The problem seems to be the speeds of the cars in each lane. In carpool lanes, traffic moves at slightly normal speed. That's the whole point of them. But cars looking to dodge in and out aren't at the same speed so their slow asses cause the accident. In a study in Texas, it was found that carpool lanes increased accidents by 50%. No study has been done here yet (thus the study) but a look at numbers in SoCal showed that were definite patterns out there suggesting problems.

Turns out that carpool lanes are different up here than they are down there. In SoCal, a barrier separates carpool lanes and so it's harder to jump back and forth from lanes. The only way to go from lane to lane is by the occasional entrance/exit that pops up. The increase of accidents in SoCal that are possibly caused by the carpool lane comes from people going in and out of those exits/entrances. Why things are different here we don't know. Maybe it has something to do with us not adding "the" in front of highways while they do that down south.