I Introduction
DialARide Problem (DARP) addresses the issue of doortodoor transportation service for the customers with high customer satisfaction. Nowadays, transportation services have increasing need in our daily life, and it started to directly impact our environment as well as quality of living. According to a study conducted by University of British Columbia, the road pricing or payperuse is the most effective way to reduce emissions and traffic [1]. DARP has many applications ranging from taxi services to autonomous cargo and ground operations at the airports.
DARP is an extension of pickup and delivery problem under the class of vehicle routing problem (VRP) [2]
. It is a combinatorial optimization problem with an objective function to minimise the overall cost while satisfying a specific set of constraints such as timewindow, maximum waiting time and maximum ride time to ensure highquality customer service. In this problem, a set of customers makes a request for pickup and dropoff at certain locations within a predefined timewindow. An approach to solve DARP based on dynamic programming has been proposed in
[3], in which divide and conquer method is used to solve the problem. However, the exponential relation between computational complexity and instance size severely limits the applicability to small scale instances.Branch and cut is a classical way of solving mathematical optimization problems. A new branchandcut method
[4]was proposed which employs cutting planes to produce mathematical formulation for DARP. The problem was initially formulated using Mixed Integer Linear Programming (MILP). Later Cordeau et. al.
[5] have revisited the mathematical formulation for DARP and the research community has addressed this version as standard DARP. A modified approach based on branchandcut algorithm [6] for pickup and delivery problems with time windows was solved for larger instances to achieve optimality. Recently, another variant of branchandcut [7] with less compact modeling helped to solve the previously unsolved benchmark instances for the heterogeneousDARP to optimality within a matter of seconds.Though the exact methods are useful to optimally solve the problem, the computational complexity of such methods is very high. Therefore, heuristic and metaheuristic approaches have become widely used techniques to solve DARP. One such technique proposed in [8], which has pioneered the heuristic approaches. A sequential insertion algorithm [9] has been designed for static DARP, which analyses the problem complexity, while offering flexibility to users. The literature also has archives of parallel insertion heuristics using the distributed computing technologies [10].
Dynamic fuzzy logic, a computationally efficient heuristic method was adopted to solve DARP [11]. The tabu search heuristic [5] aims to progressively explore the neighborhood structure from an initial solution while forbidding some solutions with similar attributes of recently visited solutions. It can be described as cleverly guided local search that is efficient in exploring the search space at the hope of finding global optimal solutions. However, the main drawback is the time spent while getting stuck in suboptimal solutions. In order to avoid that, a parallel tabu search heuristic was proposed in [12]. When infinite penalty is considered, the best solution is obtained irrespective of irrelevance in choice of initial static solution for the dynamic dial a ride problem.
A two phase heuristics approach [13] based on insertion and improvement phase was proposed for DARP by Beaudry et al. From the solution obtained by insertion heuristics, the algorithms selects best nontabu solution progressively while performing both inter and intraroute neighborhood evaluations. Recently, another variation of tabu search, named as granular tabu search algorithm [14]
that produces good solutions in short amount of time within 23 mins has been introduced. Also, this new method produces better results when compared to the classical tabu search, genetic algorithm and variable neighborhood search techniques.
In this paper, we propose an improved tabu search (ITS) heuristic for dialaride problem. We assume that the routing time and cost from each vertex to every other vertex are known apriori. The major contributions of the paper are summarized as follows:

Determining a computationally faster and reliable variant of tabu search for DARP by thorough investigation of various neighborhood evaluation and insertion techniques.

Proposed a new construction heuristic for tabu search to obtain good initial solution rapidly.

Designed a time window adjustment technique for faster solution convergence.

Implemented and tested the proposed techniques using various DARP test instances to verify the acceleration in convergence.
The remainder of the paper is organized as follows: Section II briefly discusses the DARP mathematical formulation. Section III details tabu search heuristic and several variants of tabu search using neighborhood evaluation and insertion techniques. Section IV presents the proposed ITS heuristic method. Section V illustrates the convergence analysis results for the proposed algorithm. Conclusions are provided in Section VI.
Ii Problem Formulation for DARP
Dialaride problem (DARP) is a variant of VRP that involves dispatching of a fleet of vehicles to transport customers between their desired pick up and drop off locations within specified time windows. The aim is to minimise the overall transportation cost of the vehicle by evading the longest route in tandem with providing superior passenger comfort and safety. DARP is mathematically formulated as an optimization problem with an objective function subjected to several constraints.
In dialaride problem, customer requests are served using vehicles. Each request consists of time window either for departure or arrival vertex. The objective is to minimise the travel cost subject to several constraints. Let =,, denotes the solution space. All solutions need to satisfy three basic constraints. Every route for a vehicle starts and ends at the depot and the departure vertex and arrival vertex must belong to the same route, and the arrival vertex is visited after departure vertex . Any solution that violate these basic set of constraints becomes infeasible. In addition, several other constraints need to be satisfied; the load of vehicle cannot exceed preset load bound at any time; the total route duration of a vehicle cannot exceed preset duration bound ; the ride time of any passenger cannot exceed the ride time bound ; the time window set by the customer must not be violated.
Four major constraints exist in dialaride problem: load, duration, time window and ride time constraints. Load constraint violation occurs when the number of passenger in a vehicle exceeds its load limit ; duration constraint violation happens when a vehicle exceeds its duration limit ; time window constraint violation appears when the time constraint is violated; ride time constraint violation occurs when a passenger is transported for a longer time than ride time limit . The constraints are given by Eq. (14).
(1)  
(2)  
(3)  
(4) 
The next section presents the variants of tabu search based on the neighborhood evaluation and insertion techniques.
Iii Tabu Search Heuristic
Tabu search is a higher level heuristic procedure to solve optimization problems, as originally defined by [15]. The methodology is efficient at escaping local optimal solutions with structured memories and tabu list. During the neighborhood transitions, cycling should be avoided to intelligently explore the search space for global optimal solutions. Intensification and diversification strategies have to be properly employed in order to attain optimal solutions. In tabu search, the recent neighborhood transition is recorded in a tabu list. On subsequent transitions, the recorded moves in tabu list is not considered. In this way, cycling of moves gets avoided. However, for some cases, if the objective function is below the best obtained cost, then aspiration level performs the move even if it is in tabu list.
[5] has successfully superimposed the methodology on local search with neighborhood reduction to solve DARP. In this work, the objective function is considered as travel cost with weighted penalties for additional constraints. At this case, the method could efficiently explore the search space through some infeasible solutions in the hope of finding global optimal solutions.
(5) 
The objective function is given by Eq. 1, where is the travel cost and , , , are the penalty coefficients which are initialized to 1. These penalty coefficients change periodically once the optimization process begins. In tabu search, the moves that result in both feasible and infeasible solutions are accepted during the optimization process. When a solution violates the constraints and become infeasible, the penalty coefficient for those constraints are increased by a factor and decreased by the same factor, when the constraint is not violated. In this way, the algorithm intelligently explores the search space in the direction where the constraints that are violated during the previous moves are relaxed.
In this paper, construction of initial solution, route optimization, intensification, diversification and the evaluation functions are adopted from Cordeau et al. [5].
Iiia Neighborhood Evaluation
An important aspect of Tabu search heuristic is the procedure for neighborhood evaluation technique. The objective of neighborhood evaluation is to reduce the constraint violations and assess the feasibility of the solution. There are three objectives in neighborhood evaluation: Reduction of timewindow constraint associated with requests (R1); Reduction of route duration constraint associated with vehicle (R2); Reduction of Ride Time Constraint associated with requests (R3). Fig. 1 depicts the three level neighborhood evaluation method for tabu search.
There are three neighborhood evaluation techniques: i) 1level ii) 2level and iii) 3level, which are ‘steps(12)’, ‘steps(16)’ and ‘full procedure’ adopted from [5] in their respective order. The objectives for each level is substratebased i.e., each level is built upon subsequent levels. 1level has the objective R1, 2Level has the objective R1 R2 and 3Level has objective R1, R2 R3.
IiiB Insertion Techniques
The size of neighborhood directly influence the computational complexity required during the optimization process. There are two insertion techniques carried out in this paper: i) onestep insertion and ii) twostep insertion. The convention was adopted due to nature of the neighborhood transition, which are: a) Single paired insertion (SPI) [16] and b) Neighborhood Reduction [5]. Fig. 2 illustrates the insertion of departure and arrival vertices into the routes.
For onestep insertion, the algorithm attempts to move each vertex pair from one vehicle to another in a single transition. After insertion of the vertex pair, the neighborhood solutions are evaluated using the objective function in Eq. 5. The neighbor with minimum cost is selected as the next move. Though the search process is very expensive, this type of move has the greatest potential for improvement in the objective function [16].
For twostep insertion, the algorithm attempts to move each vertex pair from one vehicle to another in two sequential steps. For each request, critical vertex indicates pickup or dropoff point that consists of narrower time window when compared to the noncritical vertex. The first step is the insertion of critical vertex into its best position, which gives with least value of objective function in Eq. 5. While holding the current position, the second step is the insertion of noncritical vertex into its best position. The second step consists of two possibilities: i) if critical vertex is departure node, then insert noncritical vertex only after the critical vertex, ii) if critical vertex is arrival node, then insert noncritical vertex only before the critical node. This technique significantly reduce the neighborhood from moves to , where is number of vertices in route .
One Step Insertion  Two Step Insertion  

1Level Evaluation  
2Level Evaluation  
3Level Evaluation 
Based on the mentioned neighborhood evaluation and insertion techniques, the possible combinations are listed in Table I. Each method is represented using the convention , where represents the neighborhood evaluation level and represents the insertion step.
Various benchmark instances for standard DARP are provided by Cordeau and Laporte in [5]. In these instances, the number of requests vary between 24 to 144 and the fleet size varies between 3 to 13. The capacity of each vehicle is set to 6, the maximum passenger ride time is 90 min, the maximum route duration is 480 min and the route planning horizon is 24 hrs.
The costs of the benchmark solutions provided by Parragh and Schmid [17] for the instances R1a, R3a and R6a are 190.02 , 532.00 and 785.26 respectively. Figs. 3, 4, 5 illustrate the convergence of each of these tabu search variants validated using these instances. Each colored line in the plot corresponds to a distinct variant of tabu search as listed in Table I. The xaxis of the plots corresponds to the simulation run time and the yaxis represents the gap (%) as given by (6
). We plot the median of the gap recorded from five independent simulations ran for a duration of five minutes. Where, ‘BKS’ corresponds to the best known solution for the problems. We decided to consider the median in order to restrict the effect of outliers on the analysis.
(6) 
From the convergence plots, the following inferences are observed: 1) Twostep insertion technique has faster convergence than Onestep insertion. 2) Onelevel achieves initial solution in the least time, followed by Threelevel and Twolevel neighborhood evaluation techniques. 3) Threelevel neighborhood evaluation technique converges deeper over time when compared to other techniques, which is followed by Twolevel, while Onelevel is the slowest.
We consider that as better variant as it converges to optimality in a lesser time, while employing threelevel neighborhood evaluation and twostep insertion techniques. However, takes more time to obtain a feasible initial solution. In order to address this issue, new techniques are proposed in the next section.
Iv Improved Tabu Search (ITS)
Tabu search heuristic has been extensively used in dialaride problem due to its comparatively faster execution. However, the time required is still significant, especially for larger instances. So, there is a need to optimize the algorithm in order to obtain good results in a reasonable time. From the analysis presented in Section III, it is clear that the algorithm takes longer time to converge and to obtain a feasible initial solution. In order to overcome this issue, two new methodologies are proposed: a) construction heuristic (CH) and b) time window adjustment (TW). The main objective of the improved tabu search is to obtain high quality solutions within a short time. The Sections IVA and IVB detail the proposed methodologies.
Iva Construction Heuristic
The convergence analysis presented for tabu search in Section III indicates that the time taken to find the first feasible solution for a problem highly depends on the quality of initial solution. In [5], a random initial solution is generated to start the search for global optimal solution. However, such methodology has higher tendency to random seed, and it takes longer time to find first feasible solution.
In this paper, a new construction heuristic is proposed to find high quality solutions more rapidly. In this method, an empty set of routes is created, and requests are sorted randomly. After the initial preparation, each request is inserted sequentially into the position that attains minimum objective function. Therefore, the objective function is formulated as in 5, with = = = = 1. The details of the proposed construction heuristic technique are described using Algorithm 1.
The next section discusses the proposed time window adjustment.
IvB Time Window Adjustment
According to the benchmark instances from [5], the DARP problem is modeled as set of requests with timewindow constraints for optimal allocation to vehicles. The objective is to minimise the travel cost, while satisfying the constraints such as time window and ride time constraints associated with requests, and route duration constraint associated with vehicles.
The DARP formulation given by [5] considers either inbound or outbound request. Here, one vertex is always critical, which has narrower time window and the other is noncritical with time window usually set to be between , where denotes the end of the day. In DARP, the ride time constraint is important for high user satisfaction. The service for a request starts at time . The service time at the pickup and dropoff point is indicated by . The ride time of the user as shown in Fig. 6 is bounded by the ride time constraint . In this work, a new time window adjustment (TW) method is proposed to improve the convergence of the tabu search heuristic towards global optimal solution. The methodology is illustrated using Figs. 7 and 8. Initially, the service of this request (outbound) consist of relaxed time window (at pick up point). When the time window of non critical vertex is constrained as per the proposed methodology, the direction of search is intensified towards more feasible region in the search space.
To adjust the time window of departure vertex (non critical), the earliest service time of departure vertex should not be earlier than from earliest service time of arrival vertex ; the latest service time of departure vertex should not be later than . The adjustment made to departure vertex are given as follows:
(7)  
(8) 
In Eq. 7 and 8, (=) and (=) are the earliest and latest time windows associated with the departure vertex that are adjusted to obtain and respectively.
Similarly, to adjust the time window of arrival vertex, the earliest service time of departure vertex should not be earlier than , while the latest service time of departure vertex should not be later than . The adjustments made to arrival vertex are given by Eq. 9 and 10.
(9)  
(10) 
In Eq. 9 and 10, (=) and (=) are the earliest and latest time windows associated with the arrival vertex that are adjusted to obtain and respectively.
The improved tabu search (ITS) is obtained by incorporating the two proposed techniques, i.e., construction heuristic (CH) and time window adjustment (TW) into the tabu search variant . ITS () is tested against the benchmark of [7] and the simulation results are presented in the next section.
V Simulation Results
The proposed ITS heuristic () is implemented in C++. Simulations have been carried out on a computer running 2.1 GHz Intel Xeon E52620 v4 processor with 128 GB RAM. The parameters suggested by [5] have been followed to conduct a fair comparison of the results obtained using the proposed heuristic with the existing tabu search [5] method. Alongside the variant of ; , a hybrid of with both construction heuristic and time window adjustment have been tested.
In Figs. 9(a)  9(t), we depict the progression of the solution towards the benchmark during the first sixty seconds of execution. In these plots, the blue and red lines correspond to the Cordeau’s tabu search [5] and the proposed ITS heuristic respectively. The plots show the median of gap obtained from fifteen independent simulations run for sixty seconds. From these plots, it is clearly evident that the proposed improved tabu search (ITS) heuristic outperforms tabu search (TS) [5] for all the DARP test instances.
The following inference is made from these numerical experiments: Construction heuristic (CH) along with the time window adjustment (TW) not only contributes to significant speed up in the convergence, but also finds good feasible solution in a shorter time.
In Table III, we compare the travel cost of TS and ITS at various time instances {1, 2, 5, 15, 30, 60} sec for the benchmark instances. The presentation style is adopted from [14]. It is observed that the solution quality of ITS is always better when compared to TS, especially during the first three minutes of the execution. Based on the best known solutions listed in the second column labeled with ‘BKS’ in Table III, it can be concluded that the proposed method attains near optimal solutions in shorter time. As mentioned in Section I, the proposed construction heuristic of the improved tabu search (ITS) helps to produce a good feasible initial solution rapidly. Table III provides an empirical evidence for this claim and presents a comparison of first feasible solutions for various test instances and the time at which they are obtained. The next section concludes the paper.
Vi Conclusion
In this paper, an improved tabu search heuristic has been proposed to solve static dialaride problem. Several variants of tabu search method based on the neighborhood evaluation and insertion techniques have been tested to analyse the convergence behavior. In the existing tabu search heuristic, two performance bottle necks have been identified: i) longer run time requirement to obtain first feasible solution and ii) slower convergence to the global optimum. To address these, two new techniques, i.e., construction heuristic and time window adjustment have been proposed to improve the performance. Simulation results for various test instances show that the proposed ITS heuristic not only improves the convergence, but also finds high quality solution faster. Moreover, the approach can be extended to dynamic DARP. Some other possible directions for future work could be the parallelization of the developed tabu search algorithm using GPUs to reduce the computation time.
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