Documents published in Scipedia

• Assessment of Level-Of-Service for Freeway Segments Using HCM and Microsimulation Methods

  D. Jolovic, A. Stevanovic, S. Sajjadi, P. Martin

  Collection of Transportation Research Procedia (2017).

  
  

  Abstract

  The Highway Capacity Manual (HCM) 2015 freeway facilities methodology offers a supplemental computational engine FREEVAL, which is a macroscopic/mesoscopic tool that enables users to implement HCM-based freeway analysis quickly and conveniently. On the other hand, Vissim is a microscopic simulation tool that enables users to model real-world conditions with high level of accuracy and comprehensiveness. Thus, the two tools represent quite opposite sides in freeway modelling – Vissim requires time-consuming preparation and calibration of the model but it usually provides benefits that are more comprehensive. FREEVAL requires less on input and calibration sides but its results may not be as beneficial as comprehensive and accurate as Vissim’s. The problem, that has not been addressed enough, is that we do not know how different their results are (when compared between themselves) and, at the same time, how close to the field conditions. Researchers and practitioners use both tools for freeway analysis and tend to compare the results directly. One of the commonly used performance measures is the Level of Service (LOS), which is used to quickly evaluate the freeway segment or facility performance. The HCM Freeway Facility Methodology uses density to estimate LOS. However, density is calculated differently in FREEVAL and Vissim, and comparing the estimated LOSs between the two may not represent a proper comparison. In essence, FREEVAL, in the under-saturated condition, estimates the density from the fundamental equations where the volume is estimated from the user entered demand and the speed is calculated using the statistical models provided in respective chapters of each segment type. On the other hand, Vissim tracks each individual vehicle as it moves along a freeway and calculates key performance measures by using individually modeled driver’s behavior. This paper aims to compare and contrast the methodologies behind the two tools and offer explanation and discussion of their outputs. The paper will cover four major HCM freeway segment types (basic, merge, diverge, and weaving) in under-saturated conditions. Field data will be acquired from a section of I-880 freeway in California. FREEVAL and Vissim models will be calibrated and validated using Mobile Century Data provided by University of California at Berkley and Caltrans Performance Measurement System. The output of both tools will be evaluated against the field data. The assessment should reveal the ability of each tool to replicate the real-world conditions. Paper results will contribute to the existing body of knowledge by filling the gap in the literature related to comparison and contrast of the key (LOS-related) performance measures of these two tools.

  Abstract
  The Highway Capacity Manual (HCM) 2015 freeway facilities methodology offers a supplemental computational engine FREEVAL, which is a macroscopic/mesoscopic tool that enables [...]

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• Capacity of Freeway Work Zones in Germany

  N. Heiden, J. Geistefeldt

  Collection of Transportation Research Procedia (2017).

  
  

  Abstract

  Work zones considerably influence the traffic flow quality on freeways. In work zones, the capacity can be significantly reduced due to narrower lanes, a reduced number of traffic lanes, and unfavorable roadway geometry. In order to evaluate the impact of work zones on congestion occurrence and resulting delays as well as to allow for a consideration of alternative work zone layouts in the planning process, models to estimate the capacity of work zones are required. The paper presents results from a comprehensive study of work zone capacity on German freeways. In the analysis, a large number of short-term work zones with temporary lane closures as well as long-time work zones with and without a reduction of the number of lanes were covered. Data from loop or radar detectors upstream of the work zones were analyzed. For the capacity estimation of long-term work zones, both deterministic and stochastic approaches were used. In contrast to long-term work zones, traffic flow data measured at short-term work zones include only one traffic breakdown observation in most cases. This breakdown often occurs immediately after the lane closure, which means that the pre-breakdown volume does not represent the capacity of the work zone. Hence, valid capacity estimates could only be determined for the post-breakdown congestion outflow. The estimated capacities were analyzed regarding the influence of the traffic, geometric and control conditions in the work zone. Relevant parameters are the number of lanes, the lane widths, the existence of divided lanes, the existence of a lane reduction as well as the share of commuters and heavy vehicles. For short-time work zones, also the side of the lane closure (left or right lane) has an influence on the capacity. As a result of the study, capacity estimation models for both short- and long-term work zones, which can be applied in work zone planning procedures, are provided.

  Abstract
  Work zones considerably influence the traffic flow quality on freeways. In work zones, the capacity can be significantly reduced due to narrower lanes, a reduced number of [...]

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• A Model for Estimating Free-Flow Speed on Brazilian Expressways

  G. Andrade, C. Pitombo, A. Cunha, J. Setti

  Collection of Transportation Research Procedia (2017).

  
  

  Abstract

  Free flow speed (FFS) is defined in the HCM as the “average speed of vehicles on a given segment, measured under low-volume conditions, when drivers are free to travel at their desired speed and are not constrained by the presence of other vehicles or downstream traffic control devices”. FFS is a very important parameter for the estimation of LOS and capacity for uninterrupted highways facilities. Ideally, FFS is determined using field measurements, but the HCM 2010 provides models for its estimation, when field data is not available. Such models must be recalibrated for local conditions when the HCM 2010 is adapted for use in countries outside North America. In this paper, Multivariate Analysis (MVA) techniques are used to investigate which infrastructure characteristics are related to FFS on Brazilian expressway facility segments, in order to provide a framework for the development of models to replace those used in HCM 2010 to estimate FFS. This study used a database with a large number of speed-flow observations obtained at 36 sites on Brazilian expressways (freeways and divided multilane highways), along with information on seven variables describing segment characteristics such as: expressway type, roadside environment, number of lanes, rise and fall, bendiness, access point density, and posted speed limit for passenger cars. Principal Component Analysis (PCA) was used to select the variables to be included in the proposed model. The results of the PCA indicated that three components are responsible for most of the variance on observed FFS: (1) posted speed, (2) the combined effects of road class, number of lanes and density of access points, and (3) the interaction of geometric design and roadside environment. Stepwise linear regression was used to fit a FFS model from these three components. This model was mathematically manipulated to provide a FFS that is structurally similar to the one used by HCM 2010 but in fact accounts for the interactions among variables. The proposed model can replace the HCM 2010 model and does not require the user to choose an initial value for the base FFS.

  Abstract
  Free flow speed (FFS) is defined in the HCM as the “average speed of vehicles on a given segment, measured under low-volume conditions, when drivers are free to travel at [...]

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• Proposed Data-Driven Performance Measures for Comparing and Ranking Traffic Bottlenecks

  D. Hale, A. Hajbabaie, J. Ma, J. Hu, H. Park, J. Bared

  Collection of Transportation Research Procedia (2017).

  
  

  Abstract

  To justify investments towards improved traffic operations, engineers and policy-makers need scientific and accurate methods of congestion measurement. However, status-quo methods are limited and/or outdated. Peak-hour analyses are becoming outdated as a sole source of traffic assessment, because they fail to account for changing conditions throughout the year. There has been a movement towards “reliability” modeling, which attempts to capture these annual effects. But due to significant input data and calibration requirements, the reliability models suffer from practicality issues. Next, there have been recent improvements in data-driven ITS technologies, which identify congestion in real time. However, there is room for improvement in the robustness of performance measures derived from these technologies. Finally, some engineers have compared and ranked congested locations (i.e., bottlenecks) on the basis of experience and judgment. Despite their cost-effectiveness, judgment-based qualitative assessments will lack credibility unless backed by quantitative results. In a recent Federal Highway Administration study, congestion measurement was a primary area of emphasis. This paper discusses project-specific software development, which produced new and innovative performance measures for congestion measurement. It will present concepts and evidence to imply superiority of the proposed new measures. This paper is intended to serve as a preview of a future full journal paper, which will rank ten or more real-world bottlenecks according to new and old performance measures, to demonstrate impacts of the new measures. It is hoped that the new performance measures will be adopted by states and/or commercial products, for a new level of robustness in congestion measurement.

  Abstract
  To justify investments towards improved traffic operations, engineers and policy-makers need scientific and accurate methods of congestion measurement. However, status-quo [...]

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• Identification of Factors that May Affect the Discharge Flow at a Bottleneck

  D. Yamaguchi, T. Okano, H. Sudo, M. Kobayashi, T. Ogihara, N. Izumi, A. Tanaka, T. Yoshii

  Collection of Transportation Research Procedia (2017).

  
  

  Abstract

  This study investigates various factors that cause the reduction of discharge flow at a bottleneck on an urban expressway. The current worst bottleneck on Tokyo Metropolitan Expressway Network seems to be combined complicate factors that affect the discharge flow at the bottleneck. 5 causing factors, merging ratio, longitudinal gradient, duration time involved in congestion, environment of backlight and brightness are considered, and the impact of these 5 causing factors are investigated. Then the results supported the hypothesis that all of the 5 causing factors affect the discharge flow rate.

  Abstract
  This study investigates various factors that cause the reduction of discharge flow at a bottleneck on an urban expressway. The current worst bottleneck on Tokyo Metropolitan [...]

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• Improving Traffic Flow at Long-term Roadworks

  C. Schwietering, M. Feldges

  Collection of Transportation Research Procedia (2017).

  
  

  Abstract

  Long-term roadworks on highways are vital as part of carriageway and bridge renovations. They generate bottlenecks and consequently congestion and accidents. If the number of lanes is reduced, it can also lead to significant capacity decrease. Even if it is ensured that all lanes are kept operative, narrow lanes and transitions will affect the capacity. The impact of roadworks on capacity have been analysed in detail by [BECK_2001]. Therefore, intelligent solutions to support the traffic flow at long-term roadworks bottlenecks have to be evaluated. For this purpose the TrafficChange concept was developed. TrafficChange describes the modules of tidal flows on long-term roadworks: - Module 1: Tidal Flow basic set - TrafficChange SE (quickly repositionable moveable safety barriers, so-called Quick-Change Moveable Barriers (QMB)) - Module 2: Attached Traffic Safety Equipment for Tidal Flow - TrafficChange VT including data collection, traffic monitoring, communication, energy supply and displays - Module 3: Repositionable congestion warning systems to safeguard congestion - Module 4: Core system for data analysis, monitoring and managing traffic (TrafficChange Center) as cloud-based service With these components, a dynamic assignment of lanes to higher demanded direction can be reached. Main application fields of TrafficChange are: - Maintenance of bridges - Renewal of road pavement, road surface restoration (entire carriageway) - Tunnel renovation with two-way traffic in the operational tube - Temporarily adaption of the number of operational lanes in dependency of the traffic demand in order to enlarge working space of a roadworks The use of TrafficChange has beneficial effects on capacity and thus on traffic flow. In case of traffic lane reduction due to roadworks, TrafficChange can dynamically assign the remaining traffic lanes according to the demand of both driving directions. Even if all traffic lanes are kept operational during the roadworks, TrafficChange provides several advantages. In case of roadworks on a six lane facility (three lanes in both directions), usually one lane has to be managed separately on the carriageway of the construction field. This has a massive negative impact on the roadworks progress and the completion timetable. When applying TrafficChange this single separate lane is not necessary. Simultaneously this leads to significant capacity increase for this lane, since all lanes are managed on one carriageway and overtaking opportunity is provided. Evaluations of roadworks in Germany have shown that using TrafficChange will enhance the capacity by 10-15% for the driving direction of the construction site. One the other hand, for the direction with low demand, capacity is reduced temporarily, since one lane less is available. When demand for this period is low and can be managed by using the remaining lanes, the traffic flow quality stays efficient. For an economically efficient use of TrafficChange, the demand peaks should not occur simultaneously in both directions. Therefore, for each roadworks it needs to be evaluated individually whether TrafficChange can efficiently improve the traffic flow. For evaluation of the effects, the TrafficChange system is implemented in a microscopic traffic simulation tool, which can be adapted easily for different settings of the roadworks to be evaluated. As input values, the number of available lanes, roadworks design, traffic demand for each required scenario (such as weekday, weekend, holiday) and the length and planned duration of the roadworks are required. As key values, the simulation determines the congestion length, congestion duration and delay times. With these values, the benefit of TrafficChange can be determined. The efficiency estimation is determined by accumulation of total delay times for each required scenario, multiplied by the amount of days this scenario occurs during the roadworks is active, for both the comparison case (roadworks without TC) and target scenario (roadworks with TC). By using current travel time expenses for passenger cars and heavy vehicles, the benefit can be expressed in costs. By the use of TrafficChange, the entire carriageway can be closed so that considerable savings of working period and economic losses could be achieved although the number of operative lanes is lower than in the comparison case. The TrafficChange concept leads to improved traffic safety and optimized traffic flow at roadworks as well as reduced roadworks duration and therefore high economic efficiency that can be determined by the simulation tool TCSim. Literature [BECK_2001] - Beckmann, A., Zackor, H. (2001): Study and calibration of procedures for current estimation of the duration and length of traffic jams as a result of single-day and long-term construction sites on motorways, Forschung Straßenbau und Straßenverkehrstechnik, Volume 808, Bonn, Germany, 2001 [EASY_2009] - Easyway (2009) : Mobile Congestion Warning Systems at lorm term roadworks increase traffic safety – Efficiency of Mobile Congestion Warning Systems, Offprint of the ‘Landestelle für Straßentechnik Baden-Württemberg’, Stuttgart, 2009, www.easyway-its.eu [SUEM_2012] - Sümmermann, A. (2012): Examination of traffic safety and traffic flow in work zones on German Motorways, Aachener Mitteilungen Straßenwesen, Erd- und Tunnelbau, Volume 59, Aachen 2012 [VOLK_2014] - Volkenhoff, T. (2014): Model-based derivation of operating conditions of a video-aided incident and work zone management system in construction sites on German motorways, Aachener Mitteilungen Straßenwesen, Erd- und Tunnelbau, Volume 61, Aachen 2014 [HMSV_2012] – HMSV (2012): Congestion Evaluation 2011. Hessen Mobil – Straßen- und Verkehrsmanagement. URL: http://www.staufreieshessen2015.de, requested on 04.07.2012

  Abstract
  Long-term roadworks on highways are vital as part of carriageway and bridge renovations. They generate bottlenecks and consequently congestion and accidents. If the number [...]

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• Evaluation of Variable Speed Limit Pilot Projects for Texas Department of Transportation

  B. Kuhn, K. Balke, R. Brydia, L. Theiss, I. Tsapakis, L. Ruback, M. Le

  Collection of Transportation Research Procedia (2017).

  
  

  Abstract

  In May 2013, the regular session of the 83rd Texas State Legislature passed House Bill (HB) 2204 related to the establishment of a variable speed limits (VSL) pilot program by the Texas Transportation Commission. The bill was signed into law by the governor in June 2013. In December 2013, the Texas Transportation Commission established Rule §25.27 of the Texas Administrative Code authorizing and requiring the Texas Department of Transportation (TxDOT) to implement a variable speed limit pilot program to “study the effectiveness of temporarily lowering prima facie speed limits to address inclement weather, congestion, road construction, or any other condition that affects the safe and orderly movement of traffic on a roadway.” The goal of the pilot program was to deploy VSL in up to three locations to test the concept under three operational conditions and to determine the impacts of VSL on facility operations and safety. The specific objectives of the pilot project evaluation were to determine how much congestion was reduced in the area impacted by the implementation of VSL, to understand the users’ perceptions of the VSL systems, to assess the safety impacts of VSL, and to determine the overall costs and benefits of VSLs. Up until this point in time, VSL was not a legal alternative for freeway operations in Texas. After the bill was passed, TxDOT worked with the Texas A&M Transportation Institute (TTI) in selecting pilot project sites for the development, implementation, and evaluation of VSL. The VSL systems were to be used for the purpose of controlling speeds at sites that have (a) construction work zones, (b) weather-related events, and (c) urban congestion. This paper presents the overall approach to the development, implementation, and operation of the VSL pilot projects along with the analysis of the installations with respect to their impact on congestion, safety, users’ perception, violations, and benefit-cost of the projects. It also presents a number of lessons learned throughout the course of the pilot tests that provide beneficial insight into how to improve similar projects for permanent installations. Based upon the limited data available for the VSL pilot project, it was determined that VSLs would be beneficial if implemented to address inclement weather, congestion or road construction. VSLs had a safety benefit at each location and motorists had a clear understanding of the purpose of the VSLs.

  Abstract
  In May 2013, the regular session of the 83rd Texas State Legislature passed House Bill (HB) 2204 related to the establishment of a variable speed limits (VSL) pilot program [...]

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• Safety of Characteristic Subsections of Roadwork Zones on Motorways

  J. Bakaba, J. Ortlepp

  Collection of Transportation Research Procedia (2017).

  
  

  Abstract

  Roadwork zones interrupt the flow of traffic and shall reduce road safety of motorways (autobahns), at least in specific sections. Motorways are the fastest roads in the German roadwork and have the highest level of safety – higher than either roads inside built-up areas or other roads outside built-up areas. Nevertheless, a total of 17, 172 accidents involving personal injury occurred on motorways in the year 2011, 1, 118 of them at roadwork zones. A total of 18 people were killed and 220 seriously injured. The principal aim of this research project was to find out if either roadwork zones on motorways are more unsafe than other motorways sections or they are specific subsections inside of roadwork zones or near them which are particularly unsafe. The research project consisted of an analysis of the statistics in the vicinity of all roadwork zones on German motorways (autobahns). Road accidents recorded by the police for a period of at least 12 months at 76 selected roadwork zones and a retrospective accident analysis of 12 completed motorway roadwork zones were macroscopic analyzed. In addition, a microscopic analysis of the accident occurrence was carried out of eight existing roadwork zones with a total length of 57 km and included also the analysis of the driving behavior of car and truck drivers throughout the length of the roadwork zones and/or in characteristic subsections in or near them. The results of the research project confirm that accidents at roadwork zones on motorways are generally accidents involving minor damage to property. The accident severity (fatalities per 1, 000 accidents involving personal injury) of accidents that occur on sections of motorway without roadwork zones is up to 1.6 times higher than that of accidents that occur at roadwork zones. The study shows different accident rates and accident cost rates for some characteristic subsections along the roadwork site. Nevertheless, there are problem areas in terms of road safety, particularly at the beginning and end of roadwork zones, lane realignment points, carriageway changeover points and temporary slip roads. These characteristic subsections of roadwork zones have an influence on the road safety. The same applies to the impact of how the traffic is routed, the length of the roadwork site or specific road equipment (e.g. road signs against a yellow background).

  Abstract
  Roadwork zones interrupt the flow of traffic and shall reduce road safety of motorways (autobahns), at least in specific sections. Motorways are the fastest roads in the German [...]

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• Functionally Hierarchical Road Classification Considering the Area Characteristics for the Performance-oriented Road Planning

  A. Goto, H. Nakamura

  Collection of Transportation Research Procedia (2017).

  
  

  Abstract

  Functional hierarchy of road network classifies individual roads into several levels by taking account of the priority for mobility, access or residential functions, in order to manage traffic efficiently by segregating through traffic from accessing, parking and non-motorized traffics. This concept has been widely recognized and applied to road planning, design and operation in several countries such as Germany, the US and Australia. Their guidelines state the target performances (i.e., LOS or travel speed in general) of each hierarchical levels being used for the performance evaluation. However in Japan, target performances of roads are not shown in the current guideline, since a concept of the performance-oriented planning and design is still uncommon in practice. As a result, the operational performances are still not satisfactory enough especially on highways. In such a situation, this study attempts to find out the hierarchical road classification applicable to Japan, considering the distribution of region/district centers as well as the impact of terrain. The main questions are how many hierarchical levels need to be defined and how much travel speed and road spacing (or access distance) should be set as the targets for each level, so that the network performance, namely travel time between region/district centers, can achieve the given target values. The basic idea comes from the German guideline RIN, which states the target travel times by region/district center (central place), classifies roads and define their target travel speeds based on their connecting centers. However, it does not clearly mention how the difference in distribution of region/district centers and the impact of terrain can be reflected to set the efficient hierarchical classification. For example, the characteristic of metropolitan areas is quite different from that of mountainous areas in terms of trip lengths between region/district centers of which travel times are to be evaluated as network performance. In addition, because of the impact of terrain, construction cost per unit is generally quite higher in mountainous areas than in flat areas, whereas travel demand is usually lower in mountainous areas. This study incorporates these issues into road classification and demonstrates its impact for more efficient network system planning. The methodology consists of four steps: (1) defining the region/district centers to be connected and setting their target travel time, (2) developing alternative road classification scenarios, (3) mathematically formulating origin-destination travel time in a grid road network under the certain classification and (4) evaluating the classification scenarios considering the difference of average trip lengths of each combination of centers as well as the difference of system cost due to the impact of terrain and travel demand. Although the original intention of this study is to apply the result to Japan, the methodology itself is generally applicable to any region. At the end, case studies are conducted for several simplified areas for reflecting their typical characteristics of region/district centers distribution and terrain in Japan. Through them, the hierarchical road classifications by area type are suggested as a conclusion.

  Abstract
  Functional hierarchy of road network classifies individual roads into several levels by taking account of the priority for mobility, access or residential functions, in order [...]

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• A Stochastic Car Following Model

  A. Kendziorra, P. Wagner, T. Toledo

  Collection of Transportation Research Procedia (2017).

  
  

  Abstract

  Car-following models are an essential part of microscopic traffic simulations. For research regarding traffic safety, traffic simulations need to simulate traffic safety related aspects realistically. That means, for example, accidents and near accidents shall occur in the same quantity and in the same way as in reality. Such simulations can be used to make statements about traffic safety with respect to traffic influencing factors and conditions. However, most car-following models are deterministic and do not incorporate uncertainty and fluctuations of perception and behavior. Also, they are explicitly conflict free. Therefore, they are unsuitable for simulating traffic in the desired way. For that reason, a car following model fulfilling the requirements above was developed. Inspired by [1], our car-following model is nondeterministic and data-driven. It is based on the data set of the Intelligent Cruise Control Field Operational Test [2], in which instrumented vehicles have been used by 108 voluntary drivers for several weeks yielding trajectories of approximately 88, 000 driving kilometres. In our model, the acceleration a of a vehicle depends on the following input: -v, the velocity of the vehicle, -∆v, the difference between the velocity of the preceding vehicle and its own velocity, -g, the gap between the vehicle and the preceding vehicle and -aL, the acceleration of the preceding vehicle. The acceleration a underlies a probability distribution that depends on v, ∆v, g and aL. That means, if v*, ∆v*, g* and aL* are the current values for v, ∆v, g and aL, then there exist a probability distribution F(v*, ∆v*, g*, aL*) and the actual acceleration a will be drawn from F(v*, ∆v*, g*, aL*). For each tuple (v, ∆v, g, aL), the probability distribution F(v, ∆v, g, aL) was determined by the data of the FOT. For that, the data of velocity, velocity difference, gap and acceleration of the preceding vehicle were binned, and for each tuple (v, ∆v, g, aL) of binned values, the expected acceleration, the variance and the type of distribution were computed and stored in look-up tables. During a simulation, the probability distribution F(v, ∆v, g, aL) to a given tuple (v, ∆v, g, aL) can be recovered by these look-up tables. To achieve probability distributions that result in a well behaving car-following model, the data had to be corrected (due to erroneous sensor data) and filtered (due to situations in which the driver reacted to other influences besides the preceding vehicle, e.g. red traffic lights) in numerous steps. Here, we will present the mentioned correction and filtering steps in detail. Further, we will discuss the derived probability distributions and the calibration of the model. Finally, the model will be evaluated and compared to other existing car-following models in several scenarios with respect to various criteria, e.g. the number of accidents, the distribution of surrogate safety measures, and the fundamental diagram.

  Abstract
  Car-following models are an essential part of microscopic traffic simulations. For research regarding traffic safety, traffic simulations need to simulate traffic safety related [...]

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