## 6 Conclusions  
The purpose of this study was to examine whether the commuting time is stable amid substantial changes to transportation and land use systems using the Portland, Oregon, region as a case study. We overcame two issues of previous studies by using consistent data and by combining aggregate and disaggregate analyses. The results of this study indicate that the commuting time increased and that there were large changes in mode share and travel distance from 1994 to 2011. Thus, the evidence from this study does not support the \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Mode**} & **1994 CBD vs.** & **2011 CBD vs.** & **1994 CBD vs.** & **1994 Non-CBD vs.** \\  & **1994 Non-CBD** & **2011 Non-CBD** & **2011 CBD** & **2011 Non-CBD** \\ \hline Driving & 0.99 & \(-10.64\) * & 21.17 * & 20.18 * \\ Passenger & 3.76 & 11.62 & 6.34 & 2.57 * \\ Bus & \(-7.14\) * & 0.17 & 3.46 & 10.60 * \\ Rail & \(-25.14\) & \(-6.62\) & \(-22.65\) & 2.49 \\ Walk & \(-0.08\) & 0.37 & 0.93 & 1.00 * \\ Bike & \(-0.33\) & \(-1.55\) & 4.29 & 4.62 * \\ \hline \hline \end{tabular} Notes. * indicates \(p\leq 0.05\).  
\end{table} 
Table 8: Z-test for the speed (mile/hour) difference by mode. constant commuting time hypothesis [2; 4] or the TTB concept [11; 29]. The results of this study suggest that the \"commuting paradox\" or \"travel time budget\" is now less influential than it used to be. Traditionally, these hypotheses posit that individuals tend to maintain a relatively constant travel time budget, meaning that as the commuting distances increase, people adjust their travel behavior to keep their overall commuting time stable. However, our research indicates that this trend may be shifting where the constraints of the travel time budget appear to be loosening. In addition, this study provides an additional in-depth understanding of the increased commuting and its influencing factors after decades of stability. The growth in the mode share of non-driving modes coupled with the longer average travel time of non-driving modes contributed to the increase in commuting time. The increase in the non-driving mode share was partially attributed to the improvement in the transit system, especially the expansion of the rail system. These results contradict previous research that claims that transit does not contribute to an increased commuting time because it only accounts for a very small proportion of commuting trips. Moreover, the evidence of the Portland case indicates that continuous investment in transit and bike infrastructure can help reduce private vehicle dependence and strengthen the role of the CBD as an employment center. The commuting patterns are a function of many factors, including, but not limited to, socioeconomic characteristics, social norms, transportation systems, and built environment. Between 1994 and 2011, the Portland region went through substantial changes to its land use and transportation system, as well as experienced population and economic development. The differences in commuting patterns between 1994 and 2011 represent a composite change in travel behavior resulting from changes in socioeconomic characteristics, social norms, and transportation systems. Our analysis shows that trip-related and socioeconomic characteristics explain a large portion of the variation in commuting time. The effect of trip-related and socioeconomic characteristics on the commuting time was significantly different between 1994 and 2011. In addition, the results show that the overall speed increased during the period between the two years, though there was a common perception of worsening traffic congestion. The results of our analysis provide empirical evidence for long-term commuting trends amid substantial changes to transportation and land use systems, which will help policymakers devise policies and optimize investments to improve transportation and land use systems.