Abstract A dramatic reduction in traffic demand has been observed during the COVID-19 pandemic, producing noticeable declines in traffic delays, energy consumption, and emissions. This unprecedented event provides us with the chance to investigate how limiting the number of vehicles on the transportation network can contribute to a better environment. This paper quantifies the effects of reduced traffic demand on vehicle delays, fuel consumption, and emission levels. Microscopic simulation was used to model traffic for seven different networks. Our results show that decreased traffic demand contributes significantly to reducing delays and emissions, especially in congested urban areas. The results also show that another important contributing factor is the network configuration. Specifically, networks with lower connectivity and fewer routing alternatives or networks with lower roadway density are more sensitive to traffic demand drops in terms of reducing vehicle delays and emissions.