## Isci.cl

Multiperiod Synchronization Bus Timetabling
Department of Transport Engineerig and Logistics
Transit network planning is a difficult task that is usually decomposed into different sub-
problems (Ceder, 2007; Desaulniers and Hickman, 2007) such as network design (deter-
mines the bus lines: routes, stops), frequency setting (defines a proper frequency for a
given bus line based on the transit network behavior), timetable generation (determines
the departure times of all trips of lines), vehicle scheduling (assigns vehicles to sets of trips
of each bus line), and crew scheduling (assigns trips to drivers). We study the Multiperiod
Synchronization of Bus Timetabling problem (MSBT) that determines the departure times
of trips of a whole day to maximize the number of synchronization events which allow well
timed passenger transfers and avoid bus congestion of different lines at common nodes
of the network (Ibarra-Rojas and Rios-Solis, 2012; Ibarra-Rojas et al., 2012). Such syn-
chronizations are defined as the arrival of two trips at a common node with a separation
time within a specific time window. Figure 1 shows congestion (a) and transfer (b) nodes
where large and small separation of arrivals are sought to avoid bus congestion and allow
Due to the parameters variability of the transit systems along the day, a common
approach to obtain solutions for the transit network planning for an entire day is divide
it into several planning periods. This manner a more accurate estimation of parameters
can be obtained and then the planner can implement deterministic approaches. Even so,
variability in a planning period of two hours may be large, leading to non-representative
solutions. Then, to build a timetable for the entire day, several timetables obtained from
single-period approaches must be merged using specific techniques (e.g. Ceder, 2007).

However, this kind of approaches lead to suboptimal solutions for the MSBT. Indeed,
Figure 1: Two types of synchronization nodes; congestion node and transfer node.

in MSBT each line has specific planning periods, i.e., planning periods may be different
for two lines. Moreover, synchronization events between periods are sought. Constraints
of the system are service regularity (separation times for consecutive trips limited by
headway bounds) and smooth transitions between periods (separation of last trip of a
period and first trip of the next one near to average headway). Figure 2 shows a line and
two planning periods s and s + 1 with frequency of 5 and 3 trips, respectively. The trips
are regular within each period and there is a smooth transition between s and s + 1. The
MSBT problem is NP-Hard and the different parameters for each period are a handicap
to implement tighter formulations developed for the single-period case that can be solved
with commercial solvers (Ibarra-Rojas et al., 2012).

Figure 2: Synchronization between trips belonging to different planning periods.

To solve MSBT, we implement two metaheuristic algorithms; Iterated Local Search (ILS)
and Variable Neighborhood Search (VNS) that are based on feasible departure time win-
dows obtained by the constraint propagation concept. In particular, our constraint propa-
gation takes advantage of the service regularity constraints to define feasible time intervals
of departure times for all trips. The main idea is that, given a departure time x for a trip p
of some line and assuming we have headway minimum and maximum bounds of h and H,
respectively, the departure time of a trip p > p must be within [x+(p −p)h, x+(p −p)H].

If we have more information about the departure times, we can propagate this information
to reduce these departure time intervals. In particular, we can build an initial solution
simply by randomly generating each departure time within its feasible departure time
interval. Moreover, we can also deduce feasible arrival time intervals to each node b by
shifting the feasible departure time interval by an amount of time (travel time from depot
to node b). These intervals allow to identify if a specific pair of trips could be synchronized
and to recompute the departure time for these two trips to guarantee their synchroniza-
tion. Therefore, one of the main contributions of this work is a tool to define and explore
Using the constraint propagation procedure, we define several neighborhood structures
based on shifting departure times of a single trip or an entire line to force the synchro-
nization between two different lines. The implemented local searches are first and best
improvement strategies to explore each neighborhood structure. Finally, we propose a
perturbation function that drastically modifies the current solution with the objective of
reach different zones of the solution space of MSBT. Since constructive algorithms and
local search procedures have a random factor, we implement multistart approaches for ILS
For the case of multiple periods, we generalize the instance benchmark of Ibarra-Rojas and
Rios-Solis (2012) that is based on information provided by a real bus transport company.

It is worth noting that a branch and bound algorithm (B&B) for solving a real sized
instance of MSBT usually does not give feasible solutions in an hour due to its extremely
slow convergence. Nevertheless, when the instance has the same parameters for all its
periods, then it is not hard to generalize the valid inequalities of Ibarra-Rojas et al. (2012).

This type of academic instance, allows us to have quality measures for the metaheuristic
algorithms we are proposing in this work. Therefore, we implemented the B&B algorithm
of CPLEX 12.3 for our academic instance of MSBT. We conclude that our proposed
metaheuristics are the only algorithms available for obtaining solutions for MSTB with
less than 13.5% of relative mean gap for instances up to 200 lines and 20 synchronization
Real life instances have parameter variation along the different planning periods of
the day. Moreover, the main advantage of the multiperiod approach is the chance to use
small planning periods with representative deterministic parameters. In these cases, the
synchronization between trips belonging to different planning period becomes necessary.

To illustrate this case, we designed 10 instances with 10 planning periods of one hour and
compare the solution obtained by our multiperiod approach (MSBT) and the procedure
of merging solutions of single period synchronization bus timetabling (SBT). The numer-
ical results (Figure 3) shown that the number of synchronizations increase drastically by
implementing our multiperiod approach for the synchronization of bus timetables.

Figure 3: Results of implement SBT and MSBT to obtain a timetable for 10 hours divided
into 10 planning periods of 60 minutes.

In summary, the MSBT is more suitable to model real transit systems. The flexibility
of the problem is based on considering small specific planning periods for each line and
synchronization events between trips belonging to different planning periods. Although,
our proposed solution methodology obtains high quality results in seconds, there is an
open door to explore the definition of more complex metaheuristics using our constraint
propagation procedure and the computation of tight upper bounds to measure the quality
of our algorithms for general cases.

A. Ceder. Public Transit Planning and Operation: Theory, Modeling and Practice. Else-
G. Desaulniers and M. Hickman. Public transit. Transportation, Handbooks in Operations
Research and Management Science, pages 69–127, 2007.

A. Eranki. A model to create bus timetables to attain maximum synchronization con-
sidering waiting times at transfer stops. Master’s thesis, Department of Industrial and
Management Systems Engineering, University of South Florida, 2004.

O.J. Ibarra-Rojas and Y.A. Rios-Solis. Synchronization of bus timetabling. Transportation
Research Part B: Methodological, 46(5):599–614, 2012.

O.J. Ibarra-Rojas, P. Fouilhoux, S. Kedad-Sidhoum, and Y.A. Rios-Solis. Valid inequal-
ities for the synchronization of bus timetabling. Technical Report PISIS-2012-02 78,
Graduate Program in Systems Engineering, UANL, San Nicol´

Source: http://www.isci.cl/tristan/data/Public%20Transport/TRISTAN8_paper_168.pdf

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