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Making the Case for Carpool Lanes

Research indicates that even underused carpool lanes have a smoothing effect on freeway traffic.

Do carpool lanes reduce traffic, or are they a waste of space — space that would be more efficiently used if it were accessible to all drivers and not just the ones who are carpooling, driving hybrids or riding motorcycles? There are plenty of commuters arguing for and against carpool lanes, and now both sides have research to back up their arguments.

An earlier article suggested that four general-purpose lanes on a freeway carry more people and vehicles per hour and than a freeway with one high-occupancy vehicle (HOV), or carpool, lane and three unrestricted lanes. The California Bay Area researchers argue that carpool lanes are inefficient because they are underused sometimes and barely moving at others, a phenomenon they attribute to drivers switching in and out of the HOV lane.

But research by Michael J. Cassidy, Kitae Jang and Carlos F. Daganzo, published in the February issue of Transportation Research Part A: Policy and Practice, suggests the opposite. The authors, also examining the Bay Area, found that carpool lanes have a positive effect on traffic because they actually reduce the number of lane changes drivers make.

The UC Berkeley scientists studied four-lane freeway sites boasting one diamond-clad carpool lane and three general-use lanes. The carpool lanes in their study were reserved for carpools on weekdays during morning and afternoon rush hours (5 a.m. to 9 a.m./3 p.m. to 7 p.m.).

They found that in the two lanes closest to the carpool lane, a reduction in the lane-changing rate was followed by an increase in the "queue discharge flow" when the carpool restriction went into effect. In other words, people started changing lanes less frequently and bottlenecks cleared up. The researchers argue that the fact that this occurred so close to 3 p.m. strongly suggests that the carpool lane is responsible.

By examining the whole network of carpool freeways in the Bay Area during multi-week periods, they identified places where bottlenecks occurred at least 30 minutes before and after the carpool restriction went into or out of effect. The team found that the "smoothing effect" arose in every instance: Carpool lanes appear to have reduced lane changing and in turn mitigated traffic across even double yellow lines.

Overall, Cassidy, Jang and Daganzo conclude that because its externalities outweigh its inefficiencies, even an underused carpool lane is a win-win.

As a side note, they found that traffic flows are significantly higher — and drivers are significantly more hostile — at the beginning of the afternoon rush than at the end of either the a.m. or p.m. rush. "The pattern suggests that early-afternoon drivers are more aggressive than drivers in the late stages of a rush and are therefore less affected by lane changes, perhaps because they are trying to 'beat the rush' for the remainder of their trips," they write.

While this is hardly surprising — people used to driving in rush hour may have resigned themselves to their traffic-laden fate, whereas those driving in the early afternoon are likely hoping to avoid one — it does suggest that if you're hoping to avoid aggressive drivers, you should wait until rush hour is well under way, whether you qualify for the carpool lane or not.

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