As recently as 200 years ago, city dwellers accounted for merely ca. 3% of the population. In 1950, already 30% of the human population lived in cities, whereas in 2018, according to the data published by the United Nations [1
], the share was 55%. In 2020, in Europe, 75% of people lived in urbanized areas, while in Poland, the percentage was 60.05% (according to the World Bank’s data). By 2050, the percentage of the population living in cities in the European Union is expected to increase up to 83.7% (according to the European Commission’s data [2
]). This state of affairs poses bigger and bigger challenges for contemporary cities.
Cities are obligated to provide their residents with the highest possible life quality and most favorable living conditions. This can be done by furnishing them with mobility, social life opportunities, financial freedom, career development opportunities, access to culture, a high level of healthcare, security, access to residential, municipal, and transport infrastructures, independent living conditions, well-being, and a high-quality environment [3
]. Activities aimed at providing city residents with the best living conditions lead to increased volumes of urban freight transport (UFT) to supply them with all the needed goods. In addition to that, it is also necessary to intensify the construction processes to provide residential units as well as commercial, recreational, healthcare, educational, and other facilities, which also requires transport of supplies. Thus, city development requires the intensification of transport processes within its area. However, it is associated with an increase in the negative impacts on the city.
Transport is one of the elements that are indispensable for human existence in cities, contributing to improving life quality [4
]. However, transport activity has negative effects such as congestion [5
], air emissions [6
], noise, and compromised road safety [7
]. Increased UFT volume has a significant impact on the possibilities of sustainable development in urbanized areas [8
]. Deliveries made in cities, being last mile deliveries, constitute the costliest parts of supply chains, with considerable environmental impacts. Freight transport in cities is characterized by the involvement of many entities, short routes, low traveling speed, short time of effective driving, limited space, limited traffic infrastructure compared to the high demand for transport, and inefficiency (low loading factor, empty runs) [9
]. Moreover, urban freight transport depends on local conditions and infrastructure limitations (e.g., unloading areas) and trends such as growing demand for services [10
Therefore, both local authorities and the European Union put more and more pressure on more sustainable transport. The European policy, in that respect, assumes that urban freight transport should be efficient in both economic and environmental terms. These goals are attained by cutting down the emissions and minimizing the transport operators’ and their clients’ costs. These assumptions are specified in various documents, e.g., The White Paper (2011) [11
], Study on Urban Freight Transport (2012) [12
], “The Europe 2020 Strategy: A strategy for smart, sustainable and inclusive growth”, and others. Measures taken towards more sustainable UFT must be systemic and influence all of its elements. Such elements include carried cargoes that create the demand for transport, the vehicles that constitute the supply side, and the infrastructure that creates the context [9
One of the industries that are particularly challenging to sustainable urban freight transport is the construction industry [13
]. Despite such a great interest in the subject of sustainable urban freight transport (SUFT) among scientists, the construction industry in this context is still little explored. This is related to the specificity of construction activities and works, as well as the lack of interest on the part of enterprises in establishing cooperation. The available literature on the subject often emphasizes the specificity of construction supplies, for example, construction works entail the need for deliveries to be made to construction sites that may be located in city centers and no-traffic zones. Additional hindrances ensue from the specific nature of the construction industry, involving numerous subcontractors, various project sizes, or contractors’ priorities [14
]. Deliveries made to construction sites often involve high-tonnage vehicles, they are fragmented rather than optimized, and there are often empty or “less-than-truckload” runs. In view of the above, it seems reasonable to take measures so that construction site deliveries become more effective and efficient. This may significantly reduce the negative impacts on the urban transport system [16
]. Such measures may include a selection of a procurement type that suits the particular conditions [17
] and the application of telematics solutions, such as electric vehicles [18
], Radio-frequency identification (RFID), Enterprise Resource Planning (ERP) [19
], Building Information Modeling (BIM) [20
], as well as appropriate organization of processes at logistic [21
] and consolidation centers [22
]. However, research in this field is not exhaustive, and the obtained results do not allow for an unequivocal determination of the impact.
A construction procurement type is of particular importance, as it is decisive for the way deliveries are organized. Selection of a centralized, dispersed, or mixed procurement type, or deciding whether the procurement should be carried out by logistic organizational units, must be based on the construction project implementation conditions. Each of the variants has its advantages and disadvantages [23
], and it determines whether the transport of construction supplies will be less or more sustainable. The most sustainable is centralized procurement and procurement carried out by logistic organizational units, because they have the greatest possibilities of consolidating and optimizing deliveries, creating rational schedules, and reducing irregularities.
The purpose of this paper is to analyze the construction supplies transport in the city of Szczecin and to identify good practices in that regard, which are applied elsewhere. In Section 2
, the authors analyze the impact of UFT on cities, the principles of SUFT, construction supplies in cities, measures for a more sustainable construction supply organization, good practices in construction site supplies in different countries. Section 3
presents the results of the study. In Section 4
, the authors present the possibilities of streamlining the decision-making processes regarding the implementation of good practices, based on selected examples.
2. Good Practices in Construction Site Supplies in Cities
In order to reduce the negative impacts of construction supplies transportation, steps should be taken toward more sustainable UFT. Such activities include various organizational and telematics solutions. Telematics solutions that affect sustainable urban freight transport connected with construction site supplies may be basically divided into those related to procurement planning (solutions used by construction companies) and those connected directly with transport (used by transport companies and solutions implemented by city authorities). The solutions that enable more rational planning, which translates into decreasing the transport volume, include BIM and ERP systems. BIM is the software used in production and construction data management, which enables the virtual generation of intelligent processes based on 3D models, making it possible for architects, engineers, and construction workers to plan, design, construct, and manage the buildings and infrastructure more effectively. BIM enables the construction of a structure first in the virtual space, which makes it possible to solve any potential problems prior to commencing the actual works. Moreover, BIM supports effective management and handling of the space available for moving and storing the building materials, as well as coordination of the works. Making use of the solution enables planning the demand for materials as well as preparing schedules for orders and deliveries, thus providing more possibilities for consolidation [20
]. ERP systems, in turn, are used in various industries and make it possible for users to better integrate diverse business functions such as, e.g., accounting, finance, human resources, production, or distribution. Due to the specific nature of the construction industry, ERP implementation may be hindered. Nevertheless, it makes it possible for users to optimize the processes, provides better possibilities of control and greater organizational flexibility, facilitates decision-making processes [24
], communicates the information to individual participants of the construction process, and supplies management, helps shorten the project implementation time and reduce the costs [25
]. Both ERP and BIM systems support materials flows in the course of the product life cycle before they are brought to the construction site. Combining them with RFID, Global Positioning System (GPS) or bar codes makes it possible to control transport, distribution to the construction site, or optimization of warehousing [19
Telematics solutions connected directly with transport also include the use of alternative drive vehicles [21
], fleet and freight management support systems, mapping, and vizualising software, automated vehicle access control systems, automated toll collection [26
], driver assistance systems (e.g., predictive cruise control, front assist, blind spot monitor, and others) [27
]. Making use of alternative drive vehicles is in line with measures taken by city authorities in order to establish low- and zero-emission zones. This leads to a reduction in the negative environmental impacts of UFT. Taking advantage of other solutions contributes to more rational delivery planning, increased road safety, shorter journey time, which, in turn, translates into reduced impacts of transport connected with construction site supplies in cities.
The organizational solutions that contribute to more sustainable transport of construction site supplies include solutions allowing for more rational planning and control of supply processes (Construction Logistics Plan (CLP), night deliveries, delivery consolidations) or related to the structure of goods flow (Construction Consolidation Centers (CCC), establishing permanent unloading areas or alternative storage sites). CLP is a document describing a logistics strategy for a construction project at hand. In the construction industry, logistics is often planned on a short-term basis, the process participants hardly cooperate, and the control level is not adequate for such a fragmented supply chain. Procurement focuses on the purchase volumes rather than the ways of production, warehousing, or delivery. This state of affairs leads to various irregularities that exacerbate the negative environmental impacts of the project implementation. Drawing up a CLP makes it possible to marshal the logistics of the construction project by identifying the key elements such as: Access management and journey planning, deliveries, and materials management, demand for transport [28
], construction machinery fleet, unloading, works on local roads and temporary situations, consolidation [29
]. Organizing night deliveries by suppliers makes it possible to shorten the transport time and increase effectiveness. In addition, this is one of the ways to reduce congestion and CO2
emissions. A good practice applied in some countries in the area of construction supplies is consolidation of deliveries [22
]. This concept is nothing new, still, it is effective in reducing the number of deliveries being made. However, it requires a centralized procurement system managed by logistic organizational units or a mixed procurement system. Construction Consolidation Centers are logistics facilities for the classification, consolidation, and delivery of building materials to the construction sites. CCCs make it possible to optimize deliveries and reduce traffic intensity, energy consumption, and emissions. They are aimed at reducing the number of deliveries and increasing transport effectiveness and efficiency. CCCs are built in the vicinity of roads and railway stations to streamline the operation. They can serve one construction project and be liquidated on its completion, or they can operate on a permanent basis and serve more projects. CCCs offer their customers services that create added value, e.g., warehousing services, quality inspections, order picking, and reverse logistics [30
]. They can significantly reduce the negative impacts of construction projects on cities via transhipping materials from high-tonnage vehicles to smaller ones, and cargo consolidations that help limit the number of less-than-truckload (LTL) runs, while assuring more cost-effective vehicle runs. This is confirmed by the results of the SUCCESS project [31
] as well as the results of CCCs operating in seven countries. These were located in Great Britain, Germany, and Sweden [21
]. Currently, the number of CCCs is growing, and their positive impact on logistics process implementation is appreciated in most European countries. They help reduce congestion, allow for more flexibility in deliveries, and offer the possibility of working on a just-in-time basis [31
]. This has been confirmed by the results of operations Royal Seaport Construction Consolidation Centers and Hammarby Sjöstad Logistics Centre in Stockholm [32
] and 12 such centres in London [34
Another good practice seems to be organization of permanent unloading areas, as it enables a reduction in congestion during deliveries. This is not a universal solution, as not each construction site has adequate space for that. Another advantage of unloading areas is a decreased risk of road accidents and cargo damage. Figure 1
presents permanent unloading areas set up in order to facilitate the Eiffel Tower painting in Paris and the construction of the Sagrada Familia in Barcelona.
Permanent unloading areas are often set up via (partial) obstruction of the right-of-way, which may lead to exacerbating the problems related to transport congestion. On the other hand, it has a positive effect on the safety of unprotected road traffic participants. Figure 2
shows the permanent unloading areas next to construction sites in Oslo. An interesting solution is to provide a construction site with several entry gates located in different parts, which makes it possible to better utilize the space available and to supply the cargo to the closest possible place of storage or use.
In the course of construction project implementation, it is necessary to keep stocks of materials directly at the construction site. However, in cities, this is often hindered due to limited space. A good solution in that respect is to establish alternative storage sites for materials. An example of this solution may be floating storage platforms placed on canals. This practice is often used in Amsterdam and Stockholm. An example is presented in Figure 3
. This solution allows reducing the number of completed deliveries, which naturally reduces the negative impact on modern cities.
The identified good practices are highly diverse in terms of conditions needed for their implementation, entities engaged, related costs, and the need to establish legal bases. Non-standardized interviews were conducted with urban freight transport experts from four countries (Poland, The Netherlands, Norway, Spain). The study involved nine experts representing such research and development units as: Amsterdam University of Applied Sciences, Delft University of Technology, Universitat Politècnica de Catalunya, Norwegian University of Science and Technology, Maritime University of Szczecin. Experts were asked to evaluate selected solutions in terms of the difficulty of their implementation. The evaluation criterion was implementation cost, the need to obtain permits/legal requirements, and infrastructure requirements (including transport, technical and IT infrastructure). The assessment is presented in Table 1
Implementation costs for the automated vehicle access control system were set as medium/high. It is related to the entity and place of implementation of the solution. In the case of implementing the solution directly on the construction site, these are high costs in the scale of the companies implementing them. If the solution is implemented by the city, these are medium costs. Legal requirements for the permanent unloading area have been set to low/medium. It depends on where the area is created. In the case of creating such a place directly on the construction site, the legal requirement was set to low. In the case of creating it in the adjacent areas, it was set to medium.
Interviews held with representatives of construction companies have shown that, in general, there is little interest in using solutions that contribute to the more sustainable organization of construction site supply processes. Costs were indicated as the biggest barrier to the implementation of selected solutions. CLP, night deliveries, or consolidation of deliveries are the simplest solutions to implement because they only require enterprises to change their approach to planning. Permanent unloading areas or alternative storage sites, however, also require meeting other conditions related to, among others, infrastructure and the need to obtain permits from authorities. Due to the above, in the further part of the paper, the authors focus on these two solutions.
3. Construction Site Supplies in Szczecin in the Context of Sustainable Urban Freight Transport
3.1. Methodological Assumptions of the Research
The research study described in this paper was carried out in Szczecin, the capital city of the West Pomeranian voivodeship. Szczecin takes up an area of 300.6 sq. km, and it is the largest city in the voivodeship. It is the third biggest city in Poland in terms of area and the seventh biggest in terms of population (source of data: Central Statistical Office). Szczecin features a strategic location close to Poland’s land and maritime borders. The geographical location makes it a vital transport node on a regional and national scale [36
]. Szczecin is a city where intensifying construction processes have been observed in recent years. Their implementation is associated with exerting an impact on the existing transport system. This served as a motivation to put the city in focus.
The starting point of the study was the analysis of the statistical data for 2018–2021. The next step included non-structured observation and surveys regarding construction projects being carried out in Szczecin. The non-structured observation covers the places of construction, processes around the construction site, and the method of deliveries. This allowed for the identification of the most important elements related to the implementation of supplies. Surveys regarding construction projects being carried out in Szczecin allowed for a specific analysis. Construction projects have their own logistics systems, and contractors have to adapt to them. That is why construction enterprises were included in the research, and the projects they carried out were the subject of the research. The questionnaires asked about the details of the implemented construction projects, the type of supplies, the number of deliveries, and the measures used in the field of SUFT. Moreover, PAPI interviews (Paper-and-Pencil Interviewing) were held with selected respondents among experts from the scientific community (9) and practitioners (5). The number of respondents representing practitioners is small due to the low interest of construction companies in cooperation with the scientific sector. The authors of the paper attempted interviews with more companies but did not get any response. Practitioners who took part in the interviews represented 5 construction companies implementing projects in Szczecin as the main contractor or subcontractor. They were people in management positions. Experts from the scientific community represented the countries where the observation was carried out. They are experts in the fields of UFT, SUFT, construction, and city logistics. Most of the respondents (12) were men aged 30–60. The women (2) represented the 30–50 age range. The respondents were asked about the challenges and problems of UFT carried out for construction purposes, methods of transport organization, and existing limitations. This made it possible to diagnose any problems and irregularities connected with construction site supplies within the city. The next stage of the research study included desk research and structured observations of good practices connected with construction site supplies in 5 European countries, followed by an analysis to determine the possibilities of their implementation.
Over recent years, the number of construction companies offering comprehensive services has been growing steadily in Szczecin. At the end of 2015, there were 7764 of them. According to the Central Statistical Office, as of 31 December 2021, the number was 9140, which means a rise of ca. 17.7%. Moreover, the number of all sorts of construction projects has been growing. In 2019–2021, more than 40 construction projects were implemented at the same time. This is confirmed by data published by industry portals (such as urbanity.pl) and own studies. The predominant part of the construction projects were residential projects: Buildings with two residential units and multi-unit buildings. The latter tend to be buildings of medium height (from 4 to 9 overground storeys).
Due to the diversity of construction works carried out, the specific nature of buildings being constructed, and the necessary resources and building techniques, the research sample was standardized. The study covered the projects that involved works related to building erection and specialized construction works (Polish Business Classification Code PKD F41 and F43). Moreover, only residential and non-residential buildings (Section 1
of the Polish classification of civil structures) were taken into account.
For the purposes of the research study, the homogeneous purposive sample method was used. In order to standardize the research sample, a set of characteristics was used. The construction site must be located in the city. The project size must exceed 1000 m2. Only structures at the stage of wall erection and specialized construction works were qualified for the study. Depending on particular stages of the works, the type of materials and the number of deliveries may vary. Due to this, the surveys covered 24 civil structures constructed by 12 construction companies in Szczecin, which, during the research, were at the stage of walls erection and specialized construction works. The companies functioned as both subcontractors and main contractors and included small, medium as well as large enterprises. The predominant part of the research sample comprised buildings with commercial, service, and office functions, hotels and tourist accommodations, buildings with two residential units and multi-unit buildings. The sample included 16 projects in sizes ranging from 1000 to 9999 m2 and 8 projects exceeding 10,000 m2.
3.2. Problems and Irregularities Regarding Construction Site Supplies in Szczecin
The completed surveys have shown there is little interest in more sustainable transport of construction site supplies. By means of non-structured observations, the authors of this paper attempted to identify some concrete cases of negative impacts of construction project implementation on congestion and safety level of road traffic participants. Figure 4
a,b presents photographs taken in 2019 and shows the Hanza Tower residential building with a commercial (office and retail) part, located in the city center on one of the busiest streets in Szczecin. The construction process often entailed a need to obstruct the right-of-way in order to make deliveries, which directly contributed to congestion. In addition to that, this posed a risk for other road traffic participants due to limited visibility, as well as threats connected with the risk of damage or crushing with the cargo being delivered.
, with a picture taken in 2019, shows the unloading of construction site supplies for the Stettiner Business Center office building being constructed in the City Centre close to the major communication node—Giedroyć Roundabout. The building was being constructed on a small street that joined the one with the highest traffic intensity in the city. The unloading operations prevented other vehicles from passing through, which naturally exacerbated the already present congestion problem.
a,b with pictures taken in 2020, shows the Wielka Odrzańska residential building project located in the City Centre. The construction site was located between a narrow street and a big traffic artery. That location considerably hindered the access of delivery vehicles, and additionally, any unloading operations prevented other vehicles from passing through, resulting in congestion.
, with a picture taken in 2021, presents a site of remodeling the outbuilding on Jagiellońska Street. The street is characterized by intensive traffic. The (partial) obstruction of the right-of-way led to congestion and compromised road traffic safety.
As the above examples have demonstrated, the implementation of construction projects has a negative effect on the city. However, specification of the effect is often hindered due to the gaps in data gathered by city authorities and often even the lack of such data. The city authorities of Szczecin do not collect statistical data on road traffic volume and GHG emissions (only general air quality is examined) or noise emissions. Due to the above, it is difficult to quantify the impact of construction projects on the phenomenon of congestion and air quality. The authors of this paper assume that the optimization of transport processes related to construction supplies will naturally contribute to the reduction of GHG emissions and the phenomenon of congestion. In the future, it is planned to expand the research to include measurements of this impact.
3.3. Analysis of Selected Construction Site Deliveries in Szczecin in the Context of Sustainable Urban Freight Transport Solutions Implementation
The selected type of construction procurement is of vital importance in the process of creating a logistics system for a project, and it considerably affects the volume of transport performed and the way it is organized. Therefore, the construction projects covered by the study were analyzed in this context. Among the 24 construction projects, the mixed procurement type was the most popular (carried out by the main contractor as well as by individual subcontractors on their own), and it was applied in 9 cases. The dispersed procurement type (carried out by individual subcontractors on their own) was applied in 8 projects, whereas the centralized procurement (carried out by the main contractor) was seen in 7 projects. In none of the projects was the procurement provided by logistic organizational units.
The surveys conducted with the construction companies’ representatives have demonstrated little interest in the advantages of applying particular procurement types. The selection was often intuitive rather than based on any analysis results, involving only economic criteria connected with materials purchase prices and the subcontractors’ general cost estimates of the construction works, including the materials. Moreover, centralized procurement is considered to be more complicated in terms of organization, management, and responsibility. Procurement provided by logistic organizational units turned out to be the least attractive. The reasons could be both lack of knowledge and higher costs. This approach proves that economic criteria take precedence over environmental ones in the decision-making process.
The respondents were also asked about the average number of deliveries made daily. At most construction sites (14), it was up to 4 deliveries per day. The detailed results are shown in Table 2
The research study made it possible to find correlations between the selected procurement type and the number of deliveries made. When centralized procurement was selected, in 71% of the projects, the number of deliveries was below 4 per day. In the case dispersed procurement was chosen, in 63% of the projects, the number of deliveries ranged from 4 to 6. It was not possible to find any unambiguous correlations in the case of mixed procurement. In 33% of the cases, there were 4 deliveries, in another 33%—from 7 to 9, in 22%—from 4 to 6, and more than 9 in 11% of the cases. According to the research results, centralized procurement is a more sustainable kind of procurement, as it makes it possible to consolidate deliveries and decrease the transport volume, which naturally translates into a reduction in its negative environmental impacts.
The construction projects covered by the study were also analyzed in terms of their compliance with the sustainability principles and their contribution to noise and air emissions and congestion in the city. In the first place, a question was asked whether or not the companies applied sustainability principles in their operations. One of the respondents did not know whether or not they were applying them, another stated they were not using any. Nevertheless, the subsequent replies suggested that all of the companies took into account the sustainability principles, at least to some extent. This clearly demonstrated there was a lack of knowledge about sustainability issues among the construction companies. The respondents were asked about any concrete solutions taken at particular construction sites and could choose multiple answers from the proposed list of solutions. However, the applied solutions were highly limited. The detailed results are shown in Figure 8
. The figure shows only answers marked at least once.
Where it was possible, permanent unloading areas were organized. In fewer than 50% of the analyzed projects, the deliveries were purposefully consolidated. In the case of some construction sites, the deliveries were made within specific hours (e.g., outside peak hours), the procurement was rationally planned, taking into account the external conditions and in cooperation with the road administrator. None of the construction companies covered by the study used alternative drive vehicles. They also did not arrange construction site offices or storage sites in alternative places, e.g., on water. Moreover, in none of them were telematics solutions applied. The results of the research clearly show that enterprises in Szczecin implement solutions to a small extent. In addition, the solutions they implement are solutions that do not involve additional financial outlays. These are solutions that only require a change in the approach to planning (apart from permanent unloading areas). The conducted research does not allow for an unequivocal statement as to which of the examined construction projects is more sustainable. This requires the continuation of research with measuring tools, e.g., a traffic detector, in order to determine the impact of the implemented solutions on traffic flows.
4. Applying a Petri Net in Decision-Making Regarding Selected Solution Implementation
The completed studies have shown a small interest in and a lack of knowledge about solutions contributing to more sustainable transport of construction site supplies within cities. Due to the more and more intensive transport processes taking place within cities, this may be a considerable issue in the future. It will contribute to both decreased quality of residents’ life and disturbances in construction project implementation, e.g., delays. Dissemination of knowledge about good practices and the possibilities of their implementation may be a solution to this problem.
A decision-making process regarding good practices implementation is complex, as it has to take into account the enterprise’s internal and external conditions, as well as any constraints connected with the construction site location or supply delivery times. Despite the difficulties in making such decisions, construction companies are to take action to implement as many good practices as possible, which allows for the synergy effect and the maximum possible reduction of the negative impact exerted on modern cities. The application of decision trees and their simulation in Petri nets may significantly streamline the process. For the purposes of this paper, two sample decision trees were designed to make decisions on organizing permanent unloading areas (Figure 9
) and storage sites or construction site offices on water (Figure 10
In order to make a decision on organizing permanent unloading areas, it is necessary to verify whether it is possible to organize such a place within the construction site. If this is not possible, it is necessary to find out whether such a place can be found next to the construction site. It is important to check if arranging the unloading area would contribute to additional road traffic difficulties. If no difficulties are anticipated, it is necessary to obtain permission from the road/area administrator and organize the permanent unloading area. The decision tree is presented in Figure 9
In order to make a decision to organize storage sites or construction site offices on water, it is first necessary to consider any water bodies in the vicinity. If the construction site is located near water, it is necessary to verify the technical possibilities of organizing such places and to solve any related problems. If the storage site may be organized in technical terms, it is necessary to obtain permission from the city authorities/ the administrator. The decision tree is presented in Figure 10
In order to make it easier for construction enterprises to make use of decision trees, they were simulated by means of Petri nets using the HPSim software. Petri nets were developed by Petri [37
] as part of his doctoral dissertation, and they make it possible to model concurrent and discrete systems, process synchronization, and others. This theory specifies various classes of nets and methods of analyzing them. Models represented by means of the nets enable not only a graphical representation but also a simulation of various systems operations [38
Petri nets are represented by directed bipartite graphs (containing two kinds of nodes: Places and transitions). The directed graph is an ordered 3-tuple G = (V,A,ƴ), where:
V is a set of nodes,
A is a set of arcs, V∩A = ∅
ƴ: A→V × V is a hook function that assigns nodes to arcs.
The net is an ordered 3-tuple N = (P,T,A), where:
P is a non-empty set of places;
T is a non-empty set of transitions, P∩T = ∅;
A ⊆ (P × T)∪(T × P) is a set of net arcs.
Graphically, places are marked with ellipses and transitions with rectangles. The flow relations between them are shown using arrows. The state of the net is described by marking, and it changes as a result of transitions. Marking introduces one more function to N = (P,T,A), which is M0
, i.e., initial marking, thus creating a marked net N = (P,T,A,M0
), where M0
is a function defined on a set of places, denoted as initial marking of net N. Each place in the net is assigned a positive integer of markings (tokens), which graphically are represented by dots located in ellipses (places) [39
There are dependencies between net elements, which form typical structures, including:
sequence, i.e., execution of instructions in order,
concurrency, i.e., concurrent execution of sets of sequential programs.
Modeling in Petri nets is nondeterministic, which may be a source of conflicts. In order to eliminate this phenomenon, it is necessary to apply structures of alternation, mutual exclusion, processes synchronization, and asynchronous communication [39
For this purpose, the HPSim software was applied. Places in the net are represented by questions that are required to be answered in the decision-making process. Transitions represent the possibilities of providing a positive or negative answer. In order to introduce determinism, places were added to represent affirmative (M0
= (1)) or negative (M0
= (0)) answers. Figure 11
and Figure 12
demonstrate decision trees in the HPSim software. In order to validate the net, it is necessary to enter data in appropriate places.
The proposed models allow for taking into account all the elements necessary to make a decision on the implementation of selected good practices. They are a graphical representation of the decision-making process. The enterprises surveyed by the authors showed intuitive decision-making, often dictated only by economic factors, which is a problem and hinders the implementation of good practices. Thus, the creation of instruments to support these processes may contribute to greater openness of construction companies to these solutions, will contribute to the dissemination of knowledge about them, and will allow justifying decisions based on modern knowledge. The proposed models were verified on selected examples of construction sites in Szczecin. Additionally, they were assessed as correct by experts. The authors of the paper plan to verify the models in real conditions, but establishing cooperation in this area is a difficult task due to the small interest among enterprises in cooperation. It seems reasonable to propose a set of models to be verified in the next stages of the research.
In order to study the real impact of the decisions to create a permanent unloading area or storage sites on the water on the transport system and the level of safety of road users, additional research should be carried out, for example, with the traffic detectors. Traffic flows and their fluidity should be analyzed ex ante and ex post after a permanent unloading area or storage site on water has been arranged. This approach will make it possible to unequivocally determine whether a given solution reduces the negative impact of construction supply on the transport system.
Studies presented in the literature on the subject as well as by observation of good practices show that implementation of the selected organizational solutions significantly contributes to reducing the negative impacts of construction site supplies deliveries within cities. However, as research conducted in Szczecin has shown, the implementation of any solutions is not a priority for construction companies. They point out the difficulties associated with their implementation. Application of Petri nets can support them.
Construction supplies transport has negative impacts, such as congestion, air emissions, noise emission, and compromised road traffic safety. Therefore, it seems reasonable to take measures and implement solutions aiming at a more sustainable approach to such processes. The paper presents the first stage of research aimed at building a model for the implementation of good practices in the field of construction supplies. The paper presents an analysis of the current state of construction supplies in Szczecin and an overview of good practices that may be a chance for more sustainable deliveries of construction site supplies, however, their implementation may be subject to various kinds of—predominantly economic—barriers, which may also be connected with the construction site location or legal constraints.
The surveys conducted with the construction companies’ representatives have demonstrated little interest in the advantages of applying particular procurement types. The selection was often intuitive rather than based on any analysis results, involving only economic criteria connected with materials purchase prices and the subcontractors’ general cost estimates of the construction works, including the materials. Moreover, centralized procurement is considered to be more complicated in terms of organization, management, and responsibility. Procurement provided by logistic organizational units turned out to be the least attractive. The reasons could be both lack of knowledge and higher costs. This approach proves that economic criteria take precedence over environmental ones in the decision-making process. The situation is similar to the implementation of solutions contributing to a more sustainable supply. Companies involved in construction projects in the city showed little interest in sustainability aspects. The indicated reasons for that were a lack of knowledge about sustainable urban freight transport solutions and the costs connected with their implementation. Problems of economic nature effectively divert attention from the resulting benefits that may be obtained. It appears reasonable to take up measures to disseminate knowledge about the benefits ensuing from good practices implementation and from making available the tools for streamlining the decision-making processes, e.g., decision trees and Petri net simulations.
The organization of permanent unloading areas and storage sites/offices on the water have been identified as organizational solutions allowing for reducing the negative impact of construction supplies on cities that are easy to implement. This is mainly due to the low costs of implementing solutions and the readiness of enterprises to implement them. On the other hand, conditions must be met regarding the terrain, the necessary infrastructure, and a permit should be obtained. Street View data and data about traffic flows in a given area may be helpful in the process of deciding on the location of permanent unloading areas and storage sites/offices on water.
Models created by the authors using Petri nets can be an important tool that clearly and transparently shows enterprises their possibilities in the implementation of good practices. The authors plan to continue research into the possibility of using Petri nets in decision-making processes in construction companies. The next stage will be the creation of a model that allows the selection of the type of construction procurement tailored to both internal and external conditions of construction companies, with particular emphasis on the need to reduce the negative impact of construction activities on modern cities.
Little interest in sustainability issues shown among construction companies may constitute a major problem in the future. The construction business development and the increasing number of large-scale projects will naturally contribute to hindrances in planning and making construction site deliveries. Additionally, these processes are often complicated by the intensified actions taken in contemporary cities to counteract transport congestion, noise pollution, and air emissions. The worldwide trends, requirements introduced by legislators, and increasing interest in sustainability among customers may have a positive effect on the situation. Construction companies will have to adapt to the challenges of new realities.
The obtained results confirm the need to intensify the activities aimed at raising awareness of the negative environmental impacts of transport processes on the city and finding how to counteract them. Moreover, stakeholders representing the construction sector should, to a larger extent, be involved in initiatives such as freight quality partnerships. Moreover, city decision-makers should to a larger extent account for the impacts of construction projects implemented in cities on the functioning of the city organism and city users. This aspect should constitute a key element of any decision-making and regulatory processes. The authors of the paper plan to continue the research with the use of Multi-Actor Multi-Criteria Analysis (MAMCA) in order to pay special attention to the role of stakeholders representing the construction sector.