Mating Habits of Slugs: Dynamic Carpool Formation in the I-95/I-395 Corridor of Northern Virginia

The availability in the Shirley Highway corridor in Northern Virginia of an uncongested high-occupancy vehicle (HOV) lane restricted to vehicles carrying three or more persons has led to the development of significant dynamic ridesharing activity. Each day, drivers of vehicles carrying too few occupants to qualify for HOV lane use (known as “bodysnatchers”) stop at informally established but well-known locations to seek additional passengers (known as “slugs”). As part of a study whose purpose was to quantify the likely effects of changing the HOV lane eligibility rule from three or more persons per vehicle to two or more persons per vehicle, data were collected on the patterns of dynamic carpool formation. These data were used in the examination of the hypothesis that an important factor contributing to the long-term success of dynamic ridesharing is that a single passenger need never ride with a lone driver: the 3+ occupant HOV rule ensures that there will always be two passengers in addition to the driver. Analysis of carpool formation patterns by number and gender revealed that slugs come in pairs. Changing the HOV occupancy rule would result in fewer vehicles seeking riders, but most slugs would still find acceptable rides. The primary effect on ridesharing would result from increased congestion in the HOV lanes.