## 2 Consequence of Not Using Crowd Management Measures at the PTI  
### Reduced Accessibility Conditions: Mind the Gap and Yellow Safety Line  
PST is highly sensitive to accessibility conditions at the PTI. Among these, vertical and horizontal gaps--i.e., the height and distance between the train and platform--are critical for determining whether passengers with mobility aids can board and alight efficiently and safely. Gap dimensions directly affect the ease with which passengers board and alight, thereby influencing total PST. The literature consistently reports that larger vertical and horizontal gaps increase the PST, particularly for passengers with reduced mobility. According to [PERSON] et al. [19], a gap of 2 cm (horizontal) \(\times\) 2 cm (vertical) is universally accessible for passengers with mobility aids, while gaps of 10 cm \(\times\) 10 cm render the PTI nearly inaccessible to most such users [20]. Several laboratory experiments cited in the literature review provide empirical evidence on the effect of gap sizes. For instance, experiments found that increasing the vertical gap from 5 cm to 15 cm reduced the alighting time by 9%, although this effect was highly dependent on door width and bidirectional flow dynamics [21]. [PERSON] and [PERSON] [22] also found that reducing the vertical gap could decrease PST by approximately 8%. Accessibility varies by mobility aid. For example, while passengers using manual wheelchairs were able to board in a 2 cm \(\times\) 2 cm scenario with relative ease, increasing the gaps to 5 cm \(\times\) 7 cm resulted in failures or high levels of discomfort for most users. Those using electric-powered wheelchairs encountered even more difficulty due to their inability to perform maneuvers such as wheelies [20]. Similarly, pushed wheelchair users were entirely dependent on assistants and could only manage smaller gaps (e.g., 5 cm \(\times\) 3 cm) with great effort. Therefore, different ranges can be recommended. For instance, a 2 cm \(\times\) 2 cm scenario--considered the \"desirable gap\"--enabled nearly all participants to board and alight independently. A secondary acceptable range is 5 cm \(\times\) 5 cm, although this caused discomfort or difficulty in a notable number of cases. When gaps reached or exceeded 10 cm in both dimensions, all participants reported either failure or serious difficulty, resulting in the lowest accessibility conditions [20]. The yellow safety line at the PTI is a proxy for orderly boarding and alighting behavior, which is closely tied to PST. Experimental results showed that an adhesive tape line of 5 cm width had the lowest compliance score, suggesting more erratic and risky positioningof passengers near the platform edge, leading to possible boarding conflicts and slower alighting. On the other hand, a tactile pavement material line of 40 cm width showed the highest compliance, suggesting that passengers maintained better spatial behavior and allowed smoother passenger flow [23]. Higher compliance helps reduce interference between boarding and alighting flows, which is one of the main causes of prolonged PST in crowded metro stations. In addition, participants with reduced mobility had different experiences depending on the yellow line design. For instance, wheelchair users preferred the adhesive tape (5 cm width) due to easier traversal, despite its lower safety compliance, while elderly participants with hemiparesis reported higher comfort and security with the case in which the line had a width of 24 cm and 40 cm, indicating a potential trade-off between comfort of movement and visual/spatial safety [23]. This suggests that PST is not only affected by the behavior of the general population but also by the usability of accessibility elements for specific groups, requiring universal design strategies. However, field observations of the Santiago and Valparaiso Metros reinforced the experimental data: stations with wider yellow lines and tactile pavement saw more orderly boarding patterns and greater respect for platform edge zones.