Speed-flow Relationship Research Articles

Macroscopic and microscopic analyses of managed lanes on freeway facilities in South Florida

Abstract As congestion grows in metropolitan areas, agencies tend to utilize managed lanes on their freeway systems. Managed lanes have several forms and names, such as high-occupancy vehicle (HOV) lanes, high-occupancy toll (HOT) lanes, express lanes, and bus-only lanes. Although managed lanes have received significant attention as they increased the overall throughput and improved mobility without adding more lanes, little has been known about their operational capabilities. In addition, calibrating managed lane facilities can be challenging as they do not necessarily follow the same behavior with general purpose freeway lanes. This paper presents an operational analysis of two HOT lane segments located in South Florida. The sites are one-lane and two-lane segments separated by flexible pylons (FPs). The paper includes a macroscopic capacity analysis, and a microscopic calibration of the two sites using VISSIM microsimulation. The research findings assist in determining the capacity and speed-flow relationship of these segments, and also provide guidance for microsimulation model calibration for practitioners. The results of the study indicate that the percent drop in capacity for the one-lane FP site is 7.6% while the flow did not substantially change after the breakdown in the two-lane FP site. The research findings also include guidelines for simulating the breakdown events and calibrating one-lane and two-lane managed lane facilities in VISSIM microsimulation software. The Wiedemann car-following parameters (CC0 = 3.9 ft, CC1 = 1.9 s, CC2 = 26.25 ft, CC4 = −0.35, and CC5 = 0.35) provided the best fit for the one-lane FP site, while the combination (CC0 = 4.92 ft, CC1 = 1.9 s, CC2 = 39.37 ft, CC4 = −0.7, and CC5 = 0.7) parameters is recommended for the two-lane FP site.

Model of Traffic Speed-Flow Relationship at Signal Intersections

Speed-flow relationship is the fundamental for the traffic simulation and traffic volume forecast. Traditional quadratic polynomial model can’t reflect the saturate flow at signal intersections. In order to determine the speed-flow relationship at signal intersections, the speed and time-headway of vehicles at two signal intersections were investigated and the accuracy of software used to get the speed was tested. After vehicle starting-up from queuing, the time-headway reduces gradually with the increase of speed. The relationship of power exponential function between speed and time-headway is formulated. Traffic volume can be calculated by the vehicle time-headway. Then the speed-flow relationship was developed and an S-shaped curve model was built in this paper. In the S-shaped curve model, traffic flow approaches to the saturate when the speed doesn’t increase. Thus, S-shaped curve model is better to describe the speed-flow relationship at signal intersection. The results can provide a reference to determine the parameters in traffic simulation and for the study of level of service of intersections.

Speed-Flow Relationship at Basic Segment of Federal Highway

Theoretically, at many traffic facility during peak hour, speedflow relationship is only appeared as a normal flow, represented by half of speed-flow curve. Forced flow will only occur during severe overcrowding resulted in drivers trying to use all the space in a way so that the amount of overcapacity. The purpose of this study is to analyse the relationship between speed-flow traffic flows along a basic segment. The objectives of this study is to analyse the time series of speed and flow at basic segment of A550 and A551 study location for 24 hours in a month as well as to determine the maximum flow and critical speed through the speed-flow models. Data obtained from this study were analysed using video recording and image processing techniques. In order to achieve these objectives, the Federal Highway 2 was selected as location for the study. Results show that the speed – flow diagram in one month on both A550 and A551 obtain the maximum flows and critical speeds of 4200 and 5320 veh/hr/ln and 55 and 60 km/hr, respectively.

Application of Traffic Thermostat for I-30 Managed Lanes in Dallas, Texas

Managed lanes have historically experienced an evolution in operational policies. Over time, speeds on a managed lane may degrade because of oversubscribed demand. Operators have strategies to mitigate problems and maintain optimal performance; these strategies include changing the occupancy requirement, varying tolls (if the facility is priced), and altering transit service. Assessing the potential impact of operational strategies is a challenge. The traffic thermostat tool is a software-based guide that helps select specific strategies projected to influence the metrics for each goal. This paper reports on how the tool was adapted for use on the I-30 managed lanes in Dallas, Texas, a high-occupancy-toll lane with reduced toll rates for qualifying carpools. An example scenario shows the consideration of travel speed and throughput goals, with acceptable performance thresholds of 50 mph and 5,700 persons per hour for the entire facility. The thermostat estimated values of projected speed and throughput for selected operational fixes by using the calibrated speed–flow relationship from the regional travel demand model. Results from a quantitative travel survey helped to derive mode shift elasticities for use by the tool. Overall, the dynamic nature of demand, diversity of user groups, ambiguity with exogenous factors (e.g., regional unemployment and fuel prices), and need for extensive data on the lanes led to uncertainty and difficulty with prediction capability. However, use of the traffic thermostat can show policy makers and others the inherent challenge of performance management for managed-lane facilities.

Avaliação de medidas de desempenho para rodovias de pista simples obtidas a partir de relações fluxo-velocidade

Para estimar o nível de serviço em rodovias de pista simples, o Highway Capacity Manual 2010 (HCM2010) adota como medidas de desempenho a porcentagem de tempo viajando em pelotões (PTSF) e a velocidade média de percurso (ATS). A PTSF, no entanto, é praticamente impossível de ser observada em campo. Na literatura, algumas pesquisas propõem medidas de desempenho alternativas à PTSF que podem ser coletadas diretamente da observação do tráfego. Além disso, as relações fluxo-velocidade têm um papel fundamental na determinação do nível de serviço no HCM2010 e no HBS2001. A meta deste trabalho foi avaliar medidas de desempenho provenientes de relações fluxo-velocidade que pudessem ser adequa-das para descrever a qualidade de serviço em rodovias de pista simples no Brasil. Usando dados de tráfego sintéticos produ-zidos com uma versão do CORSIM calibrada para rodovias no Brasil, foram obtidas relações fluxo-velocidade para diferentes condições (geometria, porcentagens de veículos pesados e velocidades de fluxo livre). Comparações dos valores provenientes desses modelos com dados de campo indicaram que a velocidade média de percurso dos automóveis e a densidade para automóveis poderiam substituir os critérios atuais do HCM para estimar o nível de serviço em rodovias de pista simples no Brasil.

Effects of Geometry on Speed–Flow Relationships for Two–Lane Single Carriageway Road

Speed–flow relationship is usually used as a basis for the performance and capacity analysis of a highway segment. This paper presents the preliminary result of a study carried out to evaluate the effects of road geometry on speed–flow relationship for two–lane single carriageway roads. Speed and traffic volume data at five uninterrupted two–lane single carriageway road segments located in different parts of Malaysia were collected using an automatic traffic counter equipment. Result of the analysis shows that speed and traffic flow variables for a single carriageway road were linearly related. The percentage of no passing zone, road bendiness and hilliness, and the presence of minor junctions were found to have negative effects on the speed–flow curve. The speed–flow–geometry relationship developed in the study produces the estimates of travel speed higher than the values estimated using both USHCM and British models and lower than the MHCM’s model. However, more data representing large range of traffic conditions and roadway characteristics are required to enhance the accuracy of the speed–flow–geometry relationship developed in the study.

Effect of directional distribution on stairway capacity at a suburban railway station

Flow condition on undivided stairways becomes more acute during heavy inflow of pedestrian under restricted space. The study of pedestrian movement under such conditions is inevitable for understanding overall operation at a railway station. Limited studies have been reported in the past considering pedestrian movement under different flow and direction of movement under controlled conditions. Nevertheless, to this end, this study aims at investigating the effect of bidirectional pedestrian movement on capacity of undivided stairways located inside railway transit stations, where pedestrians face heavy opposing flow particularly during arrival of train. For this purpose, analysis has been carried out to comprehend the influence of directional distribution of pedestrian flow on walking speed and hence, also on capacity of an undivided stairway, taking a case of Dadar railway station in Mumbai, India. Collected data-sets were used for developing fundamental macroscopic speed-flow relationships. Based on directional distribution ratio, available space for individual direction was determined through acquisition of coordinates based on the geometry of stairways considered for this study. Study results show that unbalanced distribution ratio reduces the average walking speed in individual direction as well as capacity of the stairway. The effect of bidirectional movement is found to be more significant, when distribution ratio decreases from 0.6 in descending direction, making ascending flow more dominant. At this ratio (downward 0.6 and upward 0.4), it is observed that descending flow is reasonably influenced due to movement of opposing minor flow (upward) indicating certain levels of bidirectional as well as unidirectional conflicts between two streams; resulting into highest difference in the walking speed of both directions (mainly due to higher speeds in downward direction); which offers maximum capacity. Methodology developed through this study and the results obtained can be useful for optimizing the pedestrian flow on a given undivided stairway coupled together with planning of other alternative pedestrian facilities at railway stations.

Analysing the Speed-flow Relationship in Urban Road Traffic

In road traffic, speed-flow curves describe the relationship between vehicle flow rates and average vehicle speeds. These functions are one of the basic elements of transport modelling and they are widely used in project appraisal. Review and re-validation of speed-flow relationship has become timely in recent years as vehicle fleet changed, average traffic volumes increased and more comprehensive, automatic data collection became available. This study used the database of the road operator in Budapest to simultaneously analyse travel times and traffic volumes. The paper aims to examine the validity of the well-known fundamental diagram for urban roads, to provide an estimation for the speed-flow curve based on the Budapest case and to assess the on-set process of congestion.

Effect of stairway width on pedestrian flow characteristics at railway stations

The overall efficiency of transit operation depends on effective operation of various facilities like transit units, transfer stations and access infrastructure. Stairways at transit stations are vital links between different interconnected elements like platforms and foot over bridges. Transfer rate of pedestrians through stairway primarily depends on its geometric design. Many previous studies have reported that the stairway dimensions such as stair gradient, height of riser and length of stairway influence walking speed significantly. The walking speed is also governed by the space available to the individual pedestrian. The movement will be restricted with reduction in space and hence, in principal it depends on the width of the facility. In order to understand the effect of available space on the pedestrian flow characteristics, present study has been carried out at six stairways with different dimensions at two railway stations; one suburban rail transit and the other intercity railway station in India. Total 32 310 pedestrian data points are collected using video-graphic technique and has been used to develop the fundamental relationships of pedestrian flow using single-regime and two-regime models particularly for speed–flow relationship. The results also show that flowrate and walking speed increase with increase in width of stairway. Further, walking speed variation pattern is analyzed in three density regimes. Speed reduces gradually with increase in density and reduction in space. At both the stations, compared with the smaller stairway, maximum flow and walking speed are observed for wider stairway. The study establishes that the average walking speed, flow and density are more influenced by width of stairway than other dimensions. The results of this study will be useful in planning, design and management of facility of stairways at railway stations and can also be used in finding safe evacuation time.

Traffic Flow Characteristics for Multilane Highways in India

The traffic composition on multilane highways in India comprises of a wide range of vehicles in terms of their type, size, engine power, manoeuvring ability, etc. This mix of vehicles with different operating capabilities results in a broad range of speed. Slow moving or non-motorized vehicles occupy the lower ranges of speed spectrum whereas the new technology cars dominate the higher ranges. To understand the real traffic behaviour, it requires quantification of some of the basic traffic flow characteristics such as Speed, Flow, Density and Occupancy through which the capacity can be derived. The radical changes in road network and vehicle technology have resulted in variations in speed-flow characteristics and subsequently road user costs. The problems arise out of three major aspects associated with several types of vehicles in the traffic mix i.e. speed and acceleration capabilities of vehicles, their manoeuvres and lateral clearance requirements within the right of way. The main objective of the present study is to estimate the basic traffic flow parameters for six lane divided traffic stream under study and to evolve speed-flow relationships for six lane divided carriageways for different vehicle types. In the present study, linear models have been developed for speed-flow equations for different vehicles on each lane of selected road sections of six-lane divided carriageway.

An Analysis of Express Lanes in Utah

This paper reports on an analysis conducted to identify ways to improve average speeds in the Express Lanes (ELs) in Utah. The research began with an investigation of the current usage of both the ELs and the General Purpose (GP) lanes by user type (e.g., single-occupant vehicle, high-occupancy vehicle (HOV) 2, HOV 3+, “C” decal, Express Pass user, motorcycle, bus, freight vehicle, and emergency vehicle) to better understand the breakdown of vehicles in the lanes and to calculate average vehicle occupancy. Violation data were examined to determine ways to reduce violator rates, while ways to control Express Pass traffic and to evaluate changes to toll rates and to estimate the impacts on speed of specific toll increases along the corridor were explored. The data collected were examined to better understand the speed-flow relationship on the ELs and to estimate the impact of volume changes on speeds in the lanes. This allowed the research team to estimate the impact on EL and GP lane volumes and speeds under a combination of EL education campaigns, increased EL enforcement, and increased EL peak period toll rates. Depending on the scenario analyzed, it was determined that the speeds in the ELs could be expected to increase as a result of the recommendations. The increase was found to be dependent upon the scenario chosen and the response of the traffic to the increases with an average maximum change in EL speed of 7.8 mph for a combination of education, enforcement, and increased tolls.

Capacity Issues in Sweden - Applications and Research

The Swedish Transport Administration (STA) is responsible for rural roads and major urban through roads, all together some 100 000 km. In 2013 30% of the yearly mileage is produced on some 2 000 km motorways, normally with posted speed limit 110 kph, and 14% on some 2700 km 2+1-barrier roads normally with 100 kph. All investment projects on state roads are assessed in a stepwise procedure including legal decisions in the feasibility and preliminary design stages as well as in a parallel socio- economic planning procedure. The STA official methodology software EVA is used to estimate life-cycle-cost analysis (LCC). Parameters such as value of time, accident costs, traffic growth and interest rates are standardized and regularly updated by STA. Travel time effects are to be estimated using the Swedish Highway Capacity Manual (SHCM) with an average passenger car vehicle hour time value of 171 SEK/hour (2010).A new edition of SHCM was published in 2014 based mainly on the METKAP-project. The paper gives an overview of up-dates such as speed-flow relationships for alternative cross-sections and speed limits, capacity procedures for weaving sections and capacity estimates in traffic signals with “shared lanes”.New Swedish speed limits for rural as well as urban roads were introduced in 2008 making it possible for road authorities also to use speed limits 40, 60, 80, 100 and 120 and not only the old 30, 50, 70, 90 and 110 km/h were introduced 2008-2009. New speed limit guidelines based on regional accessibility and development, environmental impact, traffic safety and driver acceptance were published. The overview is a long lasting procedure. Recent results are presented especially for the major target not to accept normal two-lane roads with speed limit over 80 kph. More than 50% of the two lane 90 and 100 kph network has been decreased or retrofitted to 2+1.A number of studies on speed and capacity on 2+1-roads have been conducted lately including effects of closing overtaking lanes in peak periods. A number of investment projects are also implemented with longer 1+1-sections with some efforts to predict speed and capacity behaviour. Efforts have also been made to estimate disturbance levels due to different types of break downs. An overview of findings is presented.An interesting project (Dukat) has developed capacity models for maintenance operations on motorways. The project includes a major full scale test of the model. Results are presented.A new Motorway Control System with varying speed limits has been implemented on the E4 between Stockholm and Södertälje. Capacity and level-of-service effects based on a pre/ater study are presented showing positive effects.The STA is also working with partly new formats to present the capacity situation in the state road network based on objectives for “usefulness” and “robustness”. These measures and results reported are also presented.

Mixed Traffic Speed–Flow Behavior under Influence of Road Side Friction and Non-Motorized Vehicles: A Comparative Study of Arterial Roads in India

Abstract—Present study is carried out on six lane divided urban arterial road in Patna and Pune city of India. Both the road having distinct differences in terms of the vehicle composition and the road side parking. Arterial road in Patan city has 33% of non-motorized mode, whereas Pune arterial road dominated by 65% of Two wheeler. Also road side parking is observed in Patna city. The field studies using videography techniques are carried out for traffic data collection. Data are extracted for one minute duration for vehicle composition, speed variation and flow rate on selected arterial road of the two cities. Speed flow relationship is developed and capacity is determine. Equivalency factor in terms of dynamic car unit is determine to represent the vehicle is single unit. The variation in the capacity due to side friction, presence of non motorized traffic and effective utilization of lane width is compared at concluding remarks.

Generic Speed–Flow Models for Basic Freeway Segments on General-Purpose and Managed Lanes in Undersaturated Flow Conditions

This paper presents a generic set of undersaturated speed–flow models for basic freeway segments on general purpose and managed lanes (MLs) consistent with the Highway Capacity Manual 2010 (HCM 2010). The proposed models predict segment space mean speed under a wide set of freeway operational conditions that can affect its free-flow speed (FFS) and capacity. Furthermore, the proposed models allow quantifying the impacts of nonrecurring events, such as severe weather conditions, incidents, and work zones on the speed–flow relationship. In addition, the model allows calibration of real-world facilities through adjustments to FFS and capacity. The incorporation of analyses of reliability and active traffic and demand management in the HCM context requires a set of speed–flow models capable of accounting for the effect of nonrecurring sources of congestion. Currently, the HCM 2010 provides a set of speed–flow models to predict space mean speed and consequently other freeway performance measures. This family of equations provides a limited adjustment to FFS. With guidance from NCHRP Project 3-96, separate speed–flow models are proposed for MLs through use of a different form from that in the HCM. The proposed generic equations describing the speed–flow relationship provide consistency between speed–flow relationships of managed and general purpose lanes and can incorporate any capacity or FFS adjustments to predict segment speed under different circumstances. The proposed generic equations are wholly consistent with the speed–flow models in the HCM 2010 and predict the same speed under any flow rate.

A Modelling Framework for estimating Road Segment Based On-Board Vehicle Emissions

Traditional traffic emission inventory models aim to provide overall emissions at regional level which cannot meet planners' demand for detailed and accurate traffic emissions information at the road segment level. Therefore, a road segment-based emission model for estimating light duty vehicle emissions is proposed, where floating car technology is used to collect information of traffic condition of roads. The employed analysis framework consists of three major modules: the Average Speed and the Average Acceleration Module (ASAAM), the Traffic Flow Estimation Module (TFEM) and the Traffic Emission Module (TEM). The ASAAM is used to obtain the average speed and the average acceleration of the fleet on each road segment using FCD. The TFEM is designed to estimate the traffic flow of each road segment in a given period, based on the speed-flow relationship and traffic flow spatial distribution. Finally, the TEM estimates emissions from each road segment, based on the results of previous two modules. Hourly on-road light-duty vehicle emissions for each road segment in Shenzhen's traffic network are obtained using this analysis framework. The temporal-spatial distribution patterns of the pollutant emissions of road segments are also summarized. The results show high emission road segments cluster in several important regions in Shenzhen. Also, road segments emit more emissions during rush hours than other periods. The presented case study demonstrates that the proposed approach is feasible and easy-to-use to help planners make informed decisions by providing detailed road segment-based emission information.

New Methodology to Determine Work Zone Capacity Distribution

Because the true value of work zone capacity is difficult to know with perfect confidence, work zone capacity should be represented by means of a probability distribution. This study proposes a methodology for determining work zone capacity distribution on the basis of the probabilistic speed–flow relationships. First, traffic speed and flow are modeled as random variables following the lognormal distribution. Then, the lognormal distributed speed–flow functions are established to formulize the probabilistic speed–flow relationships under uncongested and congested traffic conditions, respectively. Finally, a work zone capacity distribution model is derived on the basis of the probabilistic speed–flow relationships. A case study that uses the observed traffic data from Singapore work zones is conducted to confirm the effectiveness of the proposed methodology. The effects related to work zone configuration such as geometrical alignment and speed limit on the mean and prediction interval of work zone capacity are examined. The proposed methodology could help traffic engineers evaluate the variability of work zone capacity and the reliability of traffic management strategies.

Impact of Variable Speed Limits on Motorway Level of Service

Variable speed limits are applied on a considerable number of motorway sections. Besides the impact on road safety, variable speed limits harmonize traffic flow at high volumes and hence influence motorway capacity. The paper reports empirical findings obtained for motorway sections with different speed control conditions in Germany. The capacity of each cross section is determined by analyzing the speed-flow relationship as well as by applying methods for stochastic capacity analysis. The results show that the main effect of variable speed limits is a significantly reduced variance of the capacity distribution function. Hence, the application of variable speed limits leads to a lower risk of a traffic breakdown at moderate volumes, which significantly reduces the probability of congestion occurrence. In highway capacity guidelines, this effect can be considered by applying different thresholds of LOS E for motorway sections with and without variable speed limits.

Urban Road Short-term Traffic Flow Forecasting Based on the Delay and Nonlinear Grey Model

Concerning the delay and nonlinear properties of traffic flow in urban road systems, this paper forecasts the short-term traffic flow based on the grey GM(1,1|τ,r). Firstly, the delay factor τ is determined by the speed-flow relationship when volume is greater than it capacity. Then, the nonlinear parameter r is determined by a particle swarm optimization algorithm, where the prediction effect is unsurpassed. Finally, verification of this model is done by collecting traffic flow data on one section of Youyi Avenue and comparing the prediction value of GM(1,1|τ,r) with GM(1,1) and SVM. The results show that the prediction effect of GM(1,1|τ,r) model for short-term traffic flow is significantly improved, which plays an important role in intelligent traffic systems.

Vehicle Class-wise Speed Volume Model for Three-lane Undivided Urban Roads

Two or three lane two-way undivided roads are a ubiquitous presence in any city in India and they are typically mixed traffic roads with vehicles of widely varying static and dynamic characteristics and with poor or no lane discipline. Hence multi-class speed flow equations are more relevant to these types of facilities rather than single class flow speed models. In addition, the available research for the operation of these types of facilities in India is very limited. Hence this paper aims to study the traffic flow in a three-lane two-way undivided road in the city of Chennai through developing multi-class speed-flow relationships using both linear and Bureau of Public Roads (BPR) models. It has been found from the study that models with total volume perform the same irrespective of units used being vehicles or PCU and models with class-wise (with and without direction) consistently perform better than total volume models. It was also found that linear models work better than other models for all vehicle classes modelling 2-wheeler speeds and two-wheelers, cars and LCVs have the greatest influence on the speeds of almost all the category of vehicles and the stream. One of the other important conclusions from the study is that an optimum use of class-wise and class& direction-wise models can offer models of better fit than two individual class and class-direction models.

The impact of precipitation on speed–flow relationships along a UK motorway corridor

Although the fundamental traffic diagram provides the characteristics of a typical road traffic speed–flow relationship, little consideration has been given to the impact of adverse weather conditions on the relationship and the subsequent impact on local speed–flow. For the first time, this study uses precipitation radar along with a state-of-the art traffic information system to ascertain the relationship between speed–flow and precipitation on a UK transport corridor at the local (junction to junction) scale. It is evident that precipitation causes a significant reduction in speed and maximum flow on many links of the corridor as well as a downward reduction in the overall speed-flow relationship. With increased instances of heavy precipitation predicted in the UK as a result of climate change, these findings highlight the subsequent impact on journey travel times and associated economic costs.