Does Kicking Hybrid Drivers Out of Carpool Lanes Make Traffic Worse for Everyone?

california hybrid yellow sticker photo.jpg

Photo via Bill Selak flickr

Up until recently, California has been allowing single occupants to drive in the carpool lanes as long as they are driving gasoline-electric hybrids. But as of July 1st 2011, solo hybrid drivers were no longer entitled to the privilege of using the carpool lanes. In a new study, U.C. Berkeley researchers examined data from roadway sensors measuring speed and congestion on freeways in the San Francisco Bay area for six months before the new rule took effect, and then for two months after it went into effect. They found some interesting results: By banning solo hybrid drivers from the HOV lanes, traffic worsened not only in the adjacent lanes but for every lane on the state's freeways including, counter-intuitively, the carpool lane. Carpool Lanes And Perks for Hybrid Drivers
Between 2005 and July 1st 2011, hybrid vehicles could drive in the carpool lane despite being singly occupied; all they needed was a yellow sticker. Approximately 85,000 low-emission vehicles were part of the yellow sticker program that gave them entry into the carpool lanes. Critics of this perk objected that hybrid drivers were making traffic worse for those actual carpool drivers using the carpool lane, and that since hybrids were becoming commonplace, it was time to remove the hybrid incentive that is usually in place for early adopters. The state of California apparently agreed with the critics and removed the perk as of July 1st.

Kicking Hybrids Out of Carpool Lanes Makes Traffic Worse
What the Berkeley researchers found is that hybrid drivers now clog lanes adjacent to the carpool lanes, slowing regular freeway lanes and the carpool lane, despite the fact there are fewer vehicles in the carpool lane. Along one freeway, the 880 freeway near Hayward, the researchers found a 15% reduction in speed after the hybrids were kicked out of the HOV lanes. It makes sense why adjacent lanes of traffic move slower with the influx of thousands of hybrid cars, but why would a carpool lane, now with fewer cars, experience increased traffic?

The researchers hypothesized that the answer lies in people's driving behavior. Drivers tend to adjust their speed not just to the lane they are in, but to the adjacent lane as well. For example, if the lane next to you is going very slowly, drivers tend to slow down to adjust to adjacent speed lanes so that there won't be as large of a speed differential when they need to merge.

A Sweet Spot for Carpool Lanes?
So if kicking the hybrids out of the carpool lane made the situation worse not only for hybrid drivers, but also for regular solo drivers, as well as for carpool drivers, what is solution? Is there a sweet spot for carpool lanes? The U.C Berkeley researchers suggest reinstating the hybrid privilege and even allowing some regular car traffic in carpool lane, in summary, making the carpool lane less restrictive and not more restrictive will actually make traffic move faster. So, solo hybrid and other drivers who have looked longingly at the relatively less crowded carpool lane may rejoice at this finding.

But this assumes that California's goal in removing the hybrids was to reduce traffic. California's decision to kick hybrids out of the HOV lanes also worked to give a boost to rival technologies, including natural-gas and electric cars, such as the Nissan Leaf. Currently, the only single-occupancy vehicles allowed in California's carpool lanes are federally approved Inherently Low Emission Vehicles (ILEVs), such as those powered by hydrogen fuel cells, 100% battery electric, and compressed natural gas vehicles with white clean air vehicle stickers. A new program, pending federal approval next January, will allow 40,000 plug-in-hybrids or hydrogen-powered internal combustion engine vehicles to enter carpool lanes.

So: California's ban may not be reducing traffic, but it may be shifting the market to encourage drivers to purchase vehicles that emit even fewer emissions and that is good news for everyone, even if you might be listening to this news while sitting in traffic longer.

More on Hybrid Cars
CARB Trying to Kill Plug-in Hybrid Conversion Startups in California?
California To Extend Carpool-Lane Access To 40,000 Plug-In Hybrids
The Perverse Hybrid Incentive

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