Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
transportation management organizations (nonprofit employer associations focused on a
particular geographic area), or large employers.
Magnitude and Timing of GHG Reductions
While program-specific evaluations have been conducted more recently, some of the most
comprehensive and rigorous assessments of rideshare programs were performed in the
late 1970s when these programs were being implemented as an energy reduction measure.
An early evaluation of over 100 Federally funded carpool demonstration projects found
that approximately one out of six employees exposed to a program submitted applications
for carpool assistance; of these, 16 percent were influenced to join or expand carpools as a
result of carpool matching efforts – representing just under 1 percent of total areawide
employment. Including others who were influenced by marketing and promotion
campaigns, 2.8 percent of the areawide commuter population in six evaluated areas had
formed or expanded rideshare arrangements. These impacts translated into an estimated
reduction of 0.3 percent of areawide work trip VMT for carpool matching, or 1.2 percent
for broader programs (Wagner, 1978). Considering that work trips are just under onethird
of total VMT, this represents a reduction in areawide passenger VMT of 0.1 to
0.4 percent. A more recent literature review found that areawide ridesharing programs
have led to a reduction in regional VMT ranging from 0.1 to 2.0 percent, with the authors
developing a “maximum reasonable estimate” of 0.4 percent (Apogee, 1994). Considering
that metropolitan VMT represents about four-fifths of total U.S. VMT, the cited range of
0.1 to 0.4 percent would translate into a nationwide reduction in GHG from automobiles of
about 0.9 to 3.7 mmt CO2e in 2030.
Vanpooling programs have been effective at reducing VMT in niche markets. At its peak
in the late 1970s, about 15,000 vanpools operated in the U.S., with perhaps 10,000
operating recently. The five largest U.S. transit provider programs in 2002 had from 204 to
686 vanpools each, serving 2,400 to 7,200 average weekday passenger trips, with average
vehicle loadings of 5.2 to 7.0 passengers (Evans and Pratt, 2005). Vanpooling is currently
most prevalent in the Puget Sound region, where 2 percent of workers commute by
vanpool, although this is in part a result of unique geographic and institutional factors,
including water crossings with ferries with priority vanpool access, an extensive HOV lane
system, and employer trip reduction mandate. One study estimated the theoretical market
potential of vanpooling, based on the number of employees working for larger employers
and commuting longer distances, to be about 5 percent (COMSIS and ITE, 1993, in Evans
and Pratt, 2005).
Estimates of GHG emission benefits of vanpools must account for not only the reduction in
single-occupancy VMT but also the increase in GHG emissions from van operations and
the additional circuity of the trip. Furthermore, not all vanpool passengers are drawn out
of single-occupant vehicles. When vanpools serve central area employment in corridors
with heavy transit service, a substantial proportion of the vanpoolers may be drawn away
from carpools or transit use. One study of an early program in Massachusetts found an
average reduction in fuel use of 66 percent per participant (Evans and Pratt, 2005). The
State of Connecticut’s vanpool program registered over 3,000 commuters in 2006,
68 percent of whom are new to carpooling and transit. The State estimates that over 2.8
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