## 4 Discussion
Thus, in central areas of dense medium-sized cities, where a higher demand for short intra-urban journeys exists, it is necessary to assume that users who have shifted to other non-motorized means of transport will be unlikely to return to UPT (which is not negative in itself), and public efforts should instead focus on attracting those who chose to shift to private vehicles (supposedly temporarily) or to those who have never been users of UPT, which could be more complicated. In this sense, it seems that the most effective measures could be to restrict internal mobility in private vehicles within the cities rather than improving UPT service parameters such as frequency or capacity, especially in areas that already have a good service [64]. By way of contrast, in the case of areas with less developed UPT services, public administrations should focus their policies on offering a more developed, sustainable, comfortable, and reliable transport alternative to all those who currently lack it. This would be one of the most plausible alternatives to increase the UPT share as a whole. An unexpected result, although consistent with the previous ones, is the scarce impact that the average vehicle occupancy seems to have on the recovery of users in this time (Figure 5). Thus, although this factor was decisive for risk perception during the hardest phase of the pandemic, especially among frequent UPT users [65], it does not seem that an increase either in the frequency of the service or in the capacity of the vehicles for the sole purpose of decongesting vehicles has the effect of attracting new users [66]. Therefore, in general, the results of this study seem to indicate that, in central areas with a mature UPT network, public transport users do so more out of personal conviction than due to the lack of other viable alternatives. Thus, in these areas, the improvement of service parameters to increase the attractiveness of the UPT would not be a determining factor to attract new users, and that is the reason why public policies to promote a more sustainable urban transport should mainly their focus on reducing alternatives for private vehicle users. Any study of this nature, essentially exploratory, has many limitations that necessitate great caution when drawing conclusions. The most obvious limitation results from the impossibility to analyze different UPT network alternatives in the same territory. Thus, although the comparison of this study with others carried out in cities of similar or different characteristics has helped to interpret or even question the results obtained here, it will always be from the perspective of two different physical, social, and governance realities, compromising the generalization of the results. For example, it would be necessary to confirm whether the decrease in the number of UPT users in central zones, at least in dense medium-sized cities, results from a shift to non-motorized sustainable means of transport. Moreover, even though studies conducted during the COVID-19 pandemic have revealed the existence of abundant data for analysis, they have also detected numerous shortcomings. Thus, as it happened in this study, the wide-scope data available can usually be disaggregated at the line level only, which does not allow researchers to study individual patterns within them. This issue can be crucial in analyzing long lines serving very heterogeneous territories since, as indicated, it is paramount to differentiate UPT users according to their options when choosing among different mobility alternatives. This type of more specific analysis can only be carried out through more selective and expensive field studies, which can be generalized in the future through the monitoring of the occupancy vehicle levels or the moment at which users leave them.