You’d think that kicking thousands of solo drivers out of the carpool lane would make traffic move faster…at least for carpoolers. But you’d be wrong, according to researchers from UC Berkeley.

In 2005, California granted drivers of hybrid vehicles access to carpool lanes (regardless of the number of riders) as a way to spur adoption of low-emissions vehicles. But that program ended this summer, after critics argued that the 85,000 cars that had qualified for special lane access were too many, and all the new hybrid drivers were clogging things up for carpoolers.

But instead of returning the roads to smooth sailing for carpools, the change actually slowed traffic for everybody, reducing speed as much as 15% in the “high-occupancy vehicle” (HOV) lanes themselves, according to the Berkeley study (pdf).

The researchers found that the speed in the carpool lane is affected by the speed of cars in the adjacent lane. So, if the regular lanes are more congested and creeping along at a snail’s pace, carpoolers are less likely to be blasting past them at 70 miles per hour.

Their conclusion? Let more cars into the carpool lane, not fewer, and everyone will move faster.

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Study: Disqualifying Hybrids From Carpool Lanes Slowed Down Everybody