Listing all the problems with any idea of a bike lane along a freeway will tire me out (starting with: there's no space, high barriers are regarded as ugly, there's no way to make the interchanges work, the freeway under road overpasses will permit larger road widths only with enormous expense, we can't even build carpool lanes and light rail, people won't stand for the construction delays). Let's not do this.
Agreed mturmon; an open-air LA-freeway-adjacent bike path poses many technical, aesthetic, scale-of-access, cost and possible health issues.
I do not think a freeway-adjacent bikeway would be popular with cyclists if open-air. LA has a freeway-adjacent bus rapid transit line (the "Sliver Line") which isn't very popular at 12K avg boardings in March 2013, compared to the far more successful rail-route-replacing "Orange Line" at 30K+ avg weekday boardings for the same month.
The Orange Line has a parallel bike lane which is popular with cyclists and pedestrians, even into the evening as it's well-lit and well-landscaped.
The barriers might be there anyways because of nearby housing. I'm not sure how it works in the LA area, but along 101 in the bay area had plenty of biking options (I used them personally), and it seemed to work out. However, although I could deal with it for commuting, its not something I would do for pleasure.
