment (e.g. watching for actual bicycles and pedestrians rather than bicycle signs).
Many signs are ineffective (See Tom Vanderbilt on Children at play signs), and too many signs are counter-productive.
Kevin Krizek also comments.