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Abstract
Excess commuting is the additional journey‐to‐work travel represented by the difference between the actual average commute and the smallest possible average commute, given the spatial configuration of workplace and residential sites. Research on excess commuting has been carried out over the last 20 years since the seminal contribution of Hamilton (Citation1982). The literature has generated much debate and controversy, and the purpose of this review paper is to assess that material critically under three main headings: contextual, methodological and policy‐related issues. The key contextual questions relate to the assumptions of transport optimization or cost minimization, socio‐economic factors, and how these are linked to urban spatial structure. The methodological issues cover spatial structure, aggregation, time or distance measures, and the boundary problem, whilst the policy‐oriented questions relate to the understanding of the effects of taking particular actions, including the behavioural response to policy initiatives.
Notes
1. Hamilton used both population and employment density gradients that Macauley (Citation1982) updated from Mills’s (Citation1972) results to estimate the negative exponential density gradients. In his study, the mean optimum commute was defined as the difference between “mean distance of households from the CBD” and “mean distance of jobs from the CBD”. This mean optimum commute was compared with the mean actual commute to calculate the amount of excess commuting.
2. Hitchcock (Citation1941) was the first to develop this optimization model, which is known as the transportation problem.
3. In terms of evaluating a theoretical minimum commute, the first example of the application of the linear‐programming approach can be seen in Hamburg et al. (Citation1965). They introduce a measure called an “index of indifference”, which assesses the extent to which commuters are indifferent to travel time. In their study, indifference to travel time was reflected by the ‘probable’ distribution in which the friction of space measured in time units were zero. This index is calculated by the proportion of the difference between actual and minimum commutes compared with the difference between the probable and minimum commutes. Note that both excess commuting and indifference index measures focused on how far actual commute deviates from the minimized commute.
4. The monocentric model is based on a theory of individual household location choice, which is an extension of consumer‐behaviour theory. The residential bid‐rent function is obtained by assuming that each individual household tries to maximize their satisfaction with respect to the consumption of housing and commuting.
5. They calculated excess commuting using household utility constraints that no household’s utility is lowered during the assignment process.
6. Their case is related to boundary expansion. However, the size of the study area boundaries could decrease, in which case the amount of excess commuting is likely to increase.
7. They argued that previous studies that do not consider changes in inter‐zonal travel times when journey‐to‐work matrices are optimized can underestimate excess commuting. This is because inter‐zonal travel times can be reduced due to reductions in congestion that can be facilitated by shorter trips and changes in modal split.
8. The results also indicated that both linear and quadratic models seem to have rather unreliable intercepts due to too large fixed‐time costs that do not seem to be intuitively plausible.
9. When the excess commuting is investigated in relation to possible environmental effect, the distance measure may be better as energy consumption is more likely to be associated with trip distance. Travel time has a subsidiary importance as it is related to the speed that affects fuel consumption (Banister, Citation2005).
10. In a complete monocentric structure, the only way to reduce the actual average commute is to move workers closer to the urban centre. However, note that when using micro‐data and door‐to‐door travel cots, minimum commutes will never be zero unless individuals work from home.