Lane Capacity Research Articles

Capacity Change Mechanism of a Diverge Bottleneck

This paper offers a more complete picture of the capacity change mechanism at a diverge bottleneck caused by a queue from an exit ramp moving upstream onto the freeway. Detailed observations collected by videos during a 4-day period from an expressway in Bangkok, Thailand, showed that traffic states during bottleneck activation at the diverge site could be categorized into three distinct states with different levels of capacity according to vehicle speeds on through movement lanes and the rates of lane change maneuvers near the off-ramp queue. The data indicated that the transitions of traffic states were caused primarily by the changes in exit flows. The lower capacity was initiated by a more restrictive off-ramp flow that caused some cut-through vehicles on the adjacent lane to impede through movement traffic. Once the exit flow increased, impeding exit vehicles could move out of the adjacent through lane and higher capacity could be restored. These findings point to automatic off-ramp control strategies that would generate higher bottleneck capacities through detecting traffic speeds on freeway through lanes. These findings advance the present knowledge in traffic flow theory that will enable traffic researchers to understand more fully the traffic phenomena at a diverge bottleneck and will help traffic engineers to operate freeway traffic properly.

Impacts of Cooperative Adaptive Cruise Control on Freeway Traffic Flow

This study used microscopic simulation to estimate the effect on highway capacity of varying market penetrations of vehicles with adaptive cruise control (ACC) and cooperative adaptive cruise control (CACC). Because the simulation used the distribution of time gap settings that drivers from the general public used in a real field experiment, this study was the first on the effects of ACC and CACC on traffic to be based on real data on driver usage of these types of controls. The results showed that the use of ACC was unlikely to change lane capacity significantly. However, CACC was able to increase capacity greatly after its market penetration reached moderate to high percentages. The capacity increase could be accelerated by equipping non-ACC vehicles with vehicle awareness devices so that they could serve as the lead vehicles for CACC vehicles.

Establishing speed–flow–density relationships for exclusive motorcycle lanes

The motorcycle is a popular mode of transport in Malaysia and developing Asian countries, but its significant representation in the traffic mix results in high rates of motorcycle accidents. As a result, the Malaysian Government decided to segregate motorcycle traffic along its new federal roads as an engineering approach to reduce accidents. However, traffic engineers needed to know the maximum traffic a motorcycle lane could accommodate. Despite substantial literature related to speed–flow–density relationships and capacities of various transport facilities, there is a knowledge gap regarding motorcycle lanes. This paper establishes motorcycle speed–flow–density relationships and capacities of exclusive motorcycle lanes in Malaysia. Observations of motorcycle flows and speeds were conducted along existing and experimental motorcycle lanes. Motorcycle speed–density data were aggregated and plotted for two types of observable motorcycle riding behaviour patterns that were influenced by the widths of a motorcycle lane: the headway pattern (lane width ≤ 1.7 m) and the space pattern (lane width > 1.7 m). For both riding patterns, regression analysis of motorcycle speed–density data best fits the logarithmic model and consequently the motorcycle flow–density and speed–flow models are derived. Motorcycle lane capacities for headway and space riding patterns are estimated as 3300 mc/hr/lane and 2200 mc/hr/m, respectively.

Capacity Analysis of Signalized Intersections under Mixed Traffic Conditions

This paper presents a new approach for calculating straight lane capacity under mixed traffic conditions, especially in China. The First Discharge Headway (FDH) is applied as a novel to improve Stop Line Method model, and the influencing factors of FDH are analyzed. To calibrate the FDH probability distribution, investigation data of four intersections in Beijing are employed. The Equivalent First Discharge Headway (EFDH) is proposed to replace the current empirical value in traffic capacity calculation. Using the improved model, the paper analyzes conflicting pedestrian and non-motor volumes' impact on capacity under mixed traffic conditions. The results show that FDH is only related to vehicle type and external disturbance factors (α<0.05), and there is no mutual influence between each other. The Weibull distribution could well be applied to describe the probability distribution of the FDH. Though the EFDH correction, the calculation error of capacity is reduced to less than 5%. The traffic capacity presents exponential decline with the increase of conflict frequency between pedestrians, non-motors and motorized vehicles, the conflict influence is diminishing marginal utility.

Modelling Blockage Probability and Capacity of Shared Lanes at Signalized Intersections

At signalized intersections, turning vehicles often use the same shared lane together with the through traffic. Since a permitted left-turn movement has to give way to the opposing through movement, it has to wait if necessary and thus impedes the through movement in the same direction. In the real-world, if the left-turn movement is permitted controlled, the through movement at the same approach can be totally blocked by waiting left-turning vehicles during the green time. Thus, the green time for the shared lane cannot be efficiently utilised and the lane capacity under consideration cannot be fully received. In this paper, a mathematical model is presented for an exact calculation of the blockage probability caused by permitted turning vehicles and for the estimation of the capacity of single-shared lanes at signalized intersections. According to the probability and combinatory theory, a mathematically exact model is developed. The proposed model can be applied to shared lanes either with left-turn or with right-turn movement. Respectively, by extending the model, also the capacity for the Right-Turn-On-Right situation can be exactly calculated. Furthermore, in this paper, the model is generalized to turning movements with so-called sneakers within the intersection. The generalized extension provides a more realistic solution for real-world intersections where, in a normal case, there are several places downstream of the stop-line for turning vehicles. The mathematical formulation for the generalization is more complicated. For applications in the practice, monographs are produced for estimating the shared lane capacity under different traffic conditions. In addition, a set of approximation functions are recommended based on the mathematically exact results. Those approximation functions can be more easily used than the exact formulations. The new model is verified by extensive simulation studies. The results of the proposed model fill out a gap in the current procedures of capacity estimations at signalized intersections. Some of the results of the model will be incorporated in the new version of German Highway Capacity Manual. © 2011 Published by Elsevier Ltd.

Measuring the Quality of Service for High Occupancy Toll Lanes Operations

High Occupancy Toll (HOT) lane systems have been proposed as one of the most applicable countermeasures against freeway congestion. Under HOT lane operational scheme, a Single Occupancy Vehicle (SOV) can pay to access HOT lanes in exchange of travel time saving or enhanced trip reliability when excess HOT lane capacity is available. Compared with regular freeway facilities, HOT lane systems demonstrate unique characteristics in facility capacity, driver behavior, travel pattern, demand modeling, and trip reliability. This study aims at conducting a comprehensive performance analysis on two representative HOT lane systems of State Route 167 in Washington and I-394 MnPass in Minnesota based on the field data collected from traffic sensors and transponder toll tags. Performance measurements are proposed to quantify the quality of service for HOT lane operations. Three critical issues are addressed in this study: 1) the speed-flow relationships in HOT lane systems, 2) quantified system-wide travel time savings and travel time reliability achieved, 3) SOVs tolling incentives. Based on the empirical analysis and evaluation results for the SR 167 and I-394 MnPass HOT lane systems, operational problems and challenges are also identified. Although the HOT lane system preserves favorable travel reliability, under-utilized HOT lane capacities were observed. The existing tolling strategies may be modified for better SOV allocation for HOT lane usages and further optimize the overall HOT system operations. The research findings greatly advance our understanding on HOT lane system operation mechanisms and are complementary to the freeway facility performance analysis provided by Highway Capacity Manual 2000.

Evaluating the Impacts of Unconventional outside Left-Turn Lane Design on Traffic Operations at Signalized Intersections

This study evaluated the operational impacts of an unconventional outside left-turn lane design used at some signalized intersections in China. Using traffic data collected from 10 approaches to five signalized intersections where both traditional inside and unconventional outside left-turn lanes are installed, the research team compared turning speeds and saturation flow rates for left-turning vehicles from inside and outside left-turn lanes. The team found that vehicles turning from outside left-turn lanes had relatively higher turning speeds than those turning from inside left-turn lanes. The differences in speed were statistically significant for both passenger cars and heavy vehicles. However, the higher speeds did not result in higher left-turn lane capacity. Unconventional outside left-turn lanes provided slightly lower, but generally comparable, left-turn saturation flow rate when compared with traditional inside left-turn lanes. The differences in discharge headways from inside and outside left-turn lanes were not statistically significant. A binary logit model was developed to quantify the impacts of various factors on the probability of a driver selecting the outside left-turn lane to make a left turn. The chance of a left-turning driver choosing the outside left-turn lane would increase with an increase in the traffic volume of the major street and the queue length in the inside left-turn lane and would decrease with an increase in the distance from the upstream right-side side street to the signalized intersection. Drivers of heavy vehicles and those from upstream right-side side streets were more likely to select outside left-turn lanes.

Cooperative Adaptive Cruise Control: Driver Acceptance of Following Gap Settings Less than One Second

Currently, Adaptive Cruise Control (ACC) systems can automate vehicle speed control to maintain following time-gaps in the range of one to two seconds. In this study, a field test was conducted to determine whether or not drivers would be comfortable with the sub-second following time-gaps that could be provided by a Cooperative ACC (CACC) system. A CACC system uses vehicle-vehicle communication to enable faster system responses and shorter following time-gaps. Sixteen drivers from the general public drove both systems on their daily commuting trips, and their driving behavior and subjective opinions of the systems were recorded. The results show that the drivers were generally comfortable with and typically selected the sub-second following time-gaps offered by the CACC system, but there were significant differences between the preferences of male and female drivers. The male drivers typically preferred the shortest gap settings, while the female drivers typically preferred slightly longer gap settings. The drivers' willingness to accept the shorter following gaps adds credibility to the assertion that future CACC systems may have the potential to produce significant increases in the achievable highway lane capacity.

Multivariate regression for estimating driving behavior parameters in work zone simulation to replicate field capacities

Traffic simulation tools are being increasingly adopted by DOTs for various traffic analysis studies, including assessing work zone traffic impacts. This is due, in part, to their ability to model individual vehicle and driver behavior at a highly detailed level to assess the traffic performance. In order to accurately use the simulation models for traffic analysis of work zones, it is necessary to calibrate the models to match the field conditions (such as lane capacity and queue lengths) by adjusting the driving behavior parameters. Unfortunately, there is very little guidance on choosing the driving behavior parameters in simulation models that will replicate the actual field conditions in work zones. Empirical studies have shown that the roadway capacities at work zones are not only lower than the capacities under normal operating conditions but also vary across all states in the US. This disparity means that a unique driving behavior parameter set cannot be used by all states; instead the parameter values should be chosen so as to reproduce the state-specific capacity values. The default truck characteristics inside the VISSIM simulation model are different from the average characteristics of trucks on US freeways. The use of default truck characteristics that are not representative of the US conditions results in incorrect calibration and therefore erratic conclusions. Based on a review of national studies on truck characteristics (length, weight to power ratio, etc) we recommend US specific values for use in VISSIM. The recommended values are generic and are applicable to facilities other than work zones. In this study, we develop multivariate regression models to express the relationship between the critical driving behavior parameters of the VISSIM model, work zone lane configuration, truck percentages, and work zone capacity. The estimated statistical models are capable of generating a range of parameters' values that produce a wide range of capacities used by state DOTs in the US. Model validation is also conducted using field data from work zone sites in Ohio. The validation results showed that the model recommended values are capable of producing accurate estimates of capacity and queue lengths.

Multi-agent systems for simulating traffic behaviors

Multi-agent systems allow the simulation of complex phenomena that cannot be easily described analytically. As an efficient tool, the agen t-oriented traffic models have emerged to investigate vehicular traffic behaviors. In this article, a new agent-based traffic simulation model is proposed for solving the traffic simulation problems. A vehicle with the driver is represented as a composite autonomous agent in this model. Different from the classical car-following principles, vehicle-agent moving approaches are proposed instead of particle-hopping techniques. This model defines reasonable acceleration and deceleration rates at any certain condition. In this simulation, this can offer a natural, even cognitive way to follow the leader and switch lane. The simulation results have verified that this model has achieved more realistic results without loss of accuracy than those obtained from the cellular automata traffic models. This model can provide better simulation performance than the traditional vehicle-following models which are used to investigate the nonequilibrium traffic flow. A comparison of the real flow with the simulated freeway flow and lane capacity highlights the validness of this model.

Capacity Reliability of Signalized Intersections with Mixed Traffic Conditions

The reliability of capacity of signalized intersections in mixed traffic conditions involving vehicles, bicycles, and pedestrians was investigated to complete the conventional, deterministic capacity calculations. Simulations using VISSIM provided estimates of capacity distributions, and demonstrated the effects of the analysis intervals on the distributions. With the random vehicle arrivals taken into account, a capacity reliability assessment method was given as a function. Assessments were also performed regarding the effects of the conflicting pedestrian and bicycle volumes on capacity reliability. The simulation indicates that the pedestrians and bicycles result in greater random fluctuations of exclusive turning lane capacities, but have less effect on the variability of shared lane capacities. Normal distributions can be used to model the capacities for intervals not less than 10 min. At higher vehicular volumes, the capacity reliability is more sensitive to the mean and standard deviation of the pedestrian and bicycle volumes.

Calibration and Verification of SHAKER, a Deterministic Toll Plaza Simulation Model

A great deal of research has been conducted on Central Florida toll roads to better understand the characteristics of the tolling operation. In this paper, the use of the toll plaza queuing model SHAKER is calibrated to replicate real life toll facility traffic conditions. SHAKER is a deterministic queuing model based on classical physics equations that determines a plaza's maximum hourly throughput by assigning vehicles to lanes based on queuing conditions. An extensive field study on the Florida's Turnpike Enterprise network is conducted to determine the parameters affecting toll lane capacities. Data including traffic characteristics, vehicle characteristics, and toll plaza characteristics was collected and extracted at different toll locations, configurations, and pricing. From this data, periods of constant queuing were pinpointed and from those periods, factors such as demand, throughput, service time, etc., were extracted. Using the extracted parameters, SHAKER was then calibrated and validated to estimate the capacity of four different toll plazas along the Florida Turnpike Network.

Using Simulation to Plan Capacity Models by Lane for Two- and Three-Lane Roundabouts

Recently, capacity models have been developed for single- and double-lane roundabouts by using U.S. data from NCHRP Report 572. The model for double-lane capacity was provided primarily for critical lane (usually right-lane) capacity as a function of total circulating volume. None of the existing international capacity models separate circulating volumes by lane. This study proposes planning capacity models for two-lane and three-lane roundabouts by separate entry-lane and separate circulatory-lane traffic volumes. VISSIM microsimulation software was used first to compare with the new NCHRP models as well as with Australian (SIDRA) and German (Tanner-Wu) models. Given that predictions from VISSIM for overall capacity of one- and two-lane roundabouts are consistent with U.S. data from NCHRP Report 572, new planning capacity models were developed by using VISSIM for the left lane and right lane of double-lane roundabouts that are functions of separate circulatory-lane traffic volume. For three-lane roundabouts, planning capacity models were developed by separate entry lane (left, middle, and right lane) as functions of inner, middle, and outer circulatory-lane volumes. All variable coefficients are statistically significant, and model fits are strong.

A lane-changing model considering the maneuver process and its applications

An improved lane-changing model based on the Nagatani's method is proposed. The threshold of the safety headway in lane-changing process is calibrated. Both the lane-changing process and the impact of the lane-changing vehicle on the adjacent vehicles are incorporated into the proposed model. Therefore, simulated results using the proposed model agree with the surveyed results. The marginal decrease of lane capacity with the number of lanes is analysed and validated upon the proposed lane-changing model. We find that the vehicle velocity decreases and the lane changing probability increases with the increasing number of lanes under the congested condition of highways in cities.

An Approach to Assessing Freeway Lane Management Hot Spots

This research presents a procedure for capitalizing on the trade-off between urban freeway managed lanes and general purpose lanes that compete for limited road space. The basic goal of the procedure is to provide policy guidance for sharing any excess lane capacity on a timely and efficient basis. Potential operating policy options for these two types of lanes are categorized as “do nothing,” “lane management,” and “more than lane management.” The “lane management” condition recognizes the extent and duration of a “hot spot” as defined by underutilized managed lanes with congested general purpose lanes, or vice versa. Four major and three minor lane management hot spots are deterministically and stochastically captured along a 24-mi freeway stretch in California. The major hot spots account for 8.3% of the total time–space set. The approach, which can also be applied to predict upcoming hot spots, generates satisfying accuracy. Finally, strategies are proposed to prevent the hot spots, and the effects of lane management are estimated. The application of this approach is useful especially for managed lanes with limited access points that prohibit arbitrary lane changing.

The merits of separating cars and trucks

Truck-only lanes and tollways are under study as tools to combat road congestion, enhance safety and reduce other external costs of road traffic. This paper investigates the potential benefits from separating cars and trucks onto different lanes or routes while treating road infrastructure as given. The benefits are found to depend on several factors: the relative volumes of cars and trucks, the congestion delay and safety hazards that each vehicle type imposes, values of travel time for each type, and lane capacity indivisibilities. The optimal assignment of vehicles to road capacity can be supported using tolls that are differentiated by vehicle type and lane. Lane access restrictions usually cannot support the optimum and may well provide no benefit at all. Creating a toll lane for one vehicle type is generally more effective. The benefits of all forms of intervention are sensitive to whether the proportions of cars and trucks are commensurate with lane capacities.

Errors in Analyses for Capacity and Timing Design of Signalized Intersections in Absence of Steady Queue Discharge Rates

Many models for the analysis of capacity and signal timing of signalized intersections are based on the assumption that queue discharge will quickly reach a steady maximum rate after the green interval begins. This assumption leads to a common practice of using saturation flow and lost time as parameters in modeling the requirements for lane capacity, intersection performance, and signal timing. However, recent field data indicate that the queue discharge rate may continue to increase long after green onset. For this type of queue discharge characteristic, the traditional applications of lost time and saturation flow may result in significant errors when lane capacity and modeling optimal cycle length are estimated. The extent of such errors is assessed on the basis of the queue discharge characteristics observed in 38 urban traffic lanes.

High Occupancy/Toll Lanes: Phasing in Conjestion Pricing a Lane at a Time

A consensus is emerging among transportation economists that the best way to deal with freeway congestion is to charge for driving during peak hours. The main barrier to implementation is political: drastic change is politically unpopular. This paper proposes a way of overcoming the political obstacles by phasing in congestion pricing over a period of many years. The proposal involves modifying the current concept of High Occupancy Vehicle (HOV) lanes. Current HOV lanes are not very effective at reducing traffic; 43 percent of car-poolers are members of the same household. They cost everyone but serve few drivers. We propose replacing HOV lanes with HOT lanes: High Occupancy/Toll lanes. A HOT lane would give free passage to three-occupant vehicles (HOV3s) but permit all others to pay a peak-hour toll for access. This would utilize more of the lane's capacity, demonstrate congestion pricing on a wide scale, and generate revenues to pay for HOT lane construction. In cases where the choice is between a HOT lane or no additional lane, the HOT-lane option also promotes ridesharing.

Effects of Adaptive Cruise Control Systems on Highway Traffic Flow Capacity

The effects on traffic flow of increasing proportions of both autonomous and cooperative adaptive cruise control (ACC) vehicles relative to manually driven vehicles were studied. Such effects are difficult to estimate from field tests on highways because of the low market penetration of ACC systems. The research approach used Monte Carlo simulations based on detailed models presented in the literature to estimate the quantitative effects of varying the proportions of vehicle control types on lane capacity. The results of this study can help to provide realistic estimates of the effects of the introduction of ACC to the vehicle fleet. Transportation system managers can recognize that the autonomous ACC systems now entering the market are unlikely to have significant positive or negative effects on traffic flow. An additional value of studying ACC systems in this way is that these scenarios can represent the first steps in a deployment sequence that will lead to an automated highway system. Benefits gained at the early stages in this sequence, particularly through the introduction of cooperative ACC with priority access to designated (although not necessarily dedicated) lanes, can help support further investment in and development of automated highway systems.

Simulated Travel Impacts of High-Occupancy Vehicle Lane Conversion Alternatives

A simulation model of a hypothetical highway corridor was used to analyze the effects of converting an existing high-occupancy vehicle (HOV) lane to either a high-occupancy toll (HOT) lane or a mixedflow lane. The simulation model, which uses a nested logit structure with a synthetic sample of individuals, estimates vehicle miles of travel (VMT), vehicle hours of travel, and person hours of travel in the corridor. The analysis suggests that mobility needs can best be served by using excess HOV lane capacity as an HOT lane facility. Capacity expansion alternatives are also analyzed, including adding mixed-flow travel lanes or converting existing lanes to HOV or HOT lanes. Alternative toll levels are also simulated. Results show that managing capacity as an HOT or HOV lane could provide superior mobility benefits with a net decrease in VMT if capacity must be expanded.