## 2 Literature Review
The findings were prone to error, because there were differences in travel data collection between the two travel surveys, as non-motorized non-work trips were excluded in the 1968 travel survey while they were included in the 1988 travel survey [11]. Another line of studies focuses on changes in commuting time and factors explaining the changes. These studies are inspired by the urban spatial structure theory and attempt to investigate the impact of changes in urban structure on commuting time. On one hand, urban growth increases traffic congestion and commuting distance to the city center and thus leads to a rise in commuting time. On the other hand, the suburbanization of jobs and housing decreases the commuting distance, and traffic in suburban areas is not subject to traffic congestion. Nationwide studies that used travel surveys in the early 1990s or before support the constant commuting time hypothesis. [PERSON] et al. [2] compared the auto commuting time between 1980 and 1985 in the largest twenty metropolitan areas. The results showed that the average auto commuting time decreased in eighteen of these twenty areas and kept constant in the remaining years between 1980 and 1985. The authors called this the \"Commuting Paradox\", which meant that commuting did not increase given increasing traffic congestion because of location adjustment by households and firms. [PERSON] [26] used Nationwide Personal Transportation Survey data to investigate the journey to work between 1977 and 1995. The results showed that the commuting time remained stable, because people adjusted their residential locations to maintain a reasonable commuting time, and the firms followed the labor force. These location adjustments yielded an apparent overall constant commuting time. They pointed out that location adjustment might be the cause for this phenomenon. [PERSON] and [PERSON] [27] investigated the impact of employment decentralization on commuting time by industry in major U.S. metropolitan areas with individual-level panels between 1985 and 1997. They found that employment suburbanization led to a decrease in commuting time, and the effect of employment suburbanization on commuting time varied by industry type. However, metropolitan studies, which also used travel surveys conducted in the early 1990s or before, indicate that there was an increase in commuting time. [PERSON] and [PERSON] [7] examined the impact of urban spatial restructuring on commuting in Los Angeles metropolitan areas. They found that a polycentric urban form did not lead to a shorter commuting travel time and attributed the increase in commuting time to increased income and private vehicle dependence. [PERSON] and [PERSON] [6] investigated the commuting time between 1980 and 1990 in the San Francisco Bay Area. The results showed that there was no association between a shorter commuting time and employment decentralization and sub-centering. In the late 1990s, a marked increase in commuting time was also observed in nationwide studies [8]. The nationwide average commuting time increased by 2.2 min in the 1990s after excluding the increase resulting from data collection issues. [PERSON] et al. [8] investigated the factors affecting the average commuting time. They found that demographic and transportation variables only explained a relatively small portion of the increase in commuting time and that a rise in income mainly contributed to the increase in the average commuting time. To summarize, the nationwide studies that used travel surveys conducted in the early 1990s or before support the constant commuting time hypothesis, while the metropolitan studies that used travel surveys conducted during the same period found an increase in commuting time. This might indicate that the nationwide studies ignored the variation within metropolitan areas. However, a marked increase in commuting time was also observed by nationwide studies in the late 1990s.