Does road investment reduce average travel time and travel time variability?: Empirical evidence from Japan

This study empirically examines the impact of road investment on travel time and its variability in a local Japanese city, where a new bypass was introduced parallel to an existing national highway. The average treatment effects of the bypass were estimated using propensity score matching. Travel time was defined as the daily average route-level travel time between origin and destination zones, derived from global positioning system (GPS)–based journey records. Travel time variability was measured using the day-to-day standard deviation, planning time, and buffer time of route-level travel times. The dataset includes 144 million trips observed before the bypass opening and 167 million after. To assess robustness, the effects were estimated across 18 cases with different data-screening strategies. Results consistently show statistically significant reductions in travel time and planning time. On average, travel times for the treatment group were 1.02–2.75 min lower than those of the control group, while planning time decreased by 0.55–3.21 min. Effects on travel time variability measured by standard deviation and buffer time were less robust. The findings highlight clear travel time savings from the bypass and the importance of accounting for temporal data coverage when measuring travel time variability.