Abstract Manager

A degraded High-Occupancy Vehicle (HOV) facility is defined as a managed lane segment that does not meet a minimum average operating speed of 45 miles per hour (mph) for 90 percent of the time over a 180-day monitoring period during morning and evening weekday peak hours (or both), for an HOV facility with a speed limit of 50 mph or greater, according to Section 166 of Title 23 in the United States Code.  In California, the Department of Transportation (Caltrans) develops a degradation report and a degradation action plan annually to address the degradation issues as required by Section 166.  Friction factor is identified as a cause of degradation for some HOV facilities – a phenomenon in which operations in the managed lane is affected by operations in the adjacent General-Purpose (GP) lane as the adjacent GP lane approaches capacity.  Specifically, speed in the managed lane is noted to fall below free-flow speed while the managed lane demand is below capacity.  Called the sympathy of speeds phenomenon, managed lane users reduce their speed in response to a large speed differential with the adjacent GP lane where congestion occurs.  Managed lane users reduce their speed as they may experience a reduced level of comfort due to a perceived risk related to the potential of GP lane users entering the managed lane at any point at a relatively lower speed.

A study initiated by Caltrans aims to verify the existence of the sympathy of speeds phenomenon, quantify the extent of the frictional effects of congested GP lane operations on uncongested managed lane operations, and identify other factors that may influence managed lane speed aside from adjacent GP lane traffic conditions.  A study methodology is developed based on a review of the methodologies employed in several relevant research studies, with the intent to investigate if, how, and by how much, do managed lane speed and spacing change as traffic conditions in the adjacent GP lane deteriorate.  Traffic flow characteristics of continuous-access and buffer-separated managed lane facilities will be analyzed, friction effects will be quantified, and a linear regression analysis will be conducted to identify factors that may influence managed lane speed such as geometrics, in an effort to help guide the design of future managed lane facilities with improved operations.  The assessment is anticipated to be completed in April, at which time findings and conclusions will be drawn.