While these new trends in trip making behavior point towards a sustained reduction in public transport demand, they do not necessarily mean a reduction in transit modal share. New evidence seems to suggest that there has been a significant reduction in the modal share of transit in response to COVID-19 (Apple 2021; Orro _et al.,_ 2020). This shift can be attributed, at least in part, to negative passenger perceptions of the potential health risks of riding transit (Przybylowski _et al.,_ 2021). If these negative perceptions are not properly addressed during the transition stage, the reduction in the modal share of transit may be sustained for an extended period in the aftermath of the pandemic (Przybylowski _et al.,_ 2021; Tirachini and Cats, 2020). However, there is no analytical evidence of whether intermodal competition has taken place.

Aversion to crowded public transport is likely to push more journeys onto private road-based transport, with related issues for road congestion. Traffic congestion is a perpetual problem for the sustainability of transportation development. Traffic congestion causes delays, inconvenience, and economic losses to drivers, as well as air pollution. Identification and quantification of traffic congestion are crucial for decision-makers to initiate mitigation strategies to improve the overall transportation system’s sustainability. Road traffic is in fact considered as one of the main sources of air pollution in urban environments (Chowdhury _et al._ , 2021, Hamra _et al._ , 2015). In particular, among other pollutants, internal combustion engines emit nitrogen oxides like NO and NOx (mainly from diesel engines), which generate NO2 (Taquechel _et al._ , 2020) and particulate matter (Manisalidis _et al._ , 2020) such as PM10, which also originates through mechanical processes. Both primary and secondary pollutants are transported far from roads, thus worsening the air quality of whole urban areas. In addition, transport externalities soundly affect not only the environment, but also the society and economy of cities. For all these reasons, the United Nations included the creation of sustainable cities and communities as one of the 17 Sustainable Development Goals in agenda 2030, in which sustainable transport is one of the themes of several targets and goals.

Generally, studies have found that reductions in air pollution during the pandemic were most prominent for NO2, which is notably influenced by car traffic. While many other environmental indicators of air pollution (e.g., PM2.5, O3, CO) have been tied to adverse health outcomes (see Manisalidis _et al._ , 2020 for a review), a growing amount of evidence has specifically found adverse health effects from NO2 pollution. Rossi _et al._ (2020) indicated that vehicle flows significantly affect NO, NO2, and NOx concentrations, although no evidence of a relationship between traffic and PM10 was highlighted. According to this perspective, measures to limit traffic flows seem to be effective in improving air quality only in terms of reducing nitrogen oxide. Aletta _et al._ (2020) presented the results of a simulation approach using Floating Car Data (FCD) from vehicles equipped with an On-Board Unit GPS system. During the lockdown period, the traffic in Rome (Italy) was reduced by 64.6%, which had a positive impact on noise emissions reduction. On a broader scale, Chen et al. (2020) revealed that reductions in NO2 and CO were common but heterogeneous across the United States, after adjusting for temporal trends. Another USbased study by Berman & Ebisu (2020) found similar results at the county level; using two-sample aggregated 3