COVID-19 threatens the world . Social distancing is a significant factor in determining the spread of this disease, and social distancing is strongly affected by the local travel behaviour of people in large cities . In this study, we analysed the changes in the local travel behaviour of various population groups in Hong Kong, between 1 January and 31 March 2020, by using second-by-second smartcard data obtained from the Mass Transit Railway Corporation (MTRC) system . Due to the pandemic, local travel volume decreased by 43% , 49% and 59% during weekdays, Saturdays and Sundays, respectively . The local travel volumes of adults, children, students and senior citizens decreased by 42% , 86% , 73% and 48%, respectively . The local travel behaviour changes for adults and seniors between non-pandemic and pandemic times were greater than those between weekdays and weekends . The opposite was true for children and students . During the pandemic, the daily commute flow decreased by 42% . Local trips to shopping areas, amusement areas and borders decreased by 42% , 81% and 99%, respectively . The effective reproduction number (Rt) of COVID-19 had the strongest association with daily population use of the MTR 7-8 days earlier.