1. Introduction
Sustainability of transportation systems has become a growing area of interest in practice and research. Institutions and researchers have proposed different definitions of sustainability of transportation systems, and have shown a consensus, that sustainability of transportation systems requires holistic triple bottom line accounting, including the economic, social, and environmental properties of transportation [
1]. Although there is no standard definition [
1], Cormier and Gilbert [
2] still recommend the definition proposed by the Centre for Sustainable Transportation (CST) in Canada, because this definition has a broad scope and recognizes specific transportation issues. According to this definition, the sustainability of a transportation system requires a transportation system to: (1) allow the basic access and development needs of individuals, companies, and society to be safely met; (2) operate fairly and efficiently, offering a choice of transport modes and supporting a competitive economy; and (3) limit emissions and waste within the planet’s ability to absorb them [
2]. Sustainability of transportation systems could be evaluated by a set of indicators, among which the most common indicators include transport cost, transport time, emissions, noise, traffic congestion, and safety [
3].
It is worth noting that there are differentiated concepts of “sustainable transportation” and “sustainable development of transportation” relating to the concept of sustainability of transportation. The concept of “sustainable transportation” emphasizes the effects of transport sustainability, while the concept of “sustainable development of transportation” is understood as the process of evolution in the transport sector with the property of increasing sustainability [
4]. The difference between the two concepts is characterized by Goldman and Gorham [
5] as “sustainability as an end-state vision” and “sustainability as a pathway”, respectively. The meaning of freight sustainability in this study refers to “sustainability as a pathway”; namely, the achievement of better sustainability than that of the current practice, in terms of improvements to sustainable transport indicators.
Intercity freight, which accounts for a portion of China’s overall domestic transportation, has received increasing attention, as it is recognized as one of the major considerations of the nation's sustainable development. A common and serious problem with China's intercity freight systems (IFSs) is overloaded trucking, which preoccupies the Chinese government because it enhances the economic performance of IFSs at the expense of environmental and social performance. On the one hand, overloaded trucks can cause deterioration of roads, vehicle maneuvering difficulties, and safety issues [
6], which drive sustainability in the opposite direction than the positive mobility effects. On the other hand, however, overloaded trucking enhances economic performance, as it significantly lowers freight owners’ transport costs of materials, and, further, their production costs of final goods. This is why, although the statutory limit of truck weight was explicitly regulated by the Chinese government in 2004, overloaded trucking still prevails in many regions, especially in those with resource-intensive industries [
7]. Some studies focusing on overloaded trucking in China have also, to some extent, verified this phenomenon. Hang and Li [
7] concluded that the moderate weight regulation, by which truck weight is allowed to exceed the statutory limit to a moderate degree, is the most appropriate option to reduce transport and pavement maintenance costs. Liu and Mu [
8] further investigated the influences of overloaded trucking on the sustainability of highway freight systems. The results showed that the best truck weight regulation for sustainability depends on the relative importance of economic issues compared with social and environmental issues, and that overloaded trucking is a necessary choice (unless the economic issues are unimportant). In this context, decision-makers would face the predicament of policy enforcement, because permitting overloaded trucking violates the state’s regulation. It is, thus, important to come up with a solution, whereby overloaded trucking can be eliminated without undermining the overall performance of the sustainability of IFSs.
A modal shift involves using efficient modes of transportation (e.g., rail and water) more often than less efficient modes (e.g., truck and air) [
9]. A modal shift can be achieved using fiscal measures (e.g., rail freight funding or road freight taxation), regulatory measures (e.g., regulation on truck weight or truck size, traffic control), and infrastructure construction investment (e.g., constructing new rail infrastructure or improving existing rail infrastructure) [
10]. Researchers have proven that an appropriate modal shift in policy can effectively lower the average transport cost, reduce transport emissions and congestion, and enhance traffic safety [
11,
12,
13,
14], which are all important issues in freight sustainability. Therefore, shifting away from trucks would be a potential way to achieve an increasing sustainability of IFSs, with the premise of trucks meeting the statutory limit.
The objective of this study was to evaluate and compare the sustainable effects of alternative modal shift policies with trucks meeting the statutory limit in the long-term, and then to identify the effective ones, by which increasing sustainability of IFSs could be achieved. The remainder of the paper is structured as follows:
Section 2 presents a literature review.
Section 3 proposes a policy evaluation model, including model framework and model development.
Section 4 applies the proposed model to a specific case in China.
Section 5 formulates several optional modal shift policies for the case.
Section 6 evaluates the long-term sustainable effects of the formulated policies with the model and identifies the effective modal shift policies, and then includes a discussion based on an analysis of different parameter-setting scenarios regarding more general circumstances.
Section 7 draws conclusions and suggests further research.
2. Literature Review
The literature review is presented following two basic streams. The first stream investigates the existing methods for freight sustainability evaluation, and then highlights the applicability of system dynamics (SD), an important branch of systems-based approaches, to the objective of this study. The second stream introduces SD, as well as its applications, to freight transportation. The shortcomings of these applications are then analyzed, and the contributions of this study are underlined.
2.1. Methods for Freight Sustainability Assessment
Alternative methods were used for freight sustainability evaluation [
15,
16], among which optimization models [
17,
18,
19] and simulation-based methods [
20,
21,
22] were extensively used in the assessment of different aspects of freight sustainability. However, these methods are not sufficiently applicable for analyzing the long-term sustainable effects of modal shift policies with the goal of solving the overloaded trucking problem. The reasons for this are threefold:
The above-mentioned methodologies presuppose that the exogenous stimuli, e.g., policy activities, can be clearly distinguished and, thereafter, examine the effects of the stimuli separately [
23]. Regarding the problem of this study, solving the overloaded trucking problem involves changing the truck payload, which gives rise to simultaneous variations in multiple determinants of a freight system, such as transport cost, transport time, and truck traffic volume, which cannot be clearly and individually separated and analyzed.
The strategic evaluation of freight sustainability should be performed at the macro level of freight systems, which requires including components of freight systems, as comprehensively as possible. This requirement leads to a method that facilitates accommodating a wide range of components, while simplifying the details of the components. In contrast, optimization- and simulation-based methods are characterized as high-resolution in components’ details but as low-resolution in the coverage of components [
24]. Consequently, these methods are more appropriate for the analysis of sub-systems of freight systems or of sub-objectives of freight sustainability.
The assessment of the effect of modal shift policies on the overall performance of sustainability needs to, simultaneously, take into account economic, social, and environmental objectives. The optimization- and simulation-based methods, intended to optimize individual sub-objectives, cannot reliably evaluate the overall sustainable performance of a freight system, even when these methods are used in such a way that one model’s results feed into another model’s assumption [
25]. In other words, these methods are generally inadequate for the assessment of the overall performance of sustainability when the whole is greater than the sum of its parts; namely, freight sustainability as the emergence of intertwining factors, including freight activities, policies, and time.
The sustainable effects of modal shift policies on IFSs arise from a set of coupled causes and last over time. Looking too narrowly at a particular cause or changes within a particular time, without systematically considering the sustainable effects, would lead to solutions that merely serve to move the issues of sustainability around, rather than improve the system as a whole. From this point of view, the methodological priority should be toward a systems-based approach [
26], which focuses on the system’s evolution that stems from interactions and feedback among components of the system and is capable of comprehensively and dynamically analyzing policy effects.
Regarding the applications of systems-based approaches to freight sustainability evaluation, Richardson [
27] employed a method based on influence diagramming to provide frameworks illustrating the interactions among influence indicators of freight sustainability. However, this study did not suggest any method for quantifying the relationships among the variables and indicators. Ülengin et al. [
26] developed a structural equation model to assess the consequences of possible policies on Turkey’s passenger and freight transportation system. The model quantifies interrelations among a series of variables that reflect alternative sustainable issues; however, it stresses indicators regarding environmental and social issues, with little consideration for economic issues, such as transport costs and transport time, which are important for freight owners and carriers. Maheshwari et al. [
28] built a dynamic model, which was derived by referring to the predator–prey models of biology, to capture the interdependent behavior of transportation, economics, and environmental systems over time. However, the model did not further account for the interrelations among the components within the respective systems; thus, the detailed effects of the policies could not be analyzed.
In this study, the method we employed was SD, which is another important branch of systems-based approaches. SD is well suited for the objective of this study for the following reasons:
The essence of the problem of overloaded trucking lies in the interactions of different agents’ (e.g., freight owners, infrastructure operators, and the government) decisions, based on their own interests. SD is useful in understanding these interactions by setting both multiple variables representing the behavior of those agents and equations between variables that reflect the interdependency of those agents.
The objective of this study is to evaluate policies’ effects on sustainability, regarding which both economic effects and social and environmental effects should be considered simultaneously. SD allows the cost-benefit analysis approach to be integrated [
7], which enables a consistent assessment of policies by converting both economic effects and social and environmental effects into monetary equivalents; thus, it is suitable for evaluating global effects of modal shift policies on freight sustainability.
The nature of this research is policy evaluation, which is in the strategic horizon that the long-term effects of policies are to be assessed. SD has the advantage in policy evaluation of not only analyzing the ultimate effects of policies over a long time period, but also providing a time path of policies’ impacts on systems [
24].
2.2. SD and Applications to Freight Transportation
SD was initially developed by Forrester from Massachusetts Institute of Technology in the 1950s and 1960s. It uses a standard causal loop approach to develop a qualitative model of a system, which is then transformed into a quantitative stock-flow model that consists of stocks, flows, converters, and feedback loops. The SD approach is becoming increasingly used in a hierarchical manner, which allows systems and policies to interact across space and time [
29]. Thus, it is a useful decision-making and policy evaluation tool due to its ability of interpreting the past behavior of systems and forecasting policies’ effects on systems.
SD was first applied in transportation in the 1990s for passenger transportation, and later for freight transportation. Abbas and Bell [
30] first confirmed the applicability of SD for transportation modeling and listed twelve advantages of the approach compared to traditional transport modeling. Shepherd [
29] recently proposed a comprehensive review of SD models applied to transportation within the last twenty years. However, the literature applying SD to freight transportation focuses, respectively, on a single issue of sustainability, such as carbon emission [
31,
32,
33], land use [
34], pavement condition [
7,
35], traffic safety [
36], and congestion [
37,
38], with little discussion on the integrated effects of the above-mentioned issues, namely, freight sustainability. Moreover, most existing literature does not consider the problem of overloaded trucking, except the research by Hang and Li [
7], but the objective of this research is to investigate the impacts of overloaded trucking on transport and pavement maintenance costs, not on freight sustainability. Overloaded trucking gives rise to simultaneous changes in IFSs’ performance on the above-mentioned sustainable issues. The dynamic mechanism of how overloaded trucking and modal shift policies influence the overall performance of sustainability needs to be investigated.
This research advances the literature related to freight transportation by solving the problem of overloaded trucking, meanwhile achieving the increasing sustainability of IFSs via an SD approach. In this study, we developed an SD model to evaluate the sustainable effects of potential modal shift solutions in which trucks meet the statutory limit; we then compared the effects with those of the current practice in which trucks are overloaded and identified the effective modal shift policies for increasing sustainability in terms of the reduction of IFSs’ total costs, including economic, environmental, and social costs. The results of this study provide some insight into the reciprocal mechanism between modal shift policies and the sustainable performance of IFSs. The SD model provides the decision-makers with a tool for developing the appropriate and feasible modal shift policies to achieve increasing sustainability of IFSs with the premise of eliminating overloaded trucking.
4. Model Application and Validation
The proposed SD model was applied to a specific case in China, the Cao-Tang freight system (the intercity freight from Caofeidian to Tangshan), as shown in
Figure 3. The main freight type of the case is iron ore, of which the amount consists over 90 percent of the overall freight volume, in contrast to other minor freight types (e.g., crude salt, crude sugar, and wood, etc.). Tangshan city is the largest steel industry base in China, and the iron ore the steel companies need is mainly imported from overseas and is unloaded in Caofeidian port located in Caofeidian district. There is no railway linking Caofeidian and Tangshan, although both the Caofeidian port and steel companies in Tangshan are equipped with internal railways. All of the iron ore from Caofeidian to Tangshan is transported through two highways, an express highway and a regular highway. The express highway is enclosed, with weighbridges monitoring trucks’ weights, so it is used by only a small number of trucks that meet the statutory limit. The regular highway is open and free of charge, with traffic police inspecting it randomly during the day. Therefore, all of the overloaded trucks, with the average payload of 93 tons, which is far beyond the statutory limit of 35 tons, choose to use the regular highway at night when traffic police are off-duty, causing severe pavement damage and safety issues. It is urgent for the local government to come up with a modal shift solution that prevents overloaded trucking without undermining the overall sustainability of the system.
The model was applied to this case by assigning a variety of parameters specific values, which were obtained referring to both primary data derived from an onsite survey of the Cao-Tang freight system and secondary existing data derived from the peer-reviewed literature [
48,
49], the CDRL [
39], and the SDHAP [
41]. Some parameters used in the SD model of the Cao-Tang freight system are shown in
Table 3.
The applied model was implemented using the Vensim software package (Ventana Systems, inc., Harvard, MA, USA), a predominant SD simulation platform, with parameters and functions transformed into model equations. Simulation runs were first conducted on historical years, from 2008, when the Caofeidian port was first operated, to 2015, for model validation. The validation process is exemplified on two variables. The results show that the simulated values of both variables are well correlated with the actual values, as shown in
Figure 4a,b. The validation process was also undertaken for other variables, and the results are also satisfactory, which implies that the model is qualified for policy evaluation.
5. Policy Formulation
Alternative modal shift policies were set for evaluation after the model was validated. According to the commonly used modal shift policies suggested by Woodburn et al. [
10], this study considers the regulatory measure and railway infrastructure construction for the Cao-Tang freight system.
The regulatory measure refers to the rigid weight regulation (RWR) for overloaded trucks, by which all of the overloaded trucks are required to meet the statutory limit, with the expected effect that truck flow of the regular highway is diverted to the express highway. Based on the RWR, the modal shift from road towards railway (MSR) is further formulated. Railway infrastructure construction is involved regarding MSR, as there is no existing railway linking Caofeidian and Tangshan, with the expected effect that some of the freight on highways is directed to the railway, as shown in
Figure 5.
To be specific, three MSR policies were formulated, including Modal Shift from road towards Railway of Class III (MSR III), Class II (MSR II), and Class I (MSR I), according to the railway classification defined by the CDRL.
Table 4 shows the technical specifications of the railway with different classes, which were obtained by consulting practitioners from the authorities’ department of the Qian-Cao railway, which is exclusively used for coal transportation from Qian’an, a city adjacent to Tangshan, to Caofeidian.
Based on the formulated modal shift policies, a series of scenarios were defined as follows:
- (1)
The baseline, which is the scenario with a projection of the past and current trends so that the following policy scenarios can be compared with it for evaluation. As mentioned above, there is severe overloaded trucking on the regular highway. It is assumed in the baseline that this pattern will continue in the future without any policy intervention.
- (2)
The RWR, in which all of the overloaded trucks on the regular highway are assumed to meet the statutory limit from 2016, the starting year of the scenario.
- (3)
The MSR, which includes MSR III, MSR II, and MSR I that are to be implemented individually. In the scenarios of the MSR, it is assumed that a railway is to be constructed from the starting year of 2016, and the RWR for highways is also implemented at the same time.
The simulation period of the above scenarios is set as ten years, which covers the next two five-year plans of China (the 13th five-year plan: 2016–2020, and the 14th five-year plan: 2021–2025). The model aims to calculate and compare the total cumulative cost (TCC), including the cumulative economic cost (CEC) and the cumulative social cost (CSC), within the simulation period of all the scenarios, following the principle that the policies by which increasing sustainability, in terms of a reduction of the TCC compared with the baseline, can be achieved are the effective ones.