This article is from the California Driving (and Surviving) FAQ, by "George J Wu" firstname.lastname@example.org with numerous contributions by others.
from email@example.com (David Cortesi) on 22 Jun 93:
Primarily for cars not to drive in. The law says cars cannot drive in bike
lanes, except to park where permitted, to enter or leave a roadway, or to
prepare for a turn within 200 feet of an intersection [CVC 21309].
Cyclists are supposed to use bike lanes but they are not locked into them.
CVC 21208 says:
Whenever a bicycle lane has been established on a roadway pursuant to
Section 21207, any person operating a bicycle upon the roadway at a speed
less than the normal speed of traffic moving in the same direction shall
ride within the bicycle lane, except that such person may move out of the
lane under any of the following situations:
(1) When overtaking and passing another bicycle, vehicle, or pedestrian
within the lane or about to enter the lane if such overtaking and
passing cannot be done safely within the lane.
(2) When preparing for a left turn at an intersection or into a private
road or driveway.
(3) When reasonable necessary to leave the bicycle lane to avoid debris
or other hazardous conditions.
A lot of cyclists prefer to stay in the traffic lanes because traffic keeps
them swept clear of glass, tree litter, kids on skateboards... Seriously,
cycling activists like John Forrester (author of Effective Cycling) argue
that bike lanes are a unsafe and ineffective, and that governments would do
better trying to teach cyclists and drivers how better to share the roads.
From Jym Dyer (firstname.lastname@example.org) on 1 Nov 1993:
Forrester's argument about bike lanes is based on statistical
data showing lots of bike/auto collisions when autos running
parallel with a bike lane make a right turn at an intersection.
California's law about merging into the bicycle lane before
making a right turn would appear to be an attempt to address
this problem. Unfortunately this isn't made at all clear.