Monitoring public transport crowding exposure: Stockholm before, during, and after the COVID-19 pandemic

Crowding exposure in public transport vehicles has a serious negative impact on passengers’ travel experience. Especially during the COVID-19 pandemic, exposure to crowded conditions may also increase the risk of virus transmission among passengers. To mitigate the negative impact of crowding on public transport systems, there is an increasing need to understand how crowding exposure is distributed across the service area and passenger groups, and how it changes over time. This paper provides a methodology for monitoring the equity of crowding exposure over time using longitudinal smart card data. An objective measure is proposed to quantify crowding exposure: relative crowded travel time (rCTT). We apply Lorenz curves, Gini and Suits coefficients to assess horizontal equity (across the population) and vertical equity (considering income). In our case study of the Stockholm Region, we demonstrate our method by assessing the equity of crowding exposure during different stages of the COVID-19 pandemic: pre-COVID, COVID, and post-COVID. Our findings show that the pandemic negatively impacted both horizontal and vertical equity. During the pandemic, crowding exposure became increasingly uneven across the service area. While overall ridership and crowding exposure declined during the pandemic, reductions were not uniform across social groups. Lower-income riders showed smaller decreases in travel compared to higher-income riders, resulting in greater crowding exposure among disadvantaged groups and a shift from a progressive to a regressive distribution. These findings reinforce the importance of continued monitoring of crowding exposure, especially as travel behavior and policy contexts continue to evolve. The proposed framework can help identify and target the most critical equity gaps, enabling more focused and effective interventions.with lower-income travelers experiencing more crowding than their higher-income counterparts. However, by the post-COVID stage, the equity of crowding exposure has nearly returned to pre-COVID levels.