Transportation is a major driver of the urban heat island (UHI) effect, significantly contributing to higher urban temperatures through vehicle emissions and heat generated by traffic congestion. Our work aims to quantify the impact of transportation on the UHI effect using thermal satellite data and in situ meteorological measurements. The research focuses on the San Francisco Bay Area and Greater Los Angeles, analyzing MODIS data in the morning and afternoon during equivalent periods in 2019 and 2020. Additionally, in situ meteorological data aid in constructing urban surface energy budgets and developing statistical models linking solar radiation to heat intensity. The study investigates the impact of reductions in traffic flow on UHI using the period of recommended and mandated movement restrictions by California during COVID-19 as a natural experiment. The implementation of movement restrictions (lockdown) corresponds to a 0.068 °C and 0.072 °C decrease in mean morning and afternoon temperatures, respectively, for Los Angeles with smaller decreases in surrounding counties. Northern California showed more mixed results, which could be due to the specific geomorphology and microclimates found in the Bay Area. Our findings provide valuable insights into the relationship between human mobility, urban thermal dynamics, and climate variability.