Transportation infrastructure networks (TINs), and the accessibility that they provide, are argued to be major determinants of land-use change. However, the spatio-temporal extent of their impact on urbanization is unclear and calls for further investigation. This paper empirically investigates the impact of both road and transit networks on urbanization in the Greater Toronto-Hamilton Area (GTHA) from 1971 to 2016, using a traffic analysis zone (TAZ) level panel database of 17,160 observations. It first documents and measures the magnitude, location and rate of urbanization and TINs’ development in the region and in relation to each other using descriptive and buffer analyses. It then estimates the share of transport accessibility in urbanization over time and space using fixed effects panel regression models. Our findings demonstrate the significant extent of the ongoing sprawl, the substantial historical and current difference between the potential transit and car travel times, and the stability of the potential car travel times despite improvements to the road network. They also highlight the importance of urban proximity in prompting urbanization and the decreasing impact of transit improvement on urban development in urban areas and over time. A policy implication is that to encourage transit-oriented development, it is more effective to introduce or reinforce connections to activity centers rather than invest in reducing travel times to reach them.