This study investigates the localized impacts of highway capacity expansions by analyzing the change in vehicle miles traveled (VMT) levels on the California State Route 4 (SR-4) corridor in the California Department of Transportation (Caltrans) District 4. Using VMT data from the California Performance Measurement System (PeMS), a generalized least squares (GLS) regression model was used to conduct an interrupted time series analysis to examine pre- and post-expansion VMT levels while also incorporating the Covid-19 pandemic to address the disruptions in travel behavior during the study period.

Since I used interrupted time series methodology, it was possible to determine that VMT levels increased due to the capacity expansion on SR-4, suggesting that the added capacity induced travel. The cause of induced travel can stem from expanded or increased capacity on a roadway, which lowers the perceived cost of driving, inducing additional trips, longer trip distances, and mode shifts (Volker & Handy, 2022). As the scope of induced travel is still being explored by other researchers, my purpose in this thesis is to contribute to the empirical literature on induced travel by answering two key questions. The first is the primary guide for this thesis, which is understanding if highway capacity expansions still create induced travel even in regions experiencing a decline in total VMT. I answer this by comparing the SR-4 VMT levels with the broader regional VMT trendlines to get a comprehensive look at the differing VMT trends in this facility compared to regional levels.

This thesis also investigates the magnitude of induced travel based on the different lane types added to the SR-4 facility, as the westbound received a general-purpose lane and the eastbound gained an HOV lane. Using the GLS model, I found that the westbound direction showed evidence of induced travel through a statistically significant 0.21% increase in VMT upon activating the new general-purpose lane sensors. The eastbound direction, which received a new high-occupancy vehicle (HOV) lane, showed a statistically insignificant post-construction effect on VMT levels, reflecting a stagnation in VMT levels.These results support induced travel demand theory by showing that general-purpose lanes may induce travel demand through increased VMT, as seen on SR-4 westbound. As for the eastbound direction, the stabilization of VMT levels may indicate that the managed lane suppresses a decline in VMT levels, mismatching the broader District 4 region. The difference becomes more apparent as the VMT levels in District 4 have declined since the COVID-19 pandemic began, unlike the SR-4 corridor, where VMT has stabilized and even increased post-expansion. Unraveling this mismatch is how this analysis will answer the primary question of this thesis.

Furthermore, by conducting separate directional estimations, the results will help determine the extent of each lane type's influence on VMT levels. Providing valuable empirical results on how each lane type impacts VMT levels post-expansion and contributes to generating induced travel on the SR-4 corridor. Answering these two key questions will contribute to the induced travel literature as we continue to understand and research capacity expansion impacts in a post-COVID-19 world.

A robust body of empirical research demonstrates that as roadway supply increases, vehicle miles traveled (VMT) generally does, too. The evidence is particularly strong with respect to major roadways, like interstate highways (class 1), other freeways and expressways (class 2), and principal arterials (class 3). However, previous literature reviews have found limited empirical evidence as to the relative magnitude of the induced travel effect of expanding minor arterials, collector streets, and local roads. Previous reviews have similarly not reported empirical research on the induced travel effects of other types of roadway facilities, such as auxiliary lanes, ramps, or other types of interchanges. In this project, the authorsconducted a systematic literature review on the induced travel effects of minor arterials, auxiliary lanes, and interchanges (including simple on/off ramps). The authors found that the empirical literature remains limited with respect to auxiliary lanes and interchanges. They found eight studies that include minor arterials in their empirical estimates of induced travel, which collectively indicate that the induced travel elasticity for class 4 minor arterials could be similar to that of class 1-3 facilities. However, none of the studies isolated the induced travel effect from minor arterials specifically. Going forward, the report suggests avenues for future research to help close these research gaps. For example, the authors recommend using case studies of individual roadway expansions to better understand the induced travel effects specific to ramps, interchanges, minor arterials, and auxiliary lanes within specific contexts, especially where larger studies (across multiple facilities, geographies, etc.) have not yet been done. 

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