Research Article  Open Access
An Improved Model for HeadwayBased Bus Service Unreliability Prevention with Vehicle Load Capacity Constraint at Bus Stops
Abstract
This paper presents an improved model for improving headwaybased bus route service reliability at bus stops using realtime preventive operation control, taking into account dynamic interaction among random passenger demand, stochastic driving conditions of route segments, and vehicle load capacity constraint. In this model, the realtime information of passenger demand and vehicle operation is involved to predict the imminent unacceptable headway deviation, in the case of which some intime preventive control strategies are deployed according to the given control rules. As a case study, a single fixed bus route with highfrequency services was simulated and different scenarios of realtime preventive operation control were performed. Headway adherence and average passenger wait time were used to measure bus service reliability. The results show that the improved model is closer to the real bus route service, and using realtime information to predict potential service unreliability and trigger intime preventive control can reduce bus bunching and avoid big gap.
1. Introduction
Giving priority to the development of urban public transit is becoming the common view on reducing urban traffic jam and improving urban travel efficiency [1]. But it is not just a policy issue to attract more and more people to choose transit for travelling. A challenging problem faced by the government, researchers, and transit agencies is how to provide better transit service by using uptodate technologies.
Reliability is one of the most important attributes of quality of transit service and always the top concerned issue for both passengers and transit agencies [2, 3]. From the perception of the passengers, service unreliability means more average wait time, which is identified by Welding’s assertion that the more regular service means the lower average wait time for potential passengers, especially on highfrequency bus routes with random passenger demand [4]. For transit agencies, service delays and disruptions have a real monetary cost in terms of lower utilization of vehicles and operators, which account for 3–5% of operating and vehicle costs by conservative estimate [5, 6]. In terms of the causes of unreliability, running time variability and passenger demand fluctuation are generally noted to be significant factors for service unreliability [7, 8]. Moreover, the initial headway irregularity, either at the beginning or the midroute, will propagate downstream and this kind of propagation tends to worsen passenger load fluctuation and contributes to worse unreliability downstream [9, 10].
In order to improve bus service reliability, the provision of realtime information technologies has been recognized as a persuasive strategy in more and more countries. In recent days, realtime information technologies like automatic vehicle location (AVL) and automatic passenger count (APC) are used to monitor service and passenger demand, based on which different operation control strategies are adopted to control bus operation [11–15]. From the existing research, most of the control strategies were studied to restore service reliability after the occurrence of disruption [16, 17]. Considering realtime information collected by the advanced public transit system, however, it is expected to forewarn big headway deviation from the beginning of the route service and adopt some immediate preventive action to avoid potential service irregularity [18–21]. In addition, in a rich information environment, modelbased intelligent vehicle systems would help drivers to achieve the effectiveness of the vehicle operation control [22, 23]. At this point, an effective model for decisionmaking using realtime information is the key issue for realtime bus operation control.
In [20], disregarding the vehicle load capacity constraint, we developed a simulation model for fixedroute transit service unreliability prevention based on AVLAPC data, whose effectiveness was extensively demonstrated on a circular bus route [21]. However, in the real world, vehicle load capacity is a very important factor affecting bus service reliability from the perspective of passengers. Based on the main framework of the model presented in [20], this paper develops an improved model for improving bus service reliability by using instantaneous prediction and preventive control strategies on highfrequency bus route, considering the dynamic interaction among random passenger demand, stochastic driving conditions of route segments and the vehicle load capacity constraint. In addition, it presents a set of decisionmaking rules for the potential intelligent dispatching module.
This rest of the paper is organized as follows. The next section presents the measures of headwaybased transit service reliability. In Section 3, a vehicleloadcapacityconstrained preventive model is developed based on the main model framework in [20]. Section 4 presents a Monte Carlo simulation with the model embedded, which applies realtime preventive control approach to a simple fixed bus route with highfrequency services. Conclusions and future work are provided in Section 5.
2. Measures of HeadwayBased Bus Service Reliability
2.1. Coefficient of Variation of Headways ()
On highfrequency bus routes, passengers generally arrive at stops randomly disregarding the schedule. Therefore, headway adherence is the most important index for measuring service reliability. In the Transit Capacity and Quality of Service Manual (TCQSM) [24], the measure is based on the coefficient of variation of headways () of transit vehicles serving a fixed route arriving at a stop and is calculated as follows:
For fixedroute highfrequency bus services, the level of service (LOS) is classified according to the coefficient of variation of headways (see Table 1).
 
Note: (a) The probability means that a given transit vehicle’s headway will be off headway by more than onehalf the scheduled headway . (b) Applies to routes with headways of 10 minutes or less. Source: transit capacity and quality of service manual2nd edition. 
2.2. Average Passenger Wait Time
Welding’s average passenger wait time is also widely used to measure headwaybased transit service reliability, which is a function of the mean and variance of the headway [3]:
In (2.2), is the expected passenger wait time, is the expected headway, and is the variance of the headway. According to (2.2), when is a constant, the smaller means the less average passenger wait time .
3. Model Development
In order to improve bus service reliability, static preventive control strategies include route planning, scheduling, and fleet and labor management [25]. On dynamic bus operation control, an intime preventive strategy could be adopted to avoid big headway deviation before the imminent bus bunching or gap, which requires realtime bus service information for intime decision making of operation control. In terms of headwaybased preventive control, it is expected to keep Var in (2.2) close to zero as possible. Therefore, as done in [20], the prediction and forewarning of potential service irregularity using realtime information and the immediate action to prevent the headway from deviating too much is the underlying principle of this improved model.
Before the improved preventive model is formulated, we use a single fixed bus route for description. General assumptions are made and the notations used in this paper are listed below.(a)Vehicles arrive at each stop frequently (e.g., headways of 5 minutes), making it proper to assume random passenger arrivals at each stop. Moreover, the arrival rate and alighting fraction at a given stop do not change over the observed time period.(b)When a vehicle arrives at a stop, boarding and alighting of passengers can occur simultaneously (usually for vehicles with a front door for boarding and a rear door for alighting). The average boarding time and alighting time of each passenger are constant for simplicity.(c)The dwell time of a vehicle at a given stop is the maximum of the boarding time and the alighting time.(d)The running time of a vehicle in between two stops is random, but still predictable and controllable to some extent.(e)Considering the realtime information of passenger demand and vehicle operation, when a vehicle leaves the current bus stop, the dwell time and running time of its preceding vehicle, as well as its own leading headway and the onboard passenger number, are known for decision making of a preventive control strategy.(f)Considering vehicle load capacity, those passengers who are not able to take the current fully loaded bus have to wait for the next bus. Vehicles are not allowed to overtake each other under preventive control.
A set of variables will be used for describing the interaction between the vehicles and passengers on this fixed bus route.= the leaving headway of the vehicle departing from the stop;= the running time of the vehicle from stop to stop;= the dwell time of the vehicle serving passengers’ boarding and alighting at stop;= the random number of passengers boarding the vehicle at stop;= the number of passengers who are not able to get on the vehicle at stop due its capacity constraint and full loading;= the random number of passengers alighting from the vehicle at stop;= the number of onboard passengers of the vehicle when it departs from stop;= the passenger arrival rate (passengers per minute) at stop;= the passenger alighting fraction of the onboard passengers at stop;= the average alighting time for each passenger;= the average boarding time for each passenger;= the vehicle load capacity.
3.1. Dynamic Interaction between Passengers and Vehicles
The routelevel bus service system is a typical discrete dynamic system. The dynamic interaction between passengers and vehicles will be described based on the main model framework presented in [20]. The transit vehicles spend time on both traveling in between stops and dwelling at stops to serve passengers’ boarding and alighting. The dwelling time of the vehicle at stop is decided by the maximum of the times of passengers’ alighting and boarding. According to the assumption (b), will be decided by
Disregarding the vehicle load capacity , and will be calculated or predicted by (3.2)
Considering the constraint of the vehicle load capacity, those passengers who are not able to get on the vehicle at stop will have to wait for the vehicle . Thus we will have , , and in (3.3) as
When the vehicle is departing from stop , the number of onboard passengers could be observed according to the realtime information. Thus, it is appropriate to predict the minimum based on the observed, say, the vehicle will have to dwell at stop at least for a total alighting time, which can be formulized as (3.4).
The boarding time will depend on the headway with given and ; while the headway depends on the trajectories of transit vehicles. One step of iteration is as follows: where is the difference in running times between bus and its preceding bus when they arrive at stop ; is the difference in dwell times between bus and its preceding bus when they dwell at stop .
The dynamic iterative calculation of the headway is as follows: Particularly, let , the iteration will begin from the starting stop to the current stop, then,
From (3.8) and (3.9), the headway depends on the previous headways, the accumulated running time differences and dwell time differences, which present a theoretical proof for that the initial headway irregularity, either at the beginning or the midroute, will propagate downstream. This kind of propagation is caused by the dynamic interactions between passengers and vehicles, and the worsen vehicle load fluctuation would contributes to the worse unreliability downstream, vice versa.
3.2. RealTime Preventive Bus Operation Control
In a preventive model, the ideal headway will be always expected to be kept close to the scheduled headway stop by stop () along the bus route. Mathematically, is kept on the interval decided by plus or minus a permitted deviation () due to the stochastic operation environment as
With the given vehicle load capacity and the initial headway, the headway will mainly depends on the total running time difference and the total dwell time difference in between the first stop and the current stop . When it comes to the operation control strategies, two options of control strategies will be decided. One option is bus holding strategy for controlling the vehicle dwell time; the other option is bus speed adjusting for controlling the vehicle running time. The bus speed adjusting strategy will be preferred on highfrequency bus route for two aspects of consideration. On one hand, the bus holding strategy works well for improving service reliability, but it has some disadvantages: wasting time of the onboard passengers, occupying the pickup location at the bus station, delaying the arrival at the destination, and so forth [24]. On the other hand, the bus speed adjusting is acceptable and operational, which is benefited from the realtime traffic information technologies. Determining how to use the two strategies will be the key issue for the preventive control problem.
To illustrate this problem clearly, a simple example bus route is used (see Figure 1). On this single fixed bus route, the bus is leaving current stop to stop ; with the observed , , , and (according to the assumption(e)), we need to predict and suggest a reasonable running time andfor bus to keep an acceptable . Determining an adaptive control strategy will follow the steps below.
Step 1. To decide the predicted dwell time . As discussed above, a minimum dwell time is the alighting time and can be calculated by (3.4); and only if the boarding time is larger than the alighting time, the dwell time will equal to the boarding time. Here, we use a dummy variable to determine the predicted dwell time , which is calculated as
where
is the expected headway of the vehicle .
As discussed above, the vehicle speed control strategy will be firstly adopted and then will be the holding strategy. Therefore, the next step is to decide a reasonable running time.
Step 2. To decide a reasonable running time . A reasonable running time is decided jointly by the feasible running time and the acceptable running time. With the transfer of (3.5), an acceptable running time is given as follows:
A feasible running time is decided by the realtime traffic conditions in between stop and stop. Along with the development of intelligent transportation technologies, it will be easy to get the realtime road traffic information, such as current traffic volume and speed. Here we still use a prediction way to decide the feasible running time . It is acceptable to predict it based on or the statistical mean running time and the standard deviation , respectively,
In (3.14), and are nonnegative percentile fractions. In (3.15), and are nonnegative parameters used to adjust the deviation and reflect the realtime traffic conditions. Considering the combination of (3.13) and (3.14) or (and) (3.15), a reasonable will be located on the interval decided jointly by and :
Those values over the interval in (3.16) are the suggested running time for the vehicle leaving for stop . Theoretically, the interval is possible to be void, which means the acceptable can not be satisfied, either because the minimum is bigger than the maximum feasible running time or because the maximum is smaller than the minimum . In practice, the former reason is impossible due to an infinite ; then the bus will be suggested to run at any . The latter situation usually means the bad traffic conditions, and bus will be requested to run at the maximum feasible speed to prevent the gap. By using the theoretical analysis, we make some decisionmaking rules for the reasonable running time as follows:(1)If then ;(2)If and and , then ;(3)If and then ;(4)If and then ;(5)If and and then ;(6)If then .
4. Simulation and Results
4.1. Parameters Description
In order to clarify this model, the example bus route in Figure 1 is also used for the case study. This bus route has 11 stops which are scattered about in a fairly uniformity of running time, with buses being dispatched from stop 0 to stop 10 at 5minute headway . The vehicle load capacity is 80 persons ().
Each bus is expected to run in between two successive stops in 3 minutes (), leading to a total expected running time of 30 minutes from stop 0 to stop 10 in one direction. The dwell time at each stop depends on the passenger demand. On highfrequency bus route, usually passenger arrival at each stop is a Poisson process and the number of passengers alighting at each stop is subject to a binomial probability distribution based on the current passenger load on the vehicle [6, 13]. Here the passenger demand is characterized with a Poisson passenger arrival rate and a binomial probability of passenger alighting fraction . In addition, the boarding time and the alighting time per passenger are assumed to be 3.6 seconds and 1.8 seconds . Table 2 gives the parameters of the operating conditions of the example bus route.

4.2. Simulation Scenarios
For illustrative purposes, in a single simulation period 30 buses from stop 0 to stop 10 are observed and a total of 20 simulation runs are performed for four cases.
Case 1. Running time is fluctuated with and = 60 seconds.
Case 2. Running time is fluctuated with and = 90 seconds.
Case 3. Running time is fluctuated with and = 60 seconds.
Case 4. Running time is fluctuated with and = 90 seconds.
The initial conditions for each single simulation run includes: each bus leaves stop 0 precisely at its given headway, and the first bus is supposed to run in between two consecutive stops precisely with 3 minutes. The preventive strategies will be triggered by checking the realtime conditions following Steps 1 and 2 in Section 3.2.
4.3. Results and Discussion
The two metrics described in Section 2 are used to measure service reliability. The first one is the observed average headway and the coefficient of variation of headways () at each stop. Table 3 gives the observed average headway (Avg hdw), the standard deviation of headways (std hdw), and the coefficient of variation of headways () for each case.

According to Table 3, the average headways in four cases are below 6 minutes and the coefficients of variation of headways are below 0.2. It also shows that, in all four cases, the biggest coefficients of variation of headways appear at stop 5 due to the highest probability of passenger boarding and alighting. The routelevel changing of the coefficient of variation of headways is further shown in Figure 2. Compared with the results in [26], these results indicate good service unreliability prevention.
Comparing Cases 1 with 2, the running time fluctuation is the same with 40% of the scheduled running time, but the permitted headway deviation is different. From the simulation as shown in Figure 2, the coefficients of variation of headways at each stop in Case 1 are smaller than those of Case 2, which indicates that better service unreliability prevention has been achieved in Case 1. The same situation appears when comparing Cases 3 with 4. These results suggest that the tougher request for keeping headway adherence promises better bus operation control. The practical reason might be that a bus driver may pay more attention to improving bus service reliability under strict performance evaluation.
Comparing Cases 1 with 3, the permitted headway deviation is the same with one minute, but the running time fluctuation is different. In Case 3, the lower limit of the running time fluctuation is half of that in Case 1. The coefficients of variation of headways at each stop in Case 3 are smaller than those in Case 1. The same situation also appears when comparing Cases 2 with 4. The results suggest that, requested by keeping the same permitted headway deviation, the smaller lower limit of the running time fluctuation is more likely to result in better headway regularity.
The other metric is average passenger wait time. Figure 3 shows the average passenger wait time at each stop in four cases.
As shown in Figure 3, the average passenger wait time at each stop is controlled at a level of less than 3 minutes in all four cases, but it still has a growing tendency. The best preventive effectiveness appears in Case 1, the worst in Case 4. Compared with the routelevel changing of the coefficient of variation of headways, the growing tendency of the average passenger wait time is an interesting finding. This is noteworthy in that the intuition from the previous studies suggests that controlling one vehicle only by referring to its preceding vehicle might has a time lag of the travel time.
5. Conclusions and Future Works
This paper presents an improved preventive model for improving headwaybased bus service reliability at bus stops, considering the constraint of the vehicle load capacity, as well as the dynamic nature of the random passenger demand and the stochastic road traffic conditions. Standing on this point, this model represents a more realistic approach to characterize the routelevel bus service system and to utilize realtime information in a realtime decisionmaking context. As demonstrated in the simulation example, using some probability distributions of the running time and the constant passenger boarding time and alighting time, the model and the preventive control strategy are effective.
By using the simulationbased methodology, the model presented in this paper mainly presents a way of how to follow the steps and the decisionmaking rules to utilize the realtime information to make better decision. Nonetheless, a more practical decisionmaking supporting system or a more intelligent decisionmaking module will be more powerful and useful in the real application. In this sense, this model can be directly embedded in the advanced public transit system, assisting the dispatching center and the operators to utilize the realtime information including vehicle running time, speed, dwell time, passenger demand, and other unreliability causes to instantaneously predict and forewarn big headway deviation of highfrequency bus routes, based on which better decisions can be made regarding the use of preventive control strategies. This job has become our ongoing work, and a real bus route will be chosen in the city of Zhuzhou in China for testing the real effectiveness. With the real application and test, the improvements of this proposed methodology could be promised. More broadly, in terms of the preventive control strategies, the routelevel multivehicle collaborative control based on synchronous simulation optimization is another important future work.
Acknowledgments
This work was supported by National Natural Science Foundation of China (NSFC) under Grants nos. 61203162 and 71171200. This work was also supported in part by the Postdoctoral Science Foundation of Central South University, the Freedom Explore Program of Central South University (Grant no. 721500036), and the Fundamental Research Funds for the Central Universities (Grant no. 2010QZZD021).
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Copyright
Copyright © 2012 Weiya Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.