Fig. 4

Statistical data of (a) population, (b) average annual income, and (c) vehicle in three target cities/counties and the control country from 2005 to 2019. Passenger Car unit PCU (PCU) is a vehicle unit used for expressing highway capacity. A single vehicle, motorcycle, bus, and truck are considered to be 1, 0.3, 2.5, and 2.5 PCU, respectively