Abstract
We estimate the effect of carsharing on travel behavior (specifically, household vehicle holdings, frequency of transit usage, and frequency of biking and walking) using data from the 2011–2012 California Household Travel Survey (CHTS). The effect of carsharing on vehicle ownership is a dynamic process that plays out over a period of time—past ownership influences enrollment decisions, which in turn influence holdings in a later period. Representing this process using cross-sectional data conflates causal effects with simultaneity bias. Further, members and non-members differ in various observed and unobserved ways—demographics, built environment of residential and workplace location, and attitudes—raising the potential for self-selection bias in comparing travel behavior between the two groups given the observational nature of the data. Drawing on established methods for dealing with each bias individually, we develop a method to help control for this joint occurrence of self-selection and simultaneity biases. Restricting the analysis to employed respondents residing in the San Francisco Bay Area, we find that 80% of the observed difference of 0.9 units in average vehicle holdings between carsharing non-members and members may be explained by the biases listed above. The remaining difference of 0.17 units reflects the estimated effect of carsharing, which is the equivalent of shedding one vehicle by about one out of every six households whose member(s) are enrolled in carsharing. The effect on transit usage and walking and biking frequency is positive, albeit small and statistically non-significant. We identify factors that may affect the internal and external validity of our results. Our methods cannot completely replace randomized experiments or panel data. However, the methods used here provide a way to help control for the joint occurrence of self-selection and simultaneity biases, and provide a ballpark estimate of causal effects, for large-scale, general-purpose, cross-sectional datasets such as the CHTS.
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Acknowledgements
This study has benefited from the input and critical comments of several anonymous referees; various researchers including Kenneth Laberteaux from the Toyota Research Institute of North America; Susan Handy, Gil Tal, Sonia Yeh, Lewis Fulton, David Bunch and Joan Ogden, all from the Institute of Transportation Studies at UC Davis; and James Sweeney from Stanford University. We are also grateful to Karim Hamza for help with obtaining accessibility scores. The views and opinions expressed in this paper are those of the authors alone. The authors declare no conflict of interest.
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Mishra, G.S., Mokhtarian, P.L., Clewlow, R.R. et al. Addressing the joint occurrence of self-selection and simultaneity biases in the estimation of program effects based on cross-sectional observational surveys: case study of travel behavior effects in carsharing. Transportation 46, 95–123 (2019). https://doi.org/10.1007/s11116-017-9791-1
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DOI: https://doi.org/10.1007/s11116-017-9791-1