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Reply to Lopez et al.: Sustainable implementation of taxi sharing requires understanding systemic effects

Paolo Santi, Giovanni Resta, Michael Szell, Stanislav Sobolevsky, Steven H. Strogatz and Carlo Ratti
PNAS December 12, 2014. 201421300; published ahead of print December 12, 2014. https://doi.org/10.1073/pnas.1421300112
Paolo Santi
aSenseable City Laboratory, Department of Urban Studies and Planning, Massachusetts Institute of Technology, Cambridge, MA 02139;bIstituto di Informatica e Telematica del Consiglio Nazionale delle Ricerche, 56124 Pisa, Italy; and
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Giovanni Resta
bIstituto di Informatica e Telematica del Consiglio Nazionale delle Ricerche, 56124 Pisa, Italy; and
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Michael Szell
aSenseable City Laboratory, Department of Urban Studies and Planning, Massachusetts Institute of Technology, Cambridge, MA 02139;
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  • ORCID record for Michael Szell
  • For correspondence: mszell@mit.edu
Stanislav Sobolevsky
aSenseable City Laboratory, Department of Urban Studies and Planning, Massachusetts Institute of Technology, Cambridge, MA 02139;
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Steven H. Strogatz
cDepartment of Mathematics, Cornell University, Ithaca, NY 14853
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Carlo Ratti
aSenseable City Laboratory, Department of Urban Studies and Planning, Massachusetts Institute of Technology, Cambridge, MA 02139;
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Extract

We have recently developed a method that allows the quantification of spare capacity in urban taxi systems through trip sharing (1), hence making it more efficient and less resource intensive—all other things being equal—in particular fares, which can be modified by public policy. In their comment on our study, Lopez et al. observe that a complete evaluation of the environmental sustainability of taxi sharing requires not only considering direct impacts, but also indirect impacts (2). We fully agree with this observation. For limiting the scope of our analysis (1) to a manageable extent, and in lack of an established methodology for quantifying the impacts of vehicle sharing at the granularity of single trips, we were not able to account for effects that can be considered as exogenous to the taxi system. Indeed, urban and regional models are composed of both exogenous and endogenous variables, and “much of the ongoing research agenda can be focused on transferring variables from the exogenous to the endogenous category” (3). The extensions suggested by Lopez et al. (2) and in our report (1) go exactly along this direction.

Footnotes

  • ↵1To whom correspondence should be addressed. Email: mszell{at}mit.edu.
  • Author contributions: P.S., G.R., M.S., S.S., S.H.S., and C.R. wrote the paper.

  • The authors declare no conflict of interest.

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Sustainable implementation of taxi sharing
Paolo Santi, Giovanni Resta, Michael Szell, Stanislav Sobolevsky, Steven H. Strogatz, Carlo Ratti
Proceedings of the National Academy of Sciences Dec 2014, 201421300; DOI: 10.1073/pnas.1421300112

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Sustainable implementation of taxi sharing
Paolo Santi, Giovanni Resta, Michael Szell, Stanislav Sobolevsky, Steven H. Strogatz, Carlo Ratti
Proceedings of the National Academy of Sciences Dec 2014, 201421300; DOI: 10.1073/pnas.1421300112
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