When ‘push’ does not come to ‘shove’: Revisiting ‘faster is slower’ in collective egress of human crowds

Highlights

• The study empirically re-examines the faster-is-slower assumption for varying exit widths.

• In the absence of aggressive pushing, faster was invariably faster regardless of exit width.

• Faster is faster, so long as ‘pushing’ does not transition to ‘explicit forceful shoving’.

• At the highest level of competitiveness, crowd density amounted to 9 ped/m².

• A ‘non-linear’ relation between door width and flowrate was observed.

Abstract

We revisit the assumption stating that greater levels of rush in pedestrians’ collective egress through narrow bottlenecks impedes the discharge process and makes it slower, commonly known as the ‘faster-is-slower’ phenomenon. The question is of great practical significance because it ultimately can translate into whether crowds of evacuees should be dissuaded from rushing at bottlenecks in order to minimise their evacuation time. Yet, there is a large mixture of evidence on this phenomenon in the existing literature. Here, we re-examine this assumption based on empirical tests with an aim to identify explanations for these discrepancies. Our experiments were conducted with a crowd of 114 individuals, under varying doorway widths (ranging from 60 cm to 120 cm) and under three different levels of (non-aggressive) rush/competitiveness. Under our most competitive condition, crowd density behind the exit frequently exceeded 8 ped/m² and even reached 9 ped/m², as may be observed in a real case of egress under severe time constraint. This elevated level of crowd pressure and competitiveness, however, never translated in slower egress even for the narrowest exit. Based on every relevant measure of movement efficiency and regardless of the door width, faster was invariably faster. Discharge rates were larger, time headways between successive exits were smaller and evacuation times were shorter when individuals pushed more intensely (compared to more orderly types of conduct). We also observed that pedestrians exited in bursts and that the burst sizes were bigger under the greater levels of rush. Overall, all measurements indicated that for moderately large crowds, as long as the competitiveness does not amount to dangerous physical pressure, and as long as individuals do not display ‘explicit’ or ‘aggressive’ forms of pushing (i.e. as long as ‘push’ does not come to ‘shove’), rushing per se does not prolong the discharge process, rather it shortens the collective discharge. A contrast between these observations and previous experiments in earlier studies indicates that the presence or absence of ‘explicit’ or ‘aggressive’ shoving in the crowd could possibly be a major determinant of faster being slower or faster. This study suggests that the ‘faster-is-slower’ term, although very common in the literature, might be an overly simplistic terminology that does not offer adequate neuance for describing a rather complex phenomenon. To determine whether faster is slower or faster, one may need to break the question down into more details and view it through context-specific influential factors such as ‘the nature of pushing’, ‘the size of the crowd behind the bottleneck’, or 'the physical characteristics of the door'. We particularly suggest that another possible factor in determining when faster is slower or faster could potentially be the ‘crowd size’, a dimension that could be systematically investigated by future studies.

Introduction

Prediction and management of pedestrian flows through narrow doorways is an important aspect of crowd control and evacuation management. Narrow doorways are common geometric features of pedestrian and public facilities such as railway station gates, stadium gates, train gates etc (Davidich et al., 2013, Fernández et al., 2015, Seriani and Fernandez, 2015, Seriani and Fujiyama, 2018, Seriani et al., 2017) where crowds may congregate at high densities (Hänseler et al., 2017, Hughes, 2002, Shahhoseini and Sarvi, 2019, Shahhoseini et al., 2017). Therefore, accurate estimation of the egress and ingress flows through doorways and bottlenecks is a crucial aspect of crowd modelling (Boltes et al., 2013, Seriani et al., 2016) along with other modelling dimensions such as walking behaviour (Antonini et al., 2006, Flötteröd and Lämmel, 2015, Robin et al., 2009), exit direction choice (Ehtamo et al., 2010, Haghani and Sarvi, 2017, Haghani and Sarvi, 2018b, Haghani and Sarvi, 2019a, Huang and Guo, 2008, Lo et al., 2006) and pathfinding (Crociani et al., 2016, Daamen et al., 2005, Guo et al., 2012, Haghani and Sarvi, 2016, Hoogendoorn and Bovy, 2004, Kim et al., 2015). Determination of egress throughout rate at bottleneck is also one of the most important elements of evacuation time estimation (Ezaki et al., 2012, Shahhoseini and Sarvi, 2018, Yanagisawa et al., 2009a, Yanagisawa et al., 2009b, Yanagisawa and Nishinari, 2007).

The collective exit of human crowds through a narrow bottleneck is one of the most studied topics in pedestrian crowd dynamics (Daamen and Hoogendoorn, 2012a, Garcimartín et al., 2017, Garcimartín et al., 2014, Guo, 2014, Hoogendoorn and Daamen, 2005b, Nicolas et al., 2017, Oh and Park, 2017, Seyfried et al., 2010, Seyfried et al., 2009, Tanimoto et al., 2010, Tobias et al., 2006, Wang et al., 2015, Zuriguel et al., 2014). Despite the great wealth of research dedicated to this topic, however, a major aspect of pedestrian flow through a bottleneck has remained controversial, in that, there is a mixture of evidence about this question in the literature. The question is whether the efficiency of crowd escape through narrow doorways drop as a result of pedestrians’ rush to exit. This problem has been known as “faster is slower” assumption since the pioneer computational study of Helbing et al. (2000b) suggested based on computer simulations that, contrary to the intuition, when exiting through a narrow doorway, an elevated desire to move faster can cause further delays due to the clogging which reduces the efficiency of the collective discharge. The problem has been studied extensively since using both computational and experimental methods (Gago et al., 2013, Garcimartín et al., 2014, Parisi and Dorso, 2007, Pastor et al., 2015, Soria et al., 2012, Sticco et al., 2017, Suzuno et al., 2013), and has often been cited as a guide to practitioners and managers to discourage people from rushing to exits in cases of emergencies (Fang et al., 2011). It is often treated as a known fact by the media or managing authorities that the crowd will be better off (in minimising their discharge time) if people do not rush to escape in case of an emergency.

This work focuses on the question of ‘competitive egress’ (Hidalgo et al., 2017) through narrow doorways based on an empirical approach. The study is mainly motivated by the mixture of evidence that has emerged in recent years on this question and the fact that the role of ‘exit width’ on this phenomenon has not been systematically explored yet. This topic in general was identified through our recent review of the literature as one of the controversial topics in this field (Haghani and Sarvi, 2018a). The existing mixture of findings will be explained in more details in the following section. We believe that the great practical implications surrounding this topic warrants further examination and testing. The main question at hand is whether higher levels of mere rushing and pushing for exit through a narrow doorway (without any explicitly aggressive or destructive pushing) hinders the process of collective egress by prolonging it. Secondly, we aim to know whether the direction of this effect is dependent on the width of exit. Our hypothesis in designing this research was that the occurrence of the faster is slower may be limited to a certain range of exit width and that for wider exits, the effect might disappear. So, the question was how narrow the exit should be in order to observe the faster-is-slower effect. And our aim was to identify the possible borderline in terms of the exit width at which faster becomes faster as opposed to slower. The answer is crucial in determining whether crowds of evacuees should be deterred to rush at exit points in order to achieve a minimum egress time. Compared to the previous body of work on this topic that has predominantly used numerical techniques or experimental techniques with non-human crowds, we perform empirical laboratory tests with crowds of humans. And compared to the previously existing experiment of this question using human crowds (Garcimartín et al., 2017), for the first time, we systematically investigate the (potential) moderating effect of the exit width by examining varying widths ranging from 60 cm to 120 cm. Empirical tests reported in this work are performed with a crowd size of 114 individuals that is marginally larger than the counterpart comparable experiment that has previously been reported on this topic (Garcimartín et al., 2016, Garcimartín et al., 2017, Garcimartín et al., 2014, Pastor et al., 2015).