Traffic Conflict Technique Research Articles

Safety impact of application of auxiliary lanes at downstream locations of Thai U-turns

To assess the safety impact of auxiliary lanes at downstream locations of U-turns, the Traffic Conflict Technique was used. On the basis of the installed components at those locations, four types of U-turns were identified: those without any auxiliary lane, those with an acceleration lane, those with outer widening, and those with both an acceleration lane and outer widening.The available crash data is unreliable, therefore to assess the level of road safety, Conflict Indexes were formulated to put more emphasis on severe crashes than on slight ones by using two types of weighting coefficients. The first coefficient was based on the subjective assessment of the seriousness of the conflict situation and the second was based on the relative speed and angle between conflicting streams.A comparatively higher Conflict Index value represents a lower level of road safety. According to the results, a lower level of road safety occurs if two components apply or if a location is without any auxiliary lane. The highest level of road safety occurs if the layout includes only a single component, either an acceleration lane or outer widening.

Cross-comparison of three surrogate safety methods to diagnose cyclist safety problems at intersections in Norway

Relying on accident records as the main data source for studying cyclists’ safety has many drawbacks, such as high degree of under-reporting, the lack of accident details and particularly of information about the interaction processes that led to the accident. It is also an ethical problem as one has to wait for accidents to happen in order to make a statement about cyclists’ (un-)safety. In this perspective, the use of surrogate safety measures based on actual observations in traffic is very promising. In this study we used video data from three intersections in Norway that were all independently analysed using three methods: the Swedish traffic conflict technique (Swedish TCT), the Dutch conflict technique (DOCTOR) and the probabilistic surrogate measures of safety (PSMS) technique developed in Canada. The first two methods are based on manual detection and counting of critical events in traffic (traffic conflicts), while the third considers probabilities of multiple trajectories for each interaction and delivers a density map of potential collision points per site. Due to extensive use of microscopic data, PSMS technique relies heavily on automated tracking of the road users in video. Across the three sites, the methods show similarities or are at least “compatible” with the accident records. The two conflict techniques agree quite well for the number, type and location of conflicts, but some differences with no obvious explanation are also found. PSMS reports many more safety-relevant interactions including less severe events. The location of the potential collision points is compatible with what the conflict techniques suggest, but the possibly significant share of false alarms due to inaccurate trajectories extracted from video complicates the comparison. The tested techniques still require enhancement, with respect to better adjustment to analysis of the situations involving cyclists (and vulnerable road users in general) and further validation. However, we believe this to be a future direction for the road safety analysis as the number of accidents is constantly decreasing and the quality of accident data does not seem to improve.

Using Czech TCT to Access Safety Impact of Deceleration Lane at Thai U-Turns

The purpose of this study is to evaluate the safety impact of the deceleration lane at the Upstream Zone of at-grade U-turns on 4-lane divided highways of Thailand. A substantial speed reduction is required by the vehicles for diverging and making U-turn, and the deceleration lanes are provided for this purpose. These are also provides a storage space for the U-turning vehicle to avoid unnecessary blockage of the through lanes and reduce the potential of rear-end collisions. The safety at the U-turn is greatly influenced by the proper or improper uses of the deceleration lanes. Depending on their length, not only either the full or partial speed adjustment can occur within the deceleration lane also it influence the road users’ behavior. To assess the safety impact, the four groups of U-turns with the varying length of deceleration lanes were identified. Due to the limitations of availability and reliability of road crash data in Thailand a surrogate approach, based on traffic conflict was adopted for the study. Although the Traffic Conflict Technique (TCT) is widely accepted as an alternative and proactive approach but subjective nature of its parameters is debatable since its origin. The U-turns’ geometric data, traffic conflicts and volume data were recorded in the field. The Severity Conflict Rate (SCR) is assessed by applying weighing factor (based on severity grades according to the Czech TCT) to the observed conflicts exposed to the conflicting traffic volumes. A comparative higher value of SCR represents a lower level of traffic safety at a U-turn facility and a significant relationship was obtained between the length of deceleration lanes of the U-turns and the level of road traffic safety.

Analysis of Road User Behavior and Safety During New York City’s Summer Streets Program

Automated computer vision video analysis techniques were used to analyze video data during the operation of New York City’s Summer Streets Program at a major signalized intersection. The main objectives of this study were to diagnose pedestrian and cyclist safety issues during the shared space operation and to demonstrate the feasibility of the automatic extraction of road user data (e.g., pedestrian, runner, rollerblader, or cyclist) required for microscopic behavior analysis. Road users’ speeds and pedestrian gait parameters (step frequency and step length) were automatically extracted and analyzed. Results show that pedestrian walking speed was highest during the Summer Streets operation (1.49 ± 0.54 m/s) because pedestrians had more street space to use and slowest during normal operations (1.30 ± 0.22 m/s). Bike speeds were low during the Summer Streets event (3.62 ± 0.97 m/s), likely because of interaction with pedestrians, but these speeds increased during normal traffic operations. Pedestrians and cyclists moving in groups tended to be slower and confirmed results found in previous studies. The safety analysis was conducted with traffic conflict techniques. It was observed that the lowest rate of conflicts between pedestrians and cyclists and between cyclists was found to be during Summer Streets operations. In addition, an analysis of spatial violations showed that some road users were not observing traffic rules in the transition period after Summer Streets ceased to operate.

Investigating the Heterogeneity of Postencroachment Time Thresholds Determined by Peak over Threshold Approach

Inconsistency in the definition of conflict is a fundamental issue of the traffic conflict technique and raises questions about the validity of this technique. A significant aspect of inconsistency is that thresholds to distinguish traffic conflicts from normal events have not been clearly determined. The study presented in this paper proposed a peak-over-threshold (POT) approach to determine postencroachment time thresholds between traffic conflicts and normal events. The determined thresholds were evaluated by testing the correlation between the defined conflicts and observed crashes. A further regression analysis was conducted to explain heterogeneity of POT-determined thresholds. The results show that traffic conflicts defined by POT-determined thresholds have a relatively strong relationship with observed crashes, and the Pearson’s correlation coefficient is .66. Moreover, the threshold heterogeneity in this study mainly stems from the variety of exposure (i.e., traffic volume). This study also implies that the POT approach, which can account for possible heterogeneity in thresholds, is promising for improving the validity of the traffic conflict technique.

Evaluation of overtaking manoeuvres on two-lane rural roads

Two-way two-lane roads constitute a major proportion of rural road networks in Iran. The work described in this paper aimed to investigate the factors that influence overtaking manoeuvres using traffic conflict techniques to collect field data related to dangerous overtaking. The process of data collection was conducted by means of video recordings and other heuristic methods using inductive loop detectors. As one of the most important underlying independent variables, the percentage of time spent following was simulated. The classification and regression tree method (a common data mining approach) was used to identify important variables and explore the cut points of continuous variables. No-passing distance, road width, mean speed and vehicle time spent following were identified as the most important variables, and the results indicated a direct relationship between no-passing distance and the number of dangerous overtaking manoeuvres. Road widths less than the standard values and an increase in mean speed were also found to lead to an increase in the number of dangerous overtakings.

Safety evaluation of signalized intersections with left-turn waiting area in China

In recent years the metropolitans in China have seen the surging installations of the left-turn waiting area (LWA) at the signalized intersections. The design allows the left-turning vehicles to enter the intersection at the onset of the through green phase (of the same approach) and wait for the exclusive left-turn signal at the LWA. The LWA layout can effectively reduce the probability of stranded and queue overflow of the left-turn vehicles, but no study is conducted yet to assess the safety performance of the signalized intersections with LWA. The paper adopts the traffic conflict technique (represented by post-encroachment time), compares the discrepancy of conflict types between intersections with LWA and without, and develops the severity models to identify the contributing factors for the left-turn conflicts. Results demonstrate that the left-turn volume, driving outside the LWA, running red light, the presence of secondary conflicts, and the rear-end conflicts significantly increase the severities of traffic conflicts at the LWA. The findings serve to provide recommendations to revise the current design standard of the LWA (GB5768-2009) and consequently improve the safety operations of signalized intersections with LWA in China.

Modelling Collision Potentials in Port Anchorages: Application of the Navigational Traffic Conflict Technique (NTCT)

Despite the extent of works done on modelling port water collisions, not much research effort has been devoted to modelling collisions at port anchorages. This paper aims to fill this important gap in the literature by applying the Navigation Traffic Conflict Technique (NTCT) to measuring the collision potentials in anchorages and for examining the factors contributing to collisions. Building on the principles of the NTCT, a collision potential measurement model and a collision potential prediction model were developed. These models were illustrated by using vessel movement data of the anchorages in Singapore port waters. Results showed that the measured collision potentials are in close agreement with those perceived by harbour pilots. Higher collision potentials were found in anchorages attached to the shoreline and international fairways, but not at those attached to confined water. Higher operating speeds, larger numbers of isolated danger marks and day conditions were associated with reduction in the collision potentials.

Behavioural analysis of interactions between pedestrians and vehicles in street designs with elements of shared space

This paper describes the development and implementation of qualitative behavioural criteria in order to analyse the conduct of pedestrians and vehicles when they are required to interact with each other, with particular interest to street designs with elements of shared space. The new behavioural analysis technique is developed by identifying the fundamental principles that underpin existing traffic analyses, such as traffic conflicts techniques, and adapting those to a qualitative framework that describes the mindset and rationale of road users. The technique is then applied to a case study in London, using video data from periods before and after the redevelopment of the Exhibition Road site from a conventional dual carriageway to a modern design with some elements of shared space. With the main goals being to assess the pedestrians’ confidence and the vehicles’ tolerance/patience when forced to interact with each other, behavioural trends are related to instantaneous characteristics of the vehicle flow (vehicle approach speed and traffic density). The data produced are used to develop and validate qualitative behavioural relationships for pedestrian–vehicle interactions, as well as location-specific conclusions for the Exhibition Road site.

Research of Vessel Traffic Safety in Ship Routeing Precautionary Areas Based on Navigational Traffic Conflict Technique

Recently, a basic pattern of coast seaway system networking has been established in China. As a result, a large number of precautionary areas will be gradually established. Because of the high frequency of ships' encounters and accidents, these precautionary areas have been a weak but important facet of ships' routeing. Navigational Traffic Conflict Technique (NTCT) was introduced to quantitatively evaluate the safety of precautionary areas. Focusing on traffic conflict, the calculation method of navigational traffic conflict points is discussed, and a comparison analysis of the two editions (2002 and 2008) of Yangtze estuary ships’ routeing is performed. Then the automatic acquisition system of navigational traffic conflict data that can be used in the quantitative analysis of the influence degree of a single traffic flow on the whole precautionary area safety is developed, and the left-turning traffic flows of Yangtze estuary ships’ routeing precautionary areas B are taken as an example. This article establishes a foundation for the precautionary area safety analysis system, and provides the decision basis for policy making in optimisation of geometric design and traffic organisation of precautionary areas.

Can Time Proximity Measures be Used as Safety Indicators in All Driving Cultures?

Limitations associated with traditional collision-based safety analysis techniques led to a growing interest in the use of surrogate safety measures such as the traffic conflict technique. This interest was facilitated by advances in automated video-based data collection methods that helped to overcome the reliability issues associated with manual collection of data on traffic conflicts. Various objective conflict indicators that measure various spatial and temporal aspects of user proximity are available to measure the severity of traffic events. These time-proximity conflict measures assume that proximity is a surrogate for conflict severity. However, this assumption may not be valid in many driving environments. The objective of this paper is to investigate whether time-proximity conflict measures can be a good indicator of safety in less-organized traffic environments with highly mixed road users. A case study of motorcycle conflicts in a highly congested shared intersection in Shanghai, China, was used as a case study. Traffic conflicts were analyzed with the use of automated video-based analysis techniques. Several traffic conflict indicators designed to detect evasive actions, such as deceleration, jerk, and yaw rate, were recommended as better able to measure traffic conflicts in such traffic environments. The results showed that indicators that measured evasive actions had higher potential to identify motorcycle conflicts in highly mixed, less-organized traffic environments than did time-proximity measures such as the time to collision.

Automated Pedestrian Safety Analysis at a Signalized Intersection in New York City: Automated Data Extraction for Safety Diagnosis and Behavioral Study

Automated computer vision techniques were used to analyze 2 h of video data collected at a major signalized intersection in New York City. The main objectives of this study were to diagnose pedestrian safety issues and identify contributing factors at the intersection and to demonstrate the feasibility of the automatic extraction of pedestrian data required for pedestrian behavior analysis—mainly pedestrian speed and gait parameters. The safety study was conducted with traffic conflict techniques. The main factor that contributed to the high number of pedestrian and vehicle conflicts was found to be pedestrian violations, mainly temporal violations in which pedestrians crossed the street during the “Don't Walk” or flashing “Don't Walk” phase. During the 2 h analyzed, about one-third of pedestrians were noncompliant with the signal timing or crosswalk boundary (17.9% spatial violations and 15.3% temporal violations). Pedestrian speed, step frequency, and step length were automatically extracted for 333 pedestrians and were found to follow the normal distribution with 95% confidence (mean and standard deviation of 1.47 ± 0.27 m/s, 1.96 ± 0.17 Hz, and 0.75 ± 0.14 m, respectively). Gait analysis showed that the walking speed for single pedestrians was 9% higher than for those who walked in groups. Males tended to be slightly faster than females, with higher step length but lower step frequency. Violators tended to have higher walking speeds compared with non-violators, and the difference in speed was dependent on step length but not on step frequency.

Assessing Safety Improvements to Pedestrian Crossings Using Automated Conflict Analysis

Surrogate safety measures, such as the traffic conflict technique, have been promoted as an alternative or complementary approach to evaluate road safety from a broader perspective than collision statistics alone. The before-and-after evaluation of safety treatments is one application that can significantly benefit from the use of surrogate safety measures. This study demonstrated the use of automated traffic conflict analysis to conduct before-and-after safety evaluations. The objective was to conduct a time-series (before-to-after) safety evaluation for an intersection in Surrey, British Columbia, Canada, where several pedestrian-related countermeasures were implemented. Treatments included one or more of the following: use of protected-only turns for left-turning vehicles, installation of pedestrian countdown timers, crosswalk realignment, and use of drop-down sidewalks. Results indicated a significant decrease in both pedestrian conflict frequency and severity at the intersection following the treatments. A greater reduction of conflicts was exhibited on the western portion of the intersection where several countermeasures were implemented, including a protected left-turn phase, dual drop-down ramps, and a shift in the crosswalk further west. This variation in conflict reduction along different segments of the same intersection gives a particularized view of the impact of the adopted countermeasures. The outcome of this research provides evidence that surrogate safety indicators can be used effectively to diagnose safety problems and evaluate countermeasures at intersections.

Effects of behavioral characteristics of taxi drivers on safety and capacity of signalized intersections

Comparative analyses were conducted to compare the effects of the behavioral characteristics of the drivers of taxis and private cars on the capacity and safety of signalized intersections. Data were collected at sixteen signalized intersections in the Nanjing area in China. The risk-taking behaviors of the drivers of taxis and private cars were compared. The results suggest that 19.9% of taxi drivers have committed at least one of the identified risky behaviors, which is 2.37 times as high as that of the drivers of private cars (8.4%). The traffic conflicts technique was used to estimate the safety effects of taxis and private cars. The overall conflict rate for taxis is 21.4% higher than that for private cars, implying that taxis are more likely to be involved in conflicts. Almost all of the identified traffic conflicts can be attributed to certain levels of risk-taking behaviors committed by either taxi drivers or the drivers of private cars, and taxi drivers are more likely to be at fault in a conflict. Failure to yield to right-of-way and improper lane change is the leading causes of the conflicts in which taxis are at-fault. The research team further studied the effects of taxis on the queue discharge characteristics at signalized intersections. The results show that the presence of taxis significantly reduces both start-up lost time and saturation headways. The effects of taxis on saturation flow rates are dependent on the proportion of taxis in the discharge flow, and the saturation flow rates increase with the increase in the proportion of taxis. The adjustment factors for various proportions of taxis for different turning movements are then calculated to quantitatively evaluate the effects of taxis on the capacity of signalized intersections.

Optimization of pedestrian phase patterns and signal timings for isolated intersection

This paper establishes quantitative criteria for selecting pedestrian phase patterns between the exclusive pedestrian phase (EPP) and the normal two-way crossing (TWC) with both safety and efficiency factors traded-off in an economic evaluation framework. The safety effect is assessed by modeling the number of pedestrian-vehicle exposures and pedestrian violations (i.e., pedestrian noncompliance) based on the traffic conflict technique (TCT); and the operational efficiency is measured with a new model considering pedestrian delay due to signal, conflicts, and detour. Both safety and efficiency performance indices are converted to monetary values in an integrated model to simultaneously select pedestrian phase patterns and optimize signal timings of an intersection with two phases. Case study results have shown the promising property of the proposed approach to assist transportation professionals in properly selecting pedestrian phase patterns at signalized intersections.

Research on Traffic Safety on Freeway Merging Sections Based on TTC and PET

Since the traffic safety of freeway interchange merging sections and the accidents occurred in this areas can not meet the requirement of statistical analysis, this paper employed traffic conflict technique to analyze the safety situation of freeway merging sections. The traffic data of vehicles through the merging sections are collected and analyzed. These data include the vehicle type, speed, time headway and others based on the features of individual vehicle. Then two methodologies are developed, the first is based on time to collision (TTC), which can calculate the rear-end conflict number, while the second is based on post encroachment time (PET), which can calculate the lane-change conflict number. The results show that these surrogate measures can quantitatively describe the rear-end conflict situation and lane-change conflict situation.

Traffic conflict techniques for road safety analysis: open questions and some insights

Developing non-crash or surrogate measures of road safety has drawn considerable research interest over the past five decades. Traffic conflict techniques, which analyze the safety situations from the aspect of more observable traffic events than crashes, are the most prominent techniques to date. This study provides a comprehensive review of previous research on traffic conflict techniques, striving to find answers to the following open questions: What is a traffic conflict? How to collect the traffic conflict data? And what is the ground to claim that traffic conflicts can be valid surrogates for crashes? The strengths and weaknesses of available answers to these questions are assessed based on methodological and empirical grounds. Directions for the future research are identified and outlined. It is believed that following recommended future directions may offer convincing answers to identified open questions.

The Optimal Design of U-Turns on Thai Highways

The purpose of this study is to evaluate optimal layout design of U-turns on basis of road safety for highways of Thailand. In Thailand, U-turns are one of major segments of highways after intersections, contributing to rural highway crashes. Different layout designs of U-turn and variation in its elements are influencing factors for drivers’ expectance and causing confusion in drivers. Therefore, the evaluation of layout designs of U-turn of rural highways is important to reduce future crash rates. For this research, initially the ‘accident based investigation’ was considered, the accident cost rates for different layout designs was planned to be evaluated. But due to underreporting of accident data in various agencies in Thailand, the ‘accident based investigation’ could not produce reliable results, so ‘event based investigation’ is considered for further study. For ‘event based investigation’ study the ‘traffic conflict technique’ is planned to use.

Shifted Gamma-Generalized Pareto Distribution model to map the safety continuum and estimate crashes

Safety continuum plays an important role in the development of traffic conflict techniques. This study proposes a shifted Gamma-Generalized Pareto Distribution (GPD) model to map the whole safety continuum and then estimate crashes. Two important model parameters, the threshold and shifted value, are discussed in detail. The threshold, which is mapped as the boundary to distinguish conflicts and normal events, is estimated simultaneously with other four Gamma-GPD parameters by Bayesian approach. The shifted value, which is introduced by shifted reciprocal mapping and mapped as the boundary to distinguish conflicts and crashes, is determined by a crash-based approach. The proposed model is applied to estimate crashes related to lane change maneuvers on freeways, and the Bayesian approach is also compared with the classical maximum likelihood estimation approach. More accurate and less uncertain estimated crashes are obtained through the Bayesian approach, and this also shows the superiority of shifted reciprocal mapping approach over the linear mapping approaches. Meanwhile, the estimated model parameters show that the boundary to distinguish conflicts and crashes is consistent while the boundaries to distinguish conflicts and normal events might be varied across different segments.

Intercultural Analyses of Time-to-Collision in Vehicle–Pedestrian Conflict on an Urban Midblock Crosswalk

In traffic conflict techniques, time-to-collision (TTC) is suggested to be a surrogate measure of conflict severity. In order to address the differences in conflict situation between driving cultures, field traffic data have been collected by video recording and image processing at urban midblock crosswalks both in Beijing, China, and Munich, Germany. Focusing on the vehicle–pedestrian (VEH-PED) conflict, by identification of field observation, conflict situation, and TTC calculation, a trajectory-based data matrix is created for understanding the entire conflict process. TTC distribution and relationships between TTC and PED/VEH-based parameters are investigated with intercultural comparison. Special cases in China, where traffic noncompliance is common, are also studied as a complement to normal VEH-PED conflicts. The research will hopefully lay the groundwork in the target of providing advanced driver assistance system adaptation databases in the future.