 Original Paper
 Open Access
Short turning pattern for relieving metro congestion during peak hours: the substance coherence of Shanghai, China
 Xueqing Ding^{1, 2},
 Shituo Guan^{3},
 Daniel Jian Sun^{1, 2, 4}Email authorView ORCID ID profile and
 Limin Jia^{4}
https://doi.org/10.1186/s1254401802939
© The Author(s). 2018
 Received: 2 October 2017
 Accepted: 5 April 2018
 Published: 15 June 2018
Abstract
Introduction
Urban metro system generally has to deal with intractable heavily passenger loading during peak hours, in which demands are extreme huge in certain stations. However, overcrowding doesn’t ubiquitously exist for all stations and mitigation measures have to be carried out on purpose, respectively. Because of the restrictions on operational costs and avoiding transportation resources wasting, simply increasing dispatch frequency is not rational to solve the problem.
Methods
Short turning pattern has been proved to be an efficient way to solve the issue, which had been mainly used in urban ground public transport systems. This paper applied short turning pattern to urban metro system and relaxed constraints of the turningback facility. A mathematical model is proposed to determine the short turning parameters, during which a load factor was introduced as a measurement of overcrowding condition. An empirical case from Shanghai Metro Line 2 was incorporated to demonstrate the effectiveness of the proposed model.
Results
The results indicated that the short turning route from Beixinjing to Longyang Rd. in Shanghai Metro Line 2 could effectively relieve overcrowding within the heavy traffic demand zones.
Conclusions
Findings of this study could provide valuable suggestions in metro system administration for potential improvement on the operational performance during peak hours.
Keywords
 Short turning
 Relieve congestion
 Metro system
 Peak hours
1 Introduction
Urban metro system is a highcapacity public transport mode aiming at providing convenient and efficient services for passengers [1, 2], which plays an essential role in commuting traffic, especially in megacities, such as New York, London, Tokyo, and Shanghai and so on. In China, metro systems have been built in many cities during recent years, for example, Shanghai has more than 600 km mileage of subway and light rails (by 2016), which is still not able to satisfy the growing demand. Early reckless urban planning and huge population (more than 24 million) [3] spawns enormous travel demand. According to statistics from Shentong Inc., the operating company of Shanghai Metro, for the five continuous weekdays from February 29 to March 4, 2016, the networkwide passenger loading of Shanghai Metro were all over 10 million, attaining 10.16 million, 10.23 million, 10.33 million, 10.25 million and 10.82 million, respectively [4]. As a result, uncomfortable travel experience occurred, including huge ontrain overcrowding during peak hours, which seriously affected the comfortableness of passengers, and even influenced their working efficiency and quality of lives.
For the overcrowding issue in metro system, an obvious problem is that the capability isn’t always being fully utilized, i.e., some sections are heavily crowded while at the same time others may be underused. Taking AM peak as an example, traffic demand is enormous from overall perspective, however, it can’t be ignored that some sections are not crowded at all. Metro system administrator can mitigate the overcrowding by increasing service frequency, which may bring additional issues, as 1) more trains are required which brings huge monetary cost; 2) the capacity may meet the peak demand but also induce the wasting of resources for segments where the demand is not large enough. Another feature of AM peak flow in metro system is directional nonuniformity. Lines connecting suburban and central areas, such as Line 5, and those across the city center, such as Line 2, all possess directional nonuniformity. Therefore, solely improving dispatching frequency is not effective in solving the traffic overcrowding problem.
Commuter trips have tidal feature in nature, spatial and temporal inhomogeneity and directional nonuniformity, which is prone to induce vehicle congestion/passenger overcrowding or resource wasting. To avoid these, transportation administrators should adjust the operating strategies to meet different demand. Similar situations have been appeared in the regular ground transit operation. Previous studies have proposed short turning schedule along with fulllength operation policy [5, 6], which were also used in railway system [7, 8]. Short turning, also called short turn, relatively with fulllength policy, means selecting two stations within the entire line as the new origin and destination to improve the transport capacity for the section in between. Short turning is a convenient tactical strategy when high demand sections need to be served while others only have relative low demand, with respect to simply increasing the dispatching frequency. As passenger demand can be met by various operation patterns, such as short turning and fulllength policy, consequently schedule coordination between these patterns is indispensable.
Short turning is an operating pattern not only for regular public transit and railway system, but also for urban metros, which is particular efficient for lines connecting suburb residential areas with city center or connecting two main transportation hubs. The strategy has been used within other situations, such as zoning, restricted, semirestricted service, and express service [9, 10]. To improve the frequency among specific zones of the metro line, short turning may be performed to reduce average waiting time within these sections [5]. Studies related to the short turning operation pattern within urban metro system have to coordinate with the traditional operating patterns with an objective of minimizing both the passengers’ waiting time [11] and the fare costs, as well as the costs of operators [12]. Other research on short turning concerned the timetabling level [13]. Taking these into consideration, this study mainly focused on the short turning zone selection in order to relieve passenger overcrowding.
The remainder of this paper is organized as follows. Section 2 presents literature review with research gap and proposes research methods of this paper. Mathematical programming model is formulated in Section 3, followed by a case study of Shanghai Metro Line 2 in Section 4. Finally, conclusions for future work are summarized in Section 5.
2 Literature review
Imbalance between urban transport supply and demand raises many problems. JaraDíaz et al. [14] analyzed the demand between each station pair within a single bus transit line by comparing with models under different demand aggregation levels (i.e. aggregate and disaggregate demand) for obtaining optimal frequency and vehicle size. Temporal differences in passenger volumes can be addressed by providing a higher service frequency during peak hours, while the spatial demand unbalances justify the implementation of fleet assignment strategies, by increasing the service frequency on the high demanded route sections to adjust the service demand with the capacity supply [15, 16]. To deal with this, Tirachini [17] developed a disaggregated short turning strategy with information at a station level. Short turning strategy in bus corridor often uses demand from station to station within a single line and certain period setting. The main objective is to increase the service frequency on sections with high demand, thus to deal with spatial concentration of overcrowding considering the costs of both transit agency and passengers. Frequencies (inside and outside the short cycle), capacity of vehicles and the location of short turn limited stations were determined through short turning model. To relieving overcrowding, Cortés et al. [18] developed a model that combines short turning and deadheading in an integrated strategy for a single transit line. Ulusoy and Chien [19] attempted to optimize bus service patterns (i.e. allstop, shortturn, and express) and frequencies, thus to minimize total cost, considering transfer demand elasticity, similar to Ulusoy et al. [20], Zhao and Zeng [21]. These studies are mainly applications of regular transit system without considering facility constraints.
When coming to railway system, short turning has not been widely used because railway system mainly concerns about timetable [7, 8, 22] and rolling stock [23]. Louwerse and Huisman [7] adjusted a railway timetable in case of large disruptions and presented integer programing formulations based on eventactivity networks for the situations of a partial and a complete blockade. Timetable determination was also studied by means of building and solving a nonlinear integer programming model that fitted the arrival and departure time to a dynamic demand [8, 22]. Lobo and Couto [24] investigated the relationship between the operational performance of metro systems and their socioeconomic contexts, but unfortunately no effective improvement measures were proposed.
Nagorsky et al. [25] described the development and evaluation of potential interventions to improve the rapid transit network’s ability and provide service to customers in addressing subway overcrowding. Canca et al. [11] proposed a shortturning policy in rail transportation to handle the passenger overloads within train service disruptions by increasing the frequency among certain stations and equilibrating the train occupancy level. Turnback points and service offsets were determined with the objective to minimize passengers’ waiting time while ensuring a certain level of quality of service. Sun et al. [12] proposed a total cost minimization model for redesigning the short turn operation by mainly relaxing the assumption that the fulllength route must be operated with subjecting to the restriction of turnback stations. Then, to meet the increased passenger demand and overcrowding around urban and suburban areas, an integrated planning model was proposed to adequate the offered capacity and system frequencies by Cadarso and Marín [26]. What these studies might neglect is that public transit is a humanoriented transport system, relieving overcrowding for a better travel experience is as important as reducing travel time. Parbo et al. [27] reviewed passenger perspectives in railway timetabling and emphasized the importance of passengeroriented railway timetabling to save the passengers waiting time.

Short turning operating policy is widely used in regular transit. However, as the ground transportation is not limited by the tracks, applications of different operating policies are relatively free.

Timetables are the main focus in the railway system. Some complex mathematical programming models are used to obtain a better schedule.

Urban metro systems have combined operating policies of traditional ground transit and railway systems, but unfortunately, haven’t given the priority to passengers.
This study mainly focuses on relieving peak hour overcrowding of urban subway based on previous studies by relaxing the restriction of turnback stations. A mathematical model was proposed to determine the short turning parameters, during which a load factor was introduced as a measurement of overcrowding condition. Finally, empirical data obtained from Shanghai Metro were used to demonstrate the capability of the proposed method.
3 Mathematical model
3.1 Problem statement
 a)
For both full length and short turning services, all trains have to stop at every station in between and operate at the same speed with fixed headway.
 b)
No turn around constraints exist along a line since turn around facilities can be built without many particular difficulties, although only a subset of stations containing shortturning facilities may be established.
 c)
All passengers waiting on the platforms take the recent arrival train and are able to aboard.
 d)
Passengers arrive uniformly during the peak period regardless of stochastic demand.
3.2 Model formulation
Without loss of generality, a metro line can be modeled as a directed graph, G = (V, L), in which V denotes station (vertex) set, L denotes link set. Station is denoted as i ∈ V = {1, 2, …, n}, and link, between any two consecutive stations, is denoted as l ∈ L = {1, 2, …, n − 1}. Route denotes a train runs back and forth, indicated by r ∈ R. Two routes are presented in Fig. 3, one (Route 1) covers Station 1 to Station n, and the other (Route 2) covers Station 2 to Station n2 overlapping with Route 1. Passenger demand between different stations is denoted by OD (OriginDestination) pairs, an n by n matrix.
3.2.1 Parameters
Ma sufficiently large constant (chosen as 9999 during the numerical experiment);
hfull length headway;
h_{ min }the minimum headway;
h^{‘}the offset between short turning and fulllength services;
q_{ ij }hourly demands from Station i to Station j;
uupstream direction;
ddownstream direction;
\( {Q}_k^u \)hourly demand of Station k in the upstream direction (uup direction);
\( {Q}_k^d \)hourly demand of Station k in the downstream direction (ddown direction);
Capcapacity of each train (310 passengers per carriage unit, 6 passengers/m^{2});
α_{ ijr } binary variable, whether trips from Station i to Station j are covered by Route r, 1yes, 0no;
β_{ ij } binary variable, whether passengers can reach destination j directly from Station i, 1yes, 0no;
γ_{ yz }, binary variable, whether the zone from Station y to Station z is selected for short turning services, 1yes, 0no;
ηload factor;
δ_{ rk }binary variable, whether Route r covers Station k, 1yes, 0no;
\( {\pi}_{r{k}^{\prime }} \)binary variable, whether turn back Station k^{′} is part of Route r, 1yes, 0no;
\( {\uptau}_{k^{\prime }} \)headway of Station k^{′} for turning back;
S_{ r }binary variable, whether Route r is selected, 1yes, 0no;
h^{‘}the offset between shortturning and fulllength services, continuous.
f_{ r }frequency of Route r, continuous;
θ_{ ij }whether OD pair i to j can be served directly without interchange.
3.2.2 Decision variables
Short turning zone: r_{ yz }, y, z ∈ V^{′} ∈ V, route is denoted as r ∈ R;
Frequency of route r: f_{ r }.
3.3 Constraints
Train capacity constraint
where, \( {Q}_k^u \) and \( {Q}_k^d \) denote the cumulatively demand of Station k in up and down direction, respectively.
If multiple operating patterns exist, such as a fulllength service together with a shortturning service, stations are actually served with different train capacity. The capacity constraints of outside and inside short turning zone are defined as follows.
V^{′} ∈ V a subset of stations containing shortturning facilities.
h^{‘} the offset between shortturning and fulllength services.
γ_{ yz } (y, z ∈ V^{′} : y < z), binary variable, 1 if the zone is selected for short turning services.
 1)
Fulllength service outside the shortturning zone:
 2)
Fulllength service inside the shortturning zone:
 3)
Shortturning service:
To avoid heavy overcrowded and prevent resources wasting simultaneously, the loading factor η_{ k } of each train must be within a certain rational range, which is general low at the initial departure station and increases gradually, and then decreases. Therefore, load factor of a train indicates the maximum shortterm load factor during oneway operating period. Capacity of each train is calculated based on full loaded with criterion of 6 passengers/m^{2}. To avoid overcrowded, the maximum load factor η_{ k } is limited to no more than 1.2. Meanwhile, to avoid resources wasting, short turning zone has to be selected from stations with an original load factor η_{ k } ≥ 0.8.
Line capacity constraint
Turn around capacity constraint
3.3.1 Objective function
As the overcrowding status of train can be controlled by a predefined load factor η, travel time is used for evaluating travel experience depending on the departure frequency, shortturning zone selection etc. As all trains are supposed to run at the same speed, and the ontrain time is identical, the only difference lies in the platform waiting time. Therefore, the passengers’ waiting time was chosen as the objective of the model.
Other parameters, α_{ ijr } indicates whether trips from Station i to Station j are covered by Route r, binary variable. S_{ r }whether Route r is selected, binary variable. δ_{ rk }whether Route r covers station k, binary variable. \( {\beta}_{ij}=\underset{r}{\mathit{\max}}\left\{{\alpha}_{ij r}{S}_r\right\},\forall i,j\in V \)whether passengers can reach destination j directly from Station i, binary variable.
As a result, the objective function of model is formulated as follows:
Objective function
Train capacity constraints: (3), (4), (5), (6).
Load factor: 0.8 ≤ max {η_{ k }, k ∈ V^{′}} ≤ 1.2.
Line capacity constraint: (7)
Turnback capacity constraint: (8)
The objective is to minimize passengers’ average waiting time under necessary constraints. The train capacity constraint ensures that the supply could meet passenger demand at certain level of service. Line capacity constraint guarantees the operating frequency does not exceed the permissible value, while the turn back constraint makes trains turn around without exceeding the turnback capacity. Load factor was highlighted here for ensuring the degree of occupancy.
4 Numerical experiment
4.1 Data sources
The eastern portion of the line, from Guanglan Rd. to Pudong International Airport, operates independently from the main line due to the infrastructure constraint, where only the 4carriage trains were run. The western portion, also the main route, runs 8carriage trains from East Xujing to Guanglan Rd., as shown in Fig. 1. Consequently, the two operating routes for Line 2 are the one from East Xujing Station to Guanglan Rd. Station (Stations 1 to 21) using the AC02a series trains (designed and manufactured by the German company  ADTranz & Si, with an operational speed limit 80 km, VVVF AC drive and 8carriages), and the other from Guanglan Rd. to Pudong International Airport (Stations 22 to 30) using the trains with four carriages.
Due to the high passenger demand along Line 2 during the AM peak, the train is overcrowded from Stations 6 to 18 (within the western portion of the line), as presented in Fig. 2. It was found that passenger demand is not extremely large along the eastern portion, and short turning zone is not necessary to cover this segment. Therefore, the main purpose of this case study is to determine a short turning zone with higher passenger demand both in up and down directions, as well as the dispatching frequency of the short turning trains. The objective will be achieved throughout the parameters found out using the model, i.e. Eq. (11).
Passenger OD data, for AM peak 7:30–8:30, Sep. 16, 2014 were obtained from Shentong Inc., from which the number of passengers taken Line 2 were estimated. As the train operated in the west portion of Line 2 is composed by 8 AC02atype carriages and each accommodates 310 passengers (6 passengers/m^{2}) in maximal. Accordingly, the total capacity of a train in the west portion is calculated as 310*8 = 2480 passengers. The present dispatching headway is 6 min, and the frequency is 10 per hour. Therefore, the total capacity of the main line is 2480*10 = 24,800 passengers per hour. If the accumulative passengers are close to or exceed this value, short turning pattern with an original departure headway as 6 min should be considered.
4.2 Results analysis
New short turning route and original route
Type  Departure Station Number  Terminal Station Number  Freq. (per hour)  Route Status 

Original  1  22  10  Fulllength (western) 
22  30  10  Fulllength (eastern)  
New  1  22  8  Fulllength (western) 
5  19  6  New  
221  30  10  Fulllength (eastern) 
Two original routes, one from East Xujing to Guanglan Rd. and the other from Guanglan Rd. to Pudong International Airport, were retained in the new operation pattern with adjustments of departure headway. A new route, namely short turning route, from Beixinjing to Longyang Rd., is proposed to increase transport capacity within the high demand zone. Dispatching frequency of the original routes is set as 10 trains per hour consistently. It is obvious that the departure frequency between Stations 5 and 19 is 8 + 6 = 14 trains per hour, higher than before while the others were kept unchanged or reduced. Accordingly, short turning pattern is reasonable to mitigate overcrowding, different from simply improving departure frequency. By this, overcrowding of the higher traffic demand zone is relieved on purpose without too much capacity wasting.
5 Conclusions

Short turning pattern is an effective method for relieving overcrowding without wasting resources.

Load factor is an influential and essential factor in determining short turning zone.

Case study of Shanghai Metro Line 2 indicates that a short turning route from Beixinjing to Longyang Rd. with a frequency of six trains per hour could significantly ease congestion without causing much wasting comparing with simply improving departure frequency.
Although the results are promising, additional studies may be conducted to improve model performance from following aspects. First, coordination of short turning pattern and existing operating patterns was not explored in this study. Consequently, results of the case study may be just a feasible solution rather than the global optimal solution for constraints of the existing operation plan and track infrastructure. Next, improving dispatching frequency and opening new short turning need monetary cost which is a practical and critical issue. The objective function of the proposed mathematical model unfortunately doesn’t provide the quantitative monetary analysis. A CBA (Cost Benefit Analysis) considering this solution compared with the improving departure frequency could also be a further research step. Finally, with the metro capacity increases, relieving overcrowding, or more flexible working hours, some passengers may change their trip time or mode, or at least both travel time and passenger demand are stochastic in reality. To this end, the proposed model could be extended as a Markovian Process to handle these potential issues.
Declarations
Acknowledgements
The authors would like to express their appreciation to Drs. ZhongRen Peng and Yi Zhu from School of Naval Architecture Ocean and Civil Engineering, Shanghai Jiao Tong University for their valuable suggestions and assistance in this study. The research was supported in part by the Shanghai Municipal Natural Science Foundation [17ZR1445500], China, the State Key Laboratory of Rail Traffic Control and Safety [RCS2018K010], Beijing Jiaotong University, and the Humanities and Social Science Research Project [15YJCZH148], Ministry of Education, China. Any opinions, findings and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the sponsors.
Authors’ contributions
XD proposed the initial model, performed the numerical experiment as well as result analysis, and drafted the manuscript. SG participated in the model implementation and calibration, as well as the design and data preparation of numerical experiment. LJ participated in the design of this study and the outcome analysis. DJS conceived of the entire framework of the study and participated in its design and coordination. All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
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