“…Noteworthy, interpreting user perceptions of crowdedness is a complex problem that has been researched over the years from psychological/behavioural angles (Stokols 1972;Kalb and Keating 1981;Mohd Mahudin, Cox, and Griffiths 2012;Li and Hensher 2013). In a wider context, human perception of numerosity is a factor here with suggested geometric or logarithmic (Weber-Fechner-like) principles (Toyosawa and Kawai 2005;Cicchini, Anobile, and Burr 2016). In the present study, from the point of view of the data assimilation approach, such "measurements" are treated as any other noisy data on crowdedness and the methodology is, in principle, applicable for assimilation of data from vehicle weighing (Frumin 2010;Nielsen et al 2014;Ball 2016), data obtained by counting distinct personal electronic devices (e.g., Schauer, Werner, and Marcus 2014), data available in smartcard-based ticketing systems (e.g., Zhang, Jenelius, and Kottenhoff 2017) or data from automatic passenger counters (APCs, e.g., Pinna, Dalla Chiara, and Deflorio 2010).…”