Earlier I explained the obvious–"pregpools" don't spare the road from an extra car. That kind of carpool doesn't do anything to cut congestion and HOV lanes in general don't do a very good job of cutting congestion.
More evidence of this comes via Bob's most recent newsletter (which Len links to in the previous post). Bob points to some work by Berkeley's Pravin Varaiya which finds that carpool lanes can actually increase congestion:
- First, by keeping non-HOV vehicles out of that reserved lane, the policy adds to congestion in the general purpose lanes. Second, in part since nearly all HOV facilities use a single lane in each direction, those lanes have less inherent capacity than regular freeway lanes, and hence operate almost as slowly as the GP lanes.
The Varaiya article is available on page 2 of this rather big pdf.