[toast0]

Were there mandatory prescriptive federal guidelines on HOV lane line colors when California introduced carpool lanes, or did they only come after California had been using yellow lines?

Personally, I do find yellow lines are a stronger border, but then I grew up in a culture of yellow carpool lines. I feel like white lines are almost always ok to drive over, and yellow lines usually aren't (although yellow dotted lines are a sometimes drive over (if safe, to pass on a two lane undivided highway), and yellow solid + dotted is also ok to drive over (if safe, to get into a shared median lane for an unprotected left turn)

[ubercore]

I think it's because yellow is supposed to indicate traffic moving in the opposite direction. That's why it's a stronger signal (in theory), but not "correct" for an HOV lane moving in the same direction.

[jrockway]

California should have closed the lane once a year for one minute to run a single vehicle down the carpool lane in the wrong direction. Then they could justify that the lane marking was the right color (since in theory, the lane can be reverse-flow at any time, and hey sometimes they even do it).

[nybble41]

> I feel like white lines are almost always ok to drive over…

Single and double solid lines parallel to the flow of traffic, of any color, almost always mean "do not cross".

A single dashed line, or a double line where one or both halves is dashed, can be crossed under at least some circumstances. The rules for crossing these vary depending on context, e.g. one-sided passing and center turn lanes use similar markings but with the solid and dashed lines flipped. (You pass from the dashed side but enter the turn lane from the solid side.)

White lines separate traffic going the same direction. Yellow lines separate traffic in opposite directions. This is independent of whether the lines can be crossed.