Hillel Bar-gera

ABSTRACT This article examines a continuous-flow analytic dynamic traffic assignment (DTA) model with consideration of isolated uncoordinated traffic signals. The dynamic network loading component of the DTA model relies on trajectories and anticipated arrival order to nodes in order to achieve consistency between flow propagation along routes and kinematic waves (physical queue) representation of single link traffic behaviour. We compare results between a model where queues with random arrivals and deterministic departure rates are captured by non-stationary cycle-to-cycle Markov chains, and a model where only exit capacity effects of traffic signals are considered. Numerical examples illustrate that overall the model with Markov chains behaves properly, and captures interesting impacts of random queuing traffic signal delays on route choice and network level DTA solutions. A particular focus of this article is the issue of solution stability and its relationship to model specification and discretisation. We discuss the connection between DTA models and general finite element models in this respect, particularly regarding lag options in the discrete form of the equilibrium condition. Our results regarding these options, known as ‘forward’ versus ‘backward’ Euler method, or as ‘reactive’ versus ‘predictive’ user-equilibrium, confirm previous findings. In addition, the results show that the model specification with Markov chain representation of traffic signals enhances solution instability, and exhibits spurious oscillations even if backward Euler method is used. The results suggest that longer route choice intervals reduce oscillations, contrary to the typical behaviour in finite element models where stability generally improves when the resolution is refined.

ABSTRACT Several urban traffic models make the convenient assumption that turning probabilities are independent, meaning that the probability of turning right (or left or going straight through) at the downstream intersection is the same for all travelers on that roadway, regardless of their origin or destination. In reality most travelers make turns according to planned routes from origins to destinations. The research reported here identifies and quantifies the deviations that result from this assumption of independent turning probabilities.An analysis of this type requires a set of reasonably realistic “original” route flows, which were obtained by a static user-equilibrium traffic assignment and an entropy maximization condition for most likely route flows. These flows are compared with those route flows resulting from the Assumption of Independent Turning Probabilities (ITP). A small subnetwork of 3 km by 5 km in Tucson, Arizona, was chosen as a case study. An overall “typical ratio” of 2.2 between original route flows and ITP route flows was obtained. Aggregating route flows to origin–destination flows led to an overall “typical ratio” of 1.7. Such deviations are particularly high for routes that go back-and-forth, reaching a ratio of more than 3 in certain time periods. Substantial deviations for origins and destinations that are on the same border of the subnetwork are also observed in the analyses. In addition, under the ITP assumption, morning rush hour traffic peaking is the same in all directions, while in the original flows some directions do not exhibit a peak in the morning rush hour period. Overall, the conclusion of the paper is that the assumption of independent turning probabilities leads to substantial deviations both at the route level and at the origin–destination level, even for such a small network of the case study. These deviations are particularly detrimental when a network is being modeled and studied for route-based measures of effectiveness such as the number and types of routes passing a point – for monitoring specified vehicles and/or managing detouring strategies.

ABSTRACT High-occupancy toll (HOT) lanes have become an increasingly prevalent form of congestion management pricing in the United States in the past few decades. The success of a HOT facility depends on the pricing scheme, which determines the utilization of the HOT lane, and the corresponding congestion relief on the parallel general purpose lanes. An additional complexity in determining HOT rates arises from the inevitable variability in travel demand inherent to transport networks. A successful tolling scheme must therefore be robust to changes in travel demand. Various tolling schemes for HOT facilities are examined in efforts to identify robust pricing policies. The expected performance and the corresponding variability of the facility are evaluated under each pricing scheme for different demand profiles. The focus is on noncorrelated demand uncertainties (i.e., number of arrivals during given time interval is independent of number of arrivals in preceding and following time intervals), which are modeled by considering the number of arrivals in each minute as an independent random variable with known distribution and time-of-day dependent mean. The performance model for a given demand realization is deterministic. Results show that a fixed toll can achieve about two-thirds of the benefit of an ideal HOT system. The performance of a prescheduled toll system is between the fixed toll and the ideal system: closer to the ideal when the coefficient of variation is below 40% and closer to the fixed toll otherwise. The performance of a relatively simple real-time system, with density-based linear adjustment to the prescheduled toll, is practically equivalent to that of the ideal system.

Most solution methods for the traffic assignment problem can be categorized as either link-based or route-based. Only a few attempts have followed the intermediate, origin-base dapproach. This paper describes the main concepts of a new, origin-based method for the static user equilibrium traffic assignment problem. Computational efficiency in time and memory makes this method suitable for large-scale networks of practical interest. Experimental results show that the new method is especially efficient in finding highly accurate solutions.

... Collaborative Colleagues: Paolo Toth: colleagues. Hillel Bar-Gera: colleagues. Diego Klabjan: colleagues. Roberto Baldacci: colleagues. Cenk Caliskan: colleagues. Christina Diakaki: colleagues. Lasse Fridström: colleagues. Vijay S. Nori: colleagues. ...

We present an origin-based algorithm for the traffic assignment problem, which is similar conceptually to the algorithm proposed by Gallager and Bertsekas for routing in telecommunication networks. Apart from being origin-based, the algorithm is different from other algorithms ...

ABSTRACT This paper presents a method to identify the set of routes and their flows in a user-equilibrium traffic assignment solution. We present a general consistency condition that is satisfied by any set of minimum-cost routes, and show how it can be used in choosing a set of routes that is likely to be similar to the set of user-equilibrium routes. The proposed consistency condition is also essential for finding the entropy-maximizing route flows solution, which may be regarded as the most likely one. An efficient method for finding the entropy-maximizing solution is presented. Numerical results on several networks, including one of large scale, demonstrate the effectiveness of the proposed method. In most cases the method achieves a duality gap of practically zero in a short computation time.

ABSTRACT The evaluation of crash severities and the estimation of crash costs are key elements in evidence-based traffic safety improvement programs. Police records include information on injury severity estimated at the scene either by a police officer or by a medical emergency unit. At a hospital, the accident injuries are examined and documented more thoroughly and thus provide a better basis for cost estimation. Using hospital injury data for priority ranking of transportation projects and in other decision-making processes requires linking medical and crash records. Unfortunately, only a modest portion of crash records can be linked to corresponding medical records, even if a person is hospitalized, because missing data hamper the linking process. This study proposes a new method to overcome these difficulties and to estimate the expected level of injury of individuals included in all police-reported crashes. This method is accomplished by developing statistical models based on the linked medical and crash records and by applying these models to the entire crash data set. A fundamental problem to overcome was the selectivity bias present in the linked data and caused by the injury criteria for directing individuals to hospitals. The concept and the development of the proposed method are presented. The method is illustrated with its components developed with Indiana linked data and applied to Indiana crash data for 2005 to 2006.

ABSTRACT This paper is concerned with the macroscopic merging behavior of traffic at fully congested freeway merges, where a queue is present at all (upstream and downstream) approaches. An existing theory states that this behavior is dictated by a fixed ratio between the two upstream merging flows, denoted as the merge-ratio. It has been further conjectured that the merge-ratio is equal to the capacity-ratio. This paper presents an effective method to estimate merge-ratios from extensive historical traffic data. The archived traffic data in the California PEMS (Performance Measurement System) from January 2004 to June 2008 are used to estimate merge-ratios at 15 different freeway-to-freeway merge sites (via connectors). Findings show that merge-ratios can be reasonably estimated by the ratios between the numbers of lanes on the merging approaches (lane-ratio), which is typically similar to the capacity-ratio. However, residual differences between merge-ratios and lane-ratios suggest that there are probably other influencing factors.

ABSTRACT The static user-equilibrium (UE) traffic assignment model is widely used in practice. One main computational challenge in this model is to obtain sufficiently precise solutions suitable for scenario comparisons, as quickly as possible. An additional computational challenge stems from the need in practice to perform analyses based on route flows, which are not uniquely determined by the UE condition. Past research focused mainly on the first aspect. The purpose of this paper is to describe an algorithm that addresses both issues. The traffic assignment by paired alternative segments (TAPAS) algorithm, focuses on pairs of alternative segments as the key building block to the UE solution. A condition of proportionality, which is practically equivalent to entropy maximization, is used to choose one stable route flow solution. Numerical results for five publicly available networks, including two large-scale realistic networks, show that the algorithm can identify highly precise solutions that maintain proportionality in relatively short computation times.

ABSTRACT User-equilibrium (UE) traffic assignment is widely used in travel forecasting models. Analyses of assignment results often require detailed information about route flows. For a given UE model, total link flows are uniquely determined, but there can be many route flow solutions. Our goal is to evaluate the potential disagreement between different route flow solutions for the same UE model. We considered a case study of a UE model for the Chicago region. We examined 1,000 different UE route flow solutions. Out of 127,248 routes used by UE solutions, 53,134 have alternatives. For 51,748 routes (97%) the flow is completely undetermined, meaning that it is zero in one UE solution and equal to the total origin-destination (O-D) flow in another UE solution. Significant differences were also found in the distribution of users of a given link among O-D pairs ("select link analysis"). We conclude that in analyses that require route flows, it is important to make a proper choice of a specific route flow solution, such as the maximum entropy user equilibrium route flow solution.

This paper examines the effect of the use of Center High Mounted Stop Lamp (CHMSL) on rear-end accidents, as reflected in Israeli police records from calendar years 1991-2002. The basic analysis, similar to that used in previous CHMSL studies, compares the involvement in accidents of passenger cars of model years 1994-1996 that are equipped with CHMSL with passenger cars of model years 1991-1993 that are not equipped with CHMSL. The number of involvements as the struck vehicle in a rear-end accident was used as the relevant measurement and the number of involvements as the striking vehicle in a rear-end accident was used as the reference measurement. The results yielded an odds ratio of 0.93. The explanation that the CHMSL is responsible for the 7% decrease is intuitively appealing and is consistent with previous findings. However, the strength of this evidence is marginal (p = 0.07). Additional analyses evaluated the model year effect in greater detail, in order to determine whether there exists a change point between 1993 and 1994 as would be expected from a CHMSL effect, or whether the effect is spurious. Detailed analyses were performed on the ratio of struck to striking involvements as well as the rates of involvement of both types. These analyses showed that (1) the chosen reference measurement is an appropriate one, but (2) the 0.93 odds ratio is quite possibly due to other reasons unrelated to the CHMSL, thus further limiting the confidence in CHMSL effectiveness.