April 24, 2015


Lessons Learned from Spatiotemporal Studies of Freeway Carpool Lanes


1:40 p.m. – 3:00 p.m.


1605 Tilia, Room 1103, West Village


The presentation explores how the segregation of distinct vehicle classes on a roadway can improve travel conditions for all of the classes.  Insights come using freeway carpool lanes as case studies.  Spatiotemporal study of real sites shows (i) how the activation of a continuous-access carpool lane triggers reductions in vehicle lane-changing maneuvers, and (ii) how the reduced lane-changing can “smooth” and increase bottleneck discharge flows in a freeway’s regular lanes.  Theoretical analysis predicts that, thanks to this smoothing effect, even underused carpool lanes can diminish both the people-hours and the vehicle-hours traveled on a freeway.  Relevance to bus lanes is briefly discussed.

Further insights come via critiques of certain practices that degrade the effectiveness of carpool lanes.  Spatiotemporal traffic data reveal that a policy aimed at improving carpool-lane speeds has backfired, owing to a friction effect.  The policy mandates the eviction of select fuel-efficient hybrid vehicles from carpool lanes.  These evictions have caused queues to expand in regular lanes during the rush.  And these expanded queues, in turn, slow vehicles in the adjacent carpool lanes.  Spatiotemporal data further show that efforts to combat the friction effect by deploying limited-access carpool lanes can also backfire, because the designs for these lanes are prone to creating bottlenecks.

Biographical Sketch

Michael Cassidy performed his graduate studies in transportation engineering at UC Berkeley and then taught for 3⅟2 years at Purdue University.  He joined the Berkeley faculty in 1994 where he is now the Robert Horonjeff Professor in Civil Engineering, and Director of the University of California Center on Economic Competiveness in Transportation.  His professional affiliations include memberships in: the International Advisory Committee of the ISTTT; the Managed Lanes Committee of TRB; and the editorial board of Transportation Research Part B.

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