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Impacts of Home-Based Telecommuting on Vehicle-Miles Traveled: A Nationwide Time Series Analysis


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Suggested Citation:
Choo, Sangho, Patricia L. Mokhtarian, Ilan Salomon (2002) Impacts of Home-Based Telecommuting on Vehicle-Miles Traveled: A Nationwide Time Series Analysis. Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-02-05

This study estimates the impact of telecommuting on personal transportation through a multi-variate time series analysis of aggregate nationwide data spanning 1966-1999 for all variables except telecommuting, and 1988-1998 for telecommuting. Three dependent variables were modeled, in direct and per-capita forms: ground vehicle-miles traveled (VMT), airline passen-ger-miles traveled (PMT), and the sum of those two variables, loosely referred to as "total miles traveled". The analysis was conducted in two stages. In the first stage (after ensuring that all series were stationary through first-differencing and log transformations), each dependent variable (1966-1999) was modeled as a function of conventional variables representing economic activity (e.g. GDP, employment, disposable income), the cost of transportation (e.g. gasoline price, fuel efficiency, CPI for transportation), transportation supply (lane-miles of roadways), and demographics (e.g. population, household size, licensed drivers, number of personal vehicles). A total of 15 explanatory variables were allowed to enter the first-stage models. In the second stage, the residuals of the first stage (1988-1998) were modeled as a function of the number of telecommuters.

The study necessarily relied on secondary data sources. In particular, we addressed some of the key issues (such as definition, quality, and quantity) associated with measuring telecommuting, and then assessed the available data. Although none of the telecommuting data sources is entirely satisfactory, the necessity of having data measured reasonably consistently over a series of years dictated the choice of data for this study. The chosen series, based on data collected by a single individual for several different market research firms across time, represents the longest series of data available on number of telecommuters, with estimates published for each year between 1988 and 1998. The estimates are based on 2,000 – 2,500 randomly-selected households interviewed by telephone each year. However, it should be stressed that these numbers, based as they are on small samples that must rely on the proper weighting in order to be representative, are in our opinion subject to a great deal of uncertainty. From various considerations, it is likely that the data used here overestimate the true number of telecommuters.

We assess the change in annual VMT per telecommuter, which can then be translated to a change in VMT per telecommuting occasion based on an assumption about the average telecommuting frequency (and hence the number of occasions in a year). Considering the stable average frequencies of telecommuting over time found in the literature, as well as the lack of complete information on frequency for each year in the sample, we assume the average frequency of telecommuting to be constant across the period of study. The results are presented for two such assumptions: 50 occasions per year (representing a frequency of about once a week, not including vacation weeks), and 75 occasions per year (about 1.5 days a week).