Article citation information:
Gill, A.,
Smoczyński, P., Ławniczak, D. Measuring the
variability of the pedestrian crossing function in the socio-technical system
of urban road transport. Scientific
Journal of Silesian University of Technology. Series Transport. 2022, 117, 57-68. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2022.117.4.
Adrian GILL[1], Piotr SMOCZYŃSKI[2], Damian ŁAWNICZAK[3]
MEASURING THE VARIABILITY OF THE PEDESTRIAN CROSSING FUNCTION IN THE
SOCIO-TECHNICAL SYSTEM OF URBAN ROAD TRANSPORT
Summary. In some areas
of transportation systems, reduction of risk using typical safety engineering
tools can be difficult due to the relatively small number of events that can be
analysed to draw conclusions for the future. One way out of this situation is
to analyse systems in their normal operation when no adverse event occurs. It
can be done, inter alia, with the Functional Resonance Analysis Method. An
important research problem in this context is how to describe the variability
of system functions. In this article, we propose an original method, based on
the number of hazard sources present in a given analysis domain and apply it to
a real pedestrian crossing. The obtained results indicate that the quantitative
coincidence measures proposed by us are a convenient way to capture
‘functional vibrations’ in real socio-technical systems. This
allows the prediction of undesired states of such systems based on their normal
operation.
Keywords: pedestrian
crossing, hazard sources, public urban transport, Functional Resonance Analysis
Method
1. INTRODUCTION
Intuitively,
the traditional three-colour traffic lights remain the best form of crossing
protection that practically eliminates the possibility of activating hazards. A
similar phenomenon can be seen concerning railway level crossings equipped with
barriers [11], although in both cases –
despite advanced safety systems – there are still many hazard sources
left. One of the most important of these sources was indicated by Krukowicz et al. [10].
Road traffic observations conducted by them in a large city in Poland made it
possible to formulate an observation indicating that despite the improvement of
traffic organisation and modernisation of traffic lights at intersections, a
large number of road accidents and collisions are caused by inappropriate,
often illegal behaviour of road users. A detailed literature
review on the assessment of pedestrian-vehicle interaction on urban roads has
been presented by Thakur and Biswas [19].
Problems in
raising an already high level of safety are noticeable in many areas of human
activity, including in rail transport [2, 14]. This is an example of a
controller paradox [21],
quoted in [4]), whose task is to minimise
system variability; however, this variability is also the only way to measure
the effectiveness of this controller. In such situations, it is now proposed to
abandon the use of traditional methods of safety engineering (FTA, FMEA) for
the benefit of new ones, allowing a comprehensive description of the system
during its operation – both when everything goes fine as well as in the
event of hazard activation.
Examples of
new methods include the Systems-Theoretic Accident Model and Processes (STAMP),
proposed by Leveson [12] and used among others for
modelling maritime [8] or railway transport systems [20], where, however, its theoretical
character was pointed out. STAMP is also used as an enhancement of the Event
Analysis of Systemic Teamwork (EAST) method [16], which was used independently,
among others, in studying the behaviour of road traffic participants during
crossing intersections [15].
Another
popular method of the ‘new approach’ (often referred to as
‘Safety-II’) is the Functional Resonance Analysis Method (FRAM)
proposed by Hollnagel [5], and still being developed [4]. The use of the FRAM method is
shown in the example of air [18] or maritime transport [13]. However, there are no
applications in urban transport [17]. Furthermore, the existing
publications focus primarily on modelling using characteristic hexagons rather
than on the attempts to define how ‘functional vibrations’ manifest
themselves in real socio-technical systems.
This article
aims to propose an original understanding of functional vibrations for the
pedestrian crossing function, as well as to determine their waveform based on
our observation of a selected real pedestrian crossing in Poznan (Poland).
Section 2 presents the necessary theoretical information on functional
vibrations and functional resonance, as well as the applied research
methodology for determining the pedestrian crossing function vibrations. While
section 3 discusses the results of the observation study and shows how to apply
these results for determining functional vibrations. Finally, section 4 contains
conclusions and directions for further research.
2.
MATERIALS AND METHODS
2.1.
Functional vibrations
The concept of
functional vibration is a key element of the Functional Resonance Analysis
Method (FRAM) used for modelling socio-technical systems [5]. An important and sometimes
overlooked aspect of the theory behind FRAM is the mere phenomenon of
functional resonance, explaining the mechanism of activating hazards. According
to this theory, adverse events occur not as a result of breaking (intentional
or accidental) applicable procedures and specifications of the system operation
but as a result of the unfavourable superposition of the functions performed in
it. This is well illustrated by the diagram shown in Figure 1.
Fig. 1. Mechanism
of hazard activation according to
the functional resonance theory [17], based on
[6]
In the theory
of functional resonance (Figure 1), it is assumed that the result of the
system's operation is the composition (superposition) of its functions. The
superposition is variable over time but usually remains below a certain limit,
which exceeding results in the activation of a hazard (an adverse event, an
accident). Such a view on the work of socio-technical systems allows them to be
improved also when there are no adverse events. Thus, one should examine the
various ways of correct system operation – the variability, as Hollnagel
calls it – and undertake actions aimed at limiting this variability.
In this study,
we suggest that variability is described using the hazard sources that appear
during the implementation of a given function. A hazard source (also called 'a
risk source') is an element, which alone or in combination with other elements,
has the potential to give rise to (typically) undesirable consequences [1, 7]. A
broader discussion on the motivation to choose such a definition was presented
in [3]. The most important is that two
or more hazard sources may not be harmful; however, when combined they interact
to become dangerous [9]. This is graphically depicted in
Figure 2.
We, therefore,
propose that variability
where V
is the value of variability determined in the observation time interval
Fig. 2. Schematic representation of
the relation between the hazard source (HS)
and the hazard (H), as considered in this study
Describing
variability using Equation (1) allows differentiating between two levels of
superposition. First, there can be many hazard sources occurring during the
performing of one function of a socio-technical system, and their number is
variable over time. The variability of a particular function will therefore be
given by the superposition of the number of occurrences of individual hazard
sources. Second, in the context of performing all functions by a
socio-technical system, the superposition concerns variations coming from all
its functions – and this superposition, following the FRAM assumptions,
determines the possibility of having an adverse event.
2.2. Selection
of case study crossing
To carry out
the variation measurement, we have selected a pedestrian crossing equipped with
traffic lights located near two significant pedestrian traffic generators
(Figure 3). The first of them is the campus of the Poznan University of
Technology, used by approx. 20,000 students. In the immediate vicinity of the
crossing, both the most important didactic buildings and dormitories are
located. The second generator is Posnania, one of the largest shopping centres
in Poland. In addition, near the pedestrian crossing, there is an intersection
of two tram routes. This makes the tram stop located at the crossing to be
often used by passengers who make transfers.
The crossing
leads through two roadways and two tram tracks between them. The street is a
fragment of the second communication frame, that is, the bypass of the city
centre. The western roadway has two lanes, the eastern – four lanes,
including one for turning into the shopping centre. The maximum valid speed for
all roadways is 50 km/h. Three sets of traffic lights are installed at all
parts of the crossing: two on the roadway crossing, and one on the tram track
crossing. The condition of the roadway surface and tracks were assessed as
good, not interfering in any way with the movement of vehicles. Visibility is
very good, as there are no buildings or advertisements near the crossing that
could limit it. Within the crossing, there is street and tram stop lighting
that is sufficient to light the roadways as well.
To facilitate
the collection of observational data at the pedestrian crossing, it was divided
into two zones (Figure 4). Zone I includes the crossing through the tram track
and the adjacent tram stop Kórnicka. While Zone II covers the rest of
the pedestrian crossing, that is, the roadways in both directions.