Like many large US metropolitan areas, the San Francisco Bay Area has experienced rapid suburban employment growth since 1980, much of it concentrated in sub-centres. This paper shows that, contrary to the co-location hypothesis, employment decentralisation has not been associated with shorter average commute distances or durations in the Bay Area. Combining statistics on shifts in modal splits and average vehicle occupancy levels reveals that parallelling the region's sub-centring trend has been a substantial increase in average commute vehicle miles travelled (VMT) per employee between 1980 and 1990. The largest increases occurred in the fastest-growing and most remote suburban centres. Using decomposition analysis, we found that increasing commute distances contributed the most to rising commute VMT per employee, and the distance factor had proportionately the greatest effect on rising commute VMT rates in the most peripheral work centres. Since shifts in commute VMT per employee are thought to be strongly associated with transport externalities, we conclude that the social and environmental implications of the Bay Area's regional growth trends deserve more public policy attention than given to date.

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