Two-way Stop-controlled Intersections Research Articles

Proposed collision warning system for right-turning vehicles at two-way stop-controlled rural intersections

At two-way stop-controlled (TWSC) rural intersections, a right-turning driver who is departing the minor road may select an improper gap and subsequently may be involved in a rear-end collision with another vehicle approaching on the rightmost lane on the major road. This paper provides perceptual framework and algorithm design of a proposed infrastructure-based collision warning system that has the potential to aid unprotected right-turning drivers at TWSC rural intersections. The proposed system utilizes a radar sensor that measures the location, speed, and acceleration of the approaching vehicle on the major road. Based on these measurements, the system’s algorithm determines if there will be any potential conflict between the approaching and the turning vehicles and warns the driver of the latter vehicle if such a conflict is found. The algorithm is based on realistic acceleration profile of the turning vehicle to estimate its acceleration rates at different times so that the system can accurately estimate the time and distance needed for the departing vehicle to accelerate to the same speed as for the approaching vehicle. That realistic acceleration profile is established using actual experimental data collected by a Global Positioning System (GPS) data logger device that was used to record the positions and instantaneous speeds of different right-turning vehicles at 1-s intervals. The algorithm also gives consideration to the time needed by the driver of the departing vehicle to perceive the message displayed by the system and react to it (to start departure) where it was found that 95% of drivers have a perception–reaction time of 1.89s or less. A methodology is also illustrated to select the maximum measurement errors suggested for the detectors in measuring the locations of the approaching vehicle on the major road where it was found that the accuracy of the system significantly deteriorates if the errors in measuring the distance and the azimuth angle exceed 0.1m and 0.2°, respectively. An application example is provided to illustrate the algorithm used by the proposed system.

Toward a Capacity Analysis Procedure for Nonstandard Two-Way Stop-Controlled Intersections

Two-way stop-controlled (TWSC) intersections are one of the most common forms of at-grade roadway junctions in the world. These intersections are typically located where a minor road intersects or joins a major roadway and stop signs are placed on the minor approaches. When the major roadway and the traffic that it carries go straight through the intersection, it is a standard TWSC intersection. In many places, however, the prevailing traffic and the major roadway do not continue straight through the intersection. In such cases, the major roadway bends at an angle at the intersection, and the stop signs on the minor streets are placed on adjacent legs of the intersection. These intersections are nonstandard TWSC intersections. Nonstandard TWSC intersections are found frequently in some countries and exist in most. These intersections are officially recognized in traffic codes and manuals in Europe, Australia, and the United States, yet the Highway Capacity Manual (HCM) TWSC procedure does not specifically handle these intersections. In fact, a literature review yielded no established procedure for the calculation of capacity at nonstandard TWSC intersections that would consider the specificities related to the layout of the approaching roadways and the characteristics of the traffic flow, such as the critical headway and the follow-up headway. This paper demonstrates the uniqueness of nonstandard TWSC intersections, reports analysis results from field data collected at several similar nonstandard TWSC T-intersections, identifies discrepancy in critical headway figures and level-of-service ratings that can result from the use of the HCM procedure for TWSC intersections, and calls for further research on the subject in other locales.

Alternative Methodology for Determining Gap Acceptance for Two-Way Stop-Controlled Intersections

Vehicles approaching a two-way, stop-controlled intersection from the minor roadway will wait for a large enough gap in the traffic on the major roadway before entering the intersection. The primary method for modeling intersections for both safety and operations is to determine a critical gap size such that a vehicle waiting at the stop sign will enter the intersection if the gap on the major roadway is larger than the critical gap. The method that is currently most accepted for measuring the critical gap is a maximum likelihood estimation developed by Troutbeck. The Troutbeck method may yield biased results and cannot be used with data sets that contain only rejected gaps. This paper proposes an alternative model for estimating the critical gap that is unbiased and can be used with naturalistic data. The proposed model requires that the gap distribution of the major street be known. This proposed model is compared with the Troutbeck model through a Monte-Carlo simulation. The proposed model yields accurate estimates of the mean critical gap as long as accurate estimates of the major street traffic are used.

Capacity of TWSC Intersection with Multilane Approaches

This paper presents an improved model for assessing capacities of vehicular movements at Two-Way Stop-Controlled (TWSC) intersections. The theoretical model was derived based on the conflict technique for estimating capacities of vehicular movements at multilane TWSC intersections under non-saturated traffic conditions. The model parameters were calibrated by referencing the related research of field observations. The effectiveness and reliability of the proposed model were verified by comparing with the methodology of Highway Capacity Manual 2000 (HCM2000) and VISSIM simulation with various lane configurations and traffic demands. In comparison with the gap acceptance model recommended in HCM2000, the proposedmodel can simplify the process of estimating capacities of TWSC intersections and extend to be applied to any multilane TWSC intersections. © 2011 Published by Elsevier Ltd.

Analysis of Platoon Impacts on Left-Turn Delay at Unsignalized Intersections

This research aimed to develop a methodology for analyzing the platoon impacts on major-street left-turn (MSLT) delay at two-way stop-controlled intersections. The main idea was to use a microscopic simulation tool to simulate different platoon scenarios in opposing through traffic and then to apply regression models to capture the impacts of platoons on the delay of MSLT. Two platoon variables were adopted as a simplification of the complex platoon scenarios to make analysis of the platoon effects on MSLT delay practical. The first two steps were to build simulation models for real-world unsignalized intersections and simulate scenarios with a combination of various factors related to platoons in VISSIM simulation. The models were calibrated with field data before simulation started. The next step was to define, derive, and calibrate two platoon variables for describing the duration and intensity of platoon arrivals in the opposing through traffic. This process effectively simplified the large combination of various factors. Finally, the two platoon variables and their relationship with the change factor of MSLT delay were modeled with regression tools. A relationship between the two variables and the delay change factor was established. The relationship indicated a positive effect by upstream platoons on MSLT delay and made it possible to quantify the impacts. The findings in this research could be used for research on platoon or signal impacts on left-turn treatments.

Modeling Impedance Effects of Left Turners from Major Streets with Shared Short Lanes at Two-Way Stop-Controlled Intersections

At two-way stop-controlled intersections without exclusive turning lanes on major streets, all three movements (right turn, through, and left turn) must share a lane. The Highway Capacity Manual (HCM) provides a special formula to deal with this lane configuration. However, this formula cannot consider short lanes such as left-turn pockets on major streets. This paper presents a new model that accounts for shared short lanes and thus delivers more realistic results for estimating the impedance effect of left turners. The new model can also deal with the effect of the so-called back-of-queue in reality. The presented theoretical concept can be extended to include the consequences of a short left-turn pocket on major streets and the corresponding capacities of the major streets with shared short lanes. Equations are derived for calculating the influence of the length of the left-turn pocket on the capacity of minor-street movements and on the probability of blockage to the through movement resulting from queued left-turn vehicles. Comprehensive simulation studies are conducted to confirm the derivation, and a general formula that considers both the short lane and back-of-queue is recommended for further applications. This formula will be incorporated into the upcoming edition of HCM.

Rural Expressway Intersection Design Guidance

A rural expressway is a high-speed, multilane, divided highway with partial access control with both at-grade intersections and grade-separated interchanges. Many state transportation agencies are converting rural two-lane undivided highways into expressways for improved safety and mobility. However, collisions at two-way stop-controlled (TWSC) expressway intersections are reducing the safety benefits that should be achieved after conversion. When the safety performance of these intersections deteriorates, the countermeasure path typically begins with several signing, marking, or lighting improvements, followed by signal-ization, and could ultimately result in grade separation. Signals are not always effective at improving safety and hamper the mobility expressways are meant to provide. Interchanges are not economically feasible at every problematic intersection and can take years to develop; therefore, more design options at TWSC rural expressway intersections are needed. Some state transportation agencies have experienced success with innovative rural expressway intersection countermeasures that are not currently included as design options within national guides. Therefore, NCHRP Report 650: Median Intersection Design for Rural High-Speed Divided Highways, was commissioned to recommend revisions to AASHTO's A Policy on Geometric Design of Highways and Streets (Green Book) and the Manual on Uniform Traffic Control Devices (MUTCD) regarding rural expressway intersection design. Project tasks included summarizing existing expressway intersection design guidance within these national guides, documenting state transportation agency experience with innovative rural expressway intersection safety treatments, and recommending revisions to the Green Book and the MUTCD.

Full Bayes Approach to Before-and-After Safety Evaluation with Matched Comparisons: Case Study of Stop-Sign In-Fill Program

The full Bayes (FB) approach has recently been proposed for evaluating road safety treatments in before-and-after studies. In recognition of the advantages of the FB method and because of the lack of sufficient data to develop the safety performance function necessary to conduct an empirical Bayes study, the FB approach is used to determine the effectiveness of the Stop-Sign In-Fill (SSIF) program of the Canadian Insurance Corporation of British Columbia. The SSIF program funds the conversion from uncontrolled residential intersections to two-way stop-controlled intersections in an alternating pattern. This alternating pattern provides consistency in the application of stop signs within a residential neighborhood. Different modeling formulations for the before-and-after evaluation were investigated, and the results were compared with those of the traditional approach. No postprocessing of the results is required to achieve the odds ratio. The FB analysis revealed an overall significant reduction in predicted collision frequency of 51.1% with the credible interval (36.8%, 62.3%) at the 0.95 confidence level. It was also found that incorporating such design features as matched yoked comparison groups in collision prediction models may significantly improve the fit, while reducing the need to account for overdispersion. The results of the traditional technique were compatible with those of the FB approach at the overall level. It seems that the random selection of sites, which reduces the regression-to-the-mean effect, is the reason that both approaches gave relatively similar overall level results. However, the two methods produced quite different results at the zone (site) level.

Impedance Effects of Left-Turning Vehicles from the Major Street at a Two-Way Stop-Controlled Intersection

At a two-way stop-controlled intersection without a turning lane on the major street, all three movements (right, through, and left) share a lane. To cover the effect of mutual impediments between these movements on the minor street capacity, the major street-shared lane equation, 17-16, is provided by the current Highway Capacity Manual (HCM 2000). This paper shows that this formula should be replaced by an improved equation to get more realistic results. This improved formula is derived. The theoretical concept can also be extended to include the consequences of a short turning lane on the major street. An equation is derived that calculates the effect of the length of the left-turn pocket on the capacity of minor movements.

Capacities of Unsignalized Intersections under Mixed Vehicular and Nonmotorized Traffic Conditions

Unsignalized intersections consist of three types–-two-way stop-controlled, all-way stop-controlled, and uncontrolled intersections–- all with different priority relationships between traffic movements according to traffic laws. A conflict technique method was used to develop capacity models for the three types of unsignalized intersections under mixed traffic conditions involving vehicular, bicycle, and pedestrian movements. With field data collected from several unsignalized intersections, the model parameters were calibrated by a comparison analysis of traffic conditions in China and were modified on the basis of actual traffic conditions. The capacities obtained by the proposed models matched well with the observed capacities and the capacities calculated by conventional methods, both of which verified the effectiveness of the proposed models. The models proved to be valuable tools for determining capacities of vehicular movements at unsignalized intersections.

Driver Performance at Two-Way Stop-Controlled Intersections on Divided Highways

The goal of this study is to examine differences in driving performance at rural expressway intersections with respect to age (younger, middle-aged, and older) and intersection (high- and low-crash area). Such intersections occur where a median-divided highway meets a two-lane, stop-controlled minor road. One intersection was identified as a high-crash area with horizontal curves; the other one was a low-crash area with no curves. All participants were asked to perform three driving maneuvers (i.e., going straight across, making a left and a right turn) from the two-lane minor roads. Age and intersection differences immediately before entering and within these intersections were examined. Sixty drivers participated in the study and were divided evenly between the younger (18 to 25), middle-aged (35 to 55), and older (65 to 80) groups. Data for each participant were collected from an on-road study in an instrumented vehicle. Results showed that both older and younger drivers were likely to run stop signs and less likely to yield at medians when compared with middle-aged drivers. The effect of age difference on driving performance before intersection entry was consistent with previous studies. Middle-aged drivers approached the high-crash intersection with significantly less brake pedal differential time and were less likely to run stop signs compared with actions by drivers in other age groups. Higher traffic volume and more complicated road geometry at the high-crash intersection may be factors contributing to the differences observed.

Queue Length Models for All-Way Stop-Controlled Intersections

The analytical procedures for all-way stop-controlled intersections in the 2000 edition of the Highway Capacity Manual (HCM) lack a model to estimate the 95th percentile queue length. This is considered a major shortcoming of the procedures, since queue length is one of the key parameters used in the traffic engineering design process. This paper provides an assessment of three queue length models for potential application at all-way stop-controlled intersections. One is the theoretically based queue length model included in the HCM for two-way stop-controlled intersections, and the others are empirically based models: a model based on simulation and a newly calibrated model based on field data. The three models were compared on the basis of field data from 18 sites. All the models produce queue length results that closely match the field data. The queue length model for two-way stop-controlled intersections was developed with a sound theoretical basis; therefore, it is recommended that this model be adop...

Queue Length Models for All-Way Stop-Controlled Intersections

The analytical procedures for all-way stop-controlled intersections in the 2000 edition of the Highway Capacity Manual (HCM) lack a model to estimate the 95th percentile queue length. This is considered a major shortcoming of the procedures, since queue length is one of the key parameters used in the traffic engineering design process. This paper provides an assessment of three queue length models for potential application at all-way stop-controlled intersections. One is the theoretically based queue length model included in the HCM for two-way stop-controlled intersections, and the others are empirically based models: a model based on simulation and a newly calibrated model based on field data. The three models were compared on the basis of field data from 18 sites. All the models produce queue length results that closely match the field data. The queue length model for two-way stop-controlled intersections was developed with a sound theoretical basis; therefore, it is recommended that this model be adopted for analyzing all-way stop-controlled intersections in the HCM. However, the empirical models are easy to use and proved to be of the same level of accuracy. The empirical models can be used as an alternative, especially when a quick estimation of the queue length is desired.

Capacity Models and Parameters for Unsignalized Urban Intersections in Poland

Road designers in Poland use capacity methods presented in subsequent issues of the Highway Capacity Manual (HCM). The new HCM 2000 gave an impulse for research aimed at evaluating adequacy of the default parameters of the HCM capacity models to the Polish conditions. Studies were conducted in several Polish towns and cities to investigate capacity of minor movements at two-way stop-controlled intersections, two-way yield-controlled intersections, and at roundabouts. The results have allowed developing equations of basic capacity parameters, i.e., critical time gap and follow-up time and calibrating the models to the local conditions. The paper presents these results and recommends how to adapt the HCM 2000 method to the Polish conditions.

Safety Impacts at Intersections on Curved Segments

Indiana geometric design policy, consistent with national standards, allows for the design of intersections on superelevated curves if other solutions are prohibitively expensive. Consequently, the Indiana Department of Transportation (DOT) has built a number of such intersections. Following a series of fatal crashes at one of these intersections, Indiana DOT made a decision to avoid designing intersections on segments with steep superelevation. This design restriction calls for expensive alternatives, such as realigning roads or adding grade separations. This research was done to determine whether superelevated intersections were more hazardous than similar intersections located on tangents and, if so, to determine what combination of factors made this true. The research focused on two-way stop-controlled intersections where the mainline was a high-speed four-lane divided highway located on a superelevated curve. An attempt was made to analyze as many factors as possible by using appropriate comparison techniques. Negative binomial models were developed to determine the statistical relationship between crash occurrence and intersection geometric characteristics, including curvature of the main road. Crash severity and the joint impact of curvature with weather and lighting conditions were examined by using binomial comparisons of proportions. Research findings show significant increases in crash frequency and severity at intersections located on superelevated curves.

Safety Impacts at Intersections on Curved Segments

Indiana geometric design policy, consistent with national standards, allows for the design of intersections on superelevated curves if other solutions are prohibitively expensive. Consequently, the Indiana Department of Transportation (DOT) has built a number of such intersections. Following a series of fatal crashes at one of these intersections, Indiana DOT made a decision to avoid designing intersections on segments with steep superelevation. This design restriction calls for expensive alternatives, such as realigning roads or adding grade separations. This research was done to determine whether superelevated intersections were more hazardous than similar intersections located on tangents and, if so, to determine what combination of factors made this true. The research focused on two-way stop-controlled intersections where the mainline was a high-speed four-lane divided highway located on a superelevated curve. An attempt was made to analyze as many factors as possible by using appropriate comparison techniques. Negative binomial models were developed to determine the statistical relationship between crash occurrence and intersection geometric characteristics, including curvature of the main road. Crash severity and the joint impact of curvature with weather and lighting conditions were examined by using binomial comparisons of proportions. Research findings show significant increases in crash frequency and severity at intersections located on superelevated curves.

Movement Capacity at Two-Way Stop-Controlled Intersections

Potential capacity at an unsignalized intersection can be described as a conditional expectation from which the capacity equations for the most common headway distributions are easily obtained and each term in the equation finds an interpretation. The movement capacity of Rank 3 streams is adjusted for both Rank 2 queues and the modified headway distribution and lower flow rates during the free-departure periods of Rank 2 streams. The resulting movement capacities are higher than those of the Highway Capacity Manual (HCM 2000), especially at low major-stream flow rates. The Rank 4 stream adjustment factors are shown to be multiplicative even across streams of different ranks. The extra correction in HCM 2000 is, in the light of this theory, unnecessary.

Why Field Measurements Differ from Model Estimates: Analysis Framework for Capacity and Level-of-Service Analysis of Unsignalized Intersections

Several questions are considered relating to the variability between field measurements and model forecasts, with a focus on the need to moderate user expectations about this variability. Considered first are the degree of variability observed in field measurements of delay and the stochastic effects in delay estimates produced by microscopic simulation. Examined next are the structure of the models of two-way stop-controlled (TWSC) intersection capacity and delay and how this structure might cause differences between field measurements and model estimates. How much the end user can moderate these differences using backcalculations, observation, and calibration and a “correct” perspective for the end user regarding these differences and variability are also discussed. These questions are considered using data collected as part of the NCHRP project used to develop the TWSC intersection capacity and level-of-service procedures contained in the Highway Capacity Manual models.

DRIVER AGE DIFFERENCES IN DAY AND NIGHT GAP ACCEPTANCE CAPABILITIES

Performance of Two-way Stop-controlled (TWSC) intersections is greatly affected by the gap acceptance capabilities of drivers. However, there have been no detailed studies conducted regarding driver age differences in gap acceptance capabilities under different light conditions. Therefore, this study was conducted to fill the lack of information in that area, by considering three driver age groups (old, middle, and young), two maneuvers (left-turn, and through), and two light conditions (daytime, and nighttime). Field observations were made at several TWSC intersections and data were collected at these sites regarding available and accepted gaps on the major street and age group of minor street drivers, both during daytime and nighttime. Statistical analysis conducted in this study with a 5% level of significance revealed that there were significant differences in gap acceptance capabilities among the three driver age groups under both light conditions. Only older drivers indicated statistically different gap acceptance capabilities depending on the light condition, where they illustrated longer critical gap values during nighttime.

Validation of Left-Turn Delay at Two-Way Stop-Controlled Intersections

Control delay for left-turning vehicles at unsignalized intersections was observed in the field and compared with average control delay calculated from the methodologies presented in the 1997 update of the Highway Capacity Manual (HCM). Unsignalized intersections with two-way left-turn lanes on the major street were observed in the peak and offpeak hours, and control delays were recorded for the one-stage and twostage left-turn processes. Next, the methodologies presented in the HCM were used to calculate the control delay for both processes and compared with the observed data. These comparisons were used as the basis for validation of the HCM methodologies regarding left-turn control delay at unsignalized intersections. From the comparisons, the calculated delay closely corresponds with the observed data, with a total approach volume at the intersection of approximately 2,500 vehicles per hour or less. Once the total approach volume increases above this level, the calculated values rapidly increase and the actual observed control delays gradually increase at a much lower rate. As a result, the observed and calculated delays are different when the intersection handles more than 2,500 approach vehicles in an hour. Statistical analyses were performed on the data to determine if the average observed control delay was related to the calculated control delay. Statistically, the observed control delay and the calculated control delay at the 95 percent confidence level show that the two data sets yield similar results for off-peak conditions. However, during the peak hour, when the total approach volumes are higher, the 95 percent confidence interval yields different results. Hence, the HCM procedures produce, on average, greater control delay estimates than the field observations when the total approach volumes are high.