Korea has many test beds where various mobility services are provided by automated vehicles. The test beds are operated in their operational design domain (ODD). However, disengagement frequently occurs, even in the ODDs of automated vehicles. In particular, human drivers have to take control of the automated vehicles at SAE Level 3 whenever the vehicles cannot drive by themselves because of an emergency or unknown factors. This study analyzed the driving safety of right turning at signalized intersections where automated vehicles face selfdriving issues because of potential conflicts with other vehicles, crossing pedestrians, and geometric factors. To conduct this analysis, we categorized right-turning intersections into two types with right-turning lanes and channelization islands and divided them into three sections, with a total of six sections. Subsequently, the six sections were compared with each other by disengagements of the automated vehicles as the key index to investigate their self-driving safety. Their significant differences indicate that ODD-related variables must be considered when designing and updating target test beds for automated vehicles.

This study aimed to investigate the factors affecting the severity of taxi traffic accidents at intersections in Busan and propose measures to improve traffic safety. This study collected data on taxi traffic accidents that occurred at intersections in the Metropolitan City of Busan during the past 3 years (2020–2022) from Traffic Accident Analysis System(TAAS) and road views, and analyzed factors affecting their severity by employing an ordered probit model. The severity of taxi traffic accidents worsened with violations of (among others) traffic signals and pedestrian protection during January, April, and September. In addition, when a major street was operated with a permissive left turn, the severity of taxi traffic accidents worsened. Measures to improve traffic safety suggested in this study included safety education that focused on particular violations for taxi drivers, mandatory education for transport employees in an experiential format, support of video storage devices for driving records, policy establishment for the promotion and certification of good and bad driving videos, time adjustment of joint safety management inspection, and left-turn signal operation with an unprotected system and P-turn guidance.

This study points out the traffic safety inhibition problem in which the yellow time length at a signal intersection can be set differently depending on the tram, and proposes a way to overcome it. This study proposes a twofold approach for trams to coordinate with the yellow signal phase designated for general vehicles, utilizing a schedule-speed-based operating speed adjustment and departure time control at stations. To validate the proposed method, simulations were conducted for a planned tram corridor in the Wirye District, Seoul. The simulation environment was developed using the simulation of the urban mobility model. The operational characteristics of tram services in the main and branch-line sections were analyzed. In the upstream section of the mainline, the operating speed was 22.04 km/h, with a total dwell time of 400 s across 10 stations, resulting in zero intersection stops. Regarding the downstream section, the operating speed was 17.71 km/h, and one intersection stop was observed. In contrast, for the upstream section of the branch line, the operating speed was 14.73 km/h and there were two intersection stops, while the downstream section yielded an operating speed of 18.12 km/h, and also comprised two intersection stops. This study examined a novel operational strategy for trams near intersections along tram corridors without directly modifying the signal system. This strategy effectively utilizes tram stop dwell time and traffic signal information. It focuses on adjusting tram speeds to ensure smooth and safe passage through signalized intersections and optimizing operations so that trams approach and cross intersections at a consistent speed. Designed as a universal model, this strategy is adaptable to various regions and environments, rather than being limited to a specific area. It is expected to support tram operation planning by considering diverse factors, such as urban structure, traffic density, and intersection characteristics. Moreover, it provides valuable guidance to ensure the punctuality and stability of tram operations in a range of urban and traffic contexts.

PURPOSES : According to government data, the Black Spot Program has resulted in an average 28.8% reduction in traffic accidents within one year of project implementation in areas where road conditions improved. However, there has been a lack of in-depth analysis of the midto- long-term effects, with a predominant focus on short-term results. This study aimed to analyze the mid-to-long-term effects of the Black Spot Program to assess the sustainability of its reported short-term impact. Additionally, the differences in the mid-to-long-term effects were investigated based on the scale of traffic accidents at intersections and the characteristics of these effects are revealed. METHODS : The mid-to-long-term effects of the Black Spot Program were analyzed at 122 intersections in Seoul, Korea, where the program was implemented between 2013 and 2017, using traffic accident data spanning five years before and after implementation. Additionally, the differences in the program's effects were analyzed at the top-100 intersections with the highest traffic accident concentration in Seoul using the chi-square test to identify these differences. To theoretically validate these differences, the Hurst exponent, commonly used in economics, was applied to analyze the regression to the mean of the intersections and reveal the correlation with improvement. RESULTS : Through the Black Spot Program at 122 intersections, a 33.3% short-term accident reduction was observed. However, the midto- long-term effect analysis showed a 25.8% reduction, indicating a slightly smaller effect than previously reported. Specifically, the top-100 intersections exhibit a 15.4% reduction. A chi-square test with a 95% confidence level indicated significant differences in the program’s effects based on the scale of traffic accidents at intersections. The Hurst index (H ) was measured for the top-100 intersections, yielding H = 0.331. This is stronger than the overall H = 0.382 for all intersections in Seoul, suggesting that the regression to the mean is more pronounced, which may lead to a lower effectiveness of the improvement. CONCLUSIONS : The mid-to-long-term effects of the Black Spot Program were relatively lower than its short-term effects, with larger differences in effectiveness observed based on the scale of traffic accidents at intersections. This indicates the need to redefine the criteria for selecting project targets by focusing on intensive improvements at intersections, where significant effects can be achieved.

PURPOSES : This study aimed to derive the factors that contribute to crash severity in mixed traffic situations and suggest policy implications for enhancing traffic safety related to these contributing factors. METHODS : California autonomous vehicle (AV) accident reports and Google Maps based on accident location were used to identify potential accident severity-contributing factors. A decision tree analysis was adopted to derive the crash severity analyses. The 24 candidate variables that affected crash severity were used as the decision tree input variables, with the output being the crash severity categorized as high, medium, and low. RESULTS : The crash severity contributing factor results showed that the number of lanes, speed limit, bus stop, AV traveling straight, AV turning left, rightmost dedicated lane, and nighttime conditions are variables that affect crash severity. In particular, the speed limit was found to be a factor that caused serious crashes, suggesting that the AV driving speed is closely related to crash severity. Therefore, a speed management strategy for mixed traffic situations is proposed to decrease crash severity and enhance traffic safety. CONCLUSIONS : This paper presents policy implications for reducing accidents caused by autonomous and manual vehicle interactions in terms of engineering, education, enforcement, and governance. The findings of this study are expected to serve as a basis for preparing preventive measures against AV-related accidents.

PURPOSES : Even when autonomous vehicles are commercialized, a situation in which autonomous vehicles and regular drivers are mixed will persist for a considerable period of time until the percentage of autonomous vehicles on the road reaches 100%. To prepare for various situations that may occur in mixed traffic, this study aimed to understand the changes in traffic flow according to the percentage of autonomous vehicles in unsignalized intersections. METHODS : We collected road information and constructed a network using the VISSIM traffic simulation program. We then configured various scenarios according to the percentage of autonomous vehicles and traffic volume to understand the changes in the traffic flow in the mixed traffic by scenario. RESULTS : The results of the analysis showed that in all scenarios, the traffic flow on major roads changed negatively with the mix of autonomous vehicles; however, the increase or decrease was small. By contrast, the traffic flow on minor roads changed positively with a mix of autonomous vehicles. CONCLUSIONS : This study is significant because it proactively examines and designs traffic flow changes in congested traffic that may occur when autonomous vehicles are introduced.

PURPOSES : This study aims to understand the characteristics of accidents involving autonomous vehicles and derive the causes of accidents from road spatial information through autonomous vehicle accident reports. METHODS : For this study, autonomous vehicle accident reports collected and managed by the CA DMV were used as data sources. In addition, spatial characteristics and geometric data for accident locations were extracted by Google maps. Based on the collected data, the study conducted general statistics, text embedding, and cross-analysis to understand the overall characteristics of autonomous vehicle accidents and their relationship with road spatial features. RESULTS : The analysis results for characteristics of autonomous vehicle accidents, applying statistical analysis and text embedding techniques, reveal that the damages caused by autonomous vehicle accidents are often minor, and approximately half of the accidents are triggered by other vehicles. It is noteworthy that accidents where autonomous vehicles are at fault are not uncommon, and when the cause of the accident is within the autonomous vehicle, the accident risk can increase. The accident analysis results using spatial data showed that the severity of accidents increases when on-street parking is present, when dedicated lanes for bicycles and buses exist, and when bus stops are present. CONCLUSIONS : Through this study, geometric and spatial elements that appear to have an impact on autonomous driving systems have been identified. The findings of this study are expected to serve as foundational data for improving the safety of autonomous vehicle operations in the future.

The safe disposal of high-level radioactive waste has become a prominent global concern, necessitating rigorous safety assessments for deep geological disposal facilities. In Korea, crystalline rock with low-permeability is considered as the host rock for radioactive waste disposal, and fluid flow and solute transport in a low-permeability rock formation predominantly occur through interconnected fracture network. To analyze and predict fluid flow and solute transport behavior within the fractures, a comprehensive understanding of solute mixing at fracture intersections is crucial. However, difficulty in direct observation of the mixing processes occurring within microscale fracture intersections has led only to analytical and numerical studies, which requires thorough experimental study based on direct observations and measurements for a fundamental understanding of the mixing processes in fracture intersections. In this study, elaborate experiments are being prepared and conducted to measure the complex flow velocity/structure and solute concentration at rough-walled fracture intersections, using a microscale visualization technique of micro Particle Image Velocimetry (micro-PIV) system. Most analytical and numerical studies have shown that at high Peclet number (Pe) > 1,000, streamlinerouting model plays a major role in redistributing solutes at the fracture intersection, at which the mixing ratio converges to zero. As opposed to the conventional mixing model, our experiments found the rebounding of the mixing ratio in the inertial flow regime, indicating an enhanced solute mixing at the intersection. Flow visualization has demonstrated that the inertial flow features, such as the development of large-scale eddies and the straightening of main streamlines, enhance the physical mixing of solutes at rough-walled fracture intersections. The findings provide insights into the influence of fracture geometry on flow dynamics and its significant impact on solute mixing at fracture intersections.

PURPOSES : The main purpose of this study is to identify directions for improvement of triangular islands installation warrants through analysis of the characteristics of crashes and severity with and without triangular islands on intersections. METHODS : The data was collected by referring to the literature and analyzed using statistical analysis tools. First, an independence test analyzed whether statistically significant differences existed between crashes depending on the installation of triangular islands. As a result of the analysis, individual prediction models were developed for cases with significant differences. In addition, each crash factor was derived by comparison with each model. RESULTS : Significant differences appeared in the "crash frequency of serious or fatal" and "crash severity" owing to the installation of triangular islands. As a result of comparing crash factors through the individual models, it was derived that the differences were dependent on the installation of the triangular islands. CONCLUSIONS : As a result of reviewing previous studies, it is found that improving the installation warrants of triangular islands is reasonable. Through this study, the need to consider the volume and composition ratio of right-turn vehicles when installing a triangular island was also derived; these results also need to be referred to when improving the triangular island installation warrants.

PURPOSES : In this study, analyze the characteristics of IOC indicator 'threshold' which is needed when evaluating the traffic signal operation status with ESPRESSO in various grade road traffic environment of Seoul metropolitan city and derive suggested value to use in field practice. METHODS : Using the computerized database program (Postgresql), we extracted data with regional characteristics (Arterial, Collector road) and temporal characteristics (peak hour, non-peak hour). Analysis of variance and Duncan's validation were performed using statistical analysis program (SPSS) to confirm whether the extracted data contains statistical significance. RESULTS: The analysis period of the main and secondary arterial roads was confirmed to be suitable from 14 days to 60 days. For the arterial, it is suggested to use 20 km/h as the critical speed for PM peak hour and weekly non peak hour. It is suggested to use 25 km/h as the critical speed for AM peak hour and night non peak hour. As for the collector road, it is suggested to use 20 km/h as the critical speed for PM peak hour and weekly non peak hour. It is suggested to use 30 km/h as the critical speed for AM peak hour and night non peak hour. CONCLUSIONS : It is meaningful from a methodological point of view that it is possible to make a reasonable comparative analysis on the signal intersection pre-post analysis when the signal operation DB is renewed by breaking the existing traffic signal operation evaluation method.

One major concern of Seoul City is the premature failure occurrence such as fatigue cracking and rutting in the pavement. Due to the acceleration at intersections and low vehicle speed at bus stops that cause higher shear and critical strain on the pavement. Because of this, there is a need to develop a new mixture that can withstand bus stop and intersection traffic while preventing premature failure. In this study, a high modulus asphalt mixture was adapted and developed to address the cracking and rutting concerns at bus stops and intersections of Seoul City. Indirect tensile (IDT) and beam fatigue testing were conducted to determine the fatigue performance of the high modulus asphalt mixture (HMB). In addition, the behaviour of the HMB considering loading speed and temperature were investigated using the IDT dynamic modulus test. It was found that the HMB performs 3 and 1.5 times better compared to conventional asphalt using IDT and beam fatigue test respectively. Moreover, it was observed that modulus value of HMB is two times better at low frequency (high temperature) compared to conventional asphalt. The dynamic modulus value of the HMB was then used as input for bus stop and intersection scenario analyses. It was found that HMB can reduce the total thickness of the pavement around 4 to 6cm compared to the conventional asphalt. It can be concluded that because of the better fatigue and rutting performance and high modulus value of HMB at low frequency, it can perform better in bus stops and intersections. It is recommended to conduct field construction to further evaluate the performance of HMB asphalt mixtures in the field.

PURPOSES: The purpose of this study is to develop a crash prediction model at signalized intersections, which can capture the randomness and uncertainty of traffic accident forecasting in order to provide more precise results. METHODS: The authors propose a random parameter (RP) approach to overcome the limitation of the Count model that cannot consider the heterogeneity of the assigned locations or road sections. For the model’s development, 55 intersections located in the Daejeon metropolitan area were selected as the scope of the study, and panel data such as the number of crashes, traffic volume, and intersection geometry at each intersection were collected for the analysis. RESULTS: Based on the results of the RP negative binomial crash prediction model developed in this study, it was found that the independent variables such as the log form of average annual traffic volume, presence or absence of left-turn lanes on major roads, presence or absence of right-turn lanes on minor roads, and the number of crosswalks were statistically significant random parameters, and this showed that the variables have a heterogeneous influence on individual intersections. CONCLUSIONS : It was found that the RP model had a better fit to the data than the fixed parameters (FP) model since the RP model reflects the heterogeneity of the individual observations and captures the inconsistent and biased effects.

PURPOSES: This study aimed to analyze traffic accidents at circular intersections, and discuss accident reduction strategies based on land use and vehicle type. METHODS : Traffic accident data from 2010 to 2014 were collected from the “traffic accident analysis system”(TAAS) data set of the Road Traffic Authority. To develop the accident rate model, a multiple linear regression model was used. Explanatory variables such as geometry and traffic volume were used to develop the models. RESULTS: The main results of the study are as follows. First, it was found that the null hypotheses that land use and vehicle type do not affect the accident rate should be rejected. Second, 16 accident rate models, which are statistically significant (with high R2 values), were developed. Finally, the area of the central island, number of speed humps, entry lane width, circulatory roadway width, bus stops, and pedestrian crossings were analyzed to determine their effect on accidents according to the type of land use and vehicle. CONCLUSIONS: Through the developed accident rate models, it was revealed that the accident factors at circular intersections changed depending on land use and vehicle type. Thus, selecting the appropriate location of bus stops for trucks, widening entry lanes for cars, and installing splitter islands and optimal lighting for motorcycles were determined to be important for reducing the accident rate. Additionally, the evaluation showed that commercial and mixed land use had a weaker effect on accidents than residential land use.

PURPOSES: The objective of this study is to establish the traffic volume-based warrants of right-turn lanes at unsignalized intersections and to introduce a risk probability methodology based on the warrants.METHODS : In this study, a risk probability of a potential rear-end collision is applied between a right-turn vehicle and the immediately following through vehicle. Using the shifted negative exponential model and the compound probability theorem, the risk probability can be expressed as the function of directional volumes and the percentage of right-turns for a two-lane and four-lane highway, respectively.RESULTS : Based on the risk probablity, guidelines for installing right-turn lanes on two-lane and four-lane highways were developed. The risk probability also showed rationality by comparing with right-turn same-direction conflicts observed in-situ.CONCLUSIONS : The results of our study define the total approaching volumes to encourage a right-turn lane as a function of operating speed, percentage of right-turn, and number of lanes.

PURPOSES :A complete signal system is not always the best solution for improving traffic operation efficiency at intersections. An alternative solution is to use a Protected Permitted Left Turn (PPLT) operation method. However, the PPLT method needs to be developed after a detailed study of driving tendencies, most notably the gap acceptance behavior, for successful implementation. In this study, the gap acceptance behavior was investigated under various variables and weather conditions, especially under rain, and the results were compared to the case of normal weather. The results of this study will be helpful in introducing the PPLT method, and are important considering the tendency of attempting unprotected left turns that is extremely common in Korean drivers.METHODS :Data was obtained by analyzing traffic footage at four intersections on a day when the precipitation was greater than 5 mm/h. The collected data was classified into seven variables for statistical analysis. Finally, we used logistic regression analysis to develop a probability distribution model.RESULTS :Gap, traffic volume, and the number of conflicting lanes were factors affecting the gap acceptance behavior of unprotected left turns under rainy conditions.CONCLUSIONS :The probability of attempting unprotected left turns is higher for larger gaps. On the other hand, the probability of attempting unprotected left turns decreases with an increase in the traffic volume. Finally, an increase in the number of conflict lanes leads to a decrease in the probability of attempting unprotected left turns.

PURPOSES :With the increasing number of older drivers in an aging society, there is a growing need for research and planning on traffic safety for the older drivers using an improved road geometry design. This study also proposed a modified urban road interchange design, which aims to keep the older drivers away from accident-prone and high-traffic areas of the city.METHODS:In this study, we examined accident data records of older drivers to identify accident-prone zones and intersections; we studied the road geometry at these zones and analyzed if it was an underlying cause for higher number of accidents. Based on the research and subsequent analysis, we suggested plans for improvement of road geometry design at these intersections.RESULTS:By studying historic data and analyzing factors that affect the likelihood of accidents of vehicles driven by older drivers and after studying suitable traffic accident prediction models, we identified the major variables that need to be modified at accident-prone intersections, such as the width of a left turn lane at an intersection and the radius of the right turn lane at a street corner. The results have a significance probability of less than 0.001 and a 95% confidence level. To improve safety at the identified intersection, this study suggests the installation of a left-turn-lane-shaped Positive Offset and a right-turn-lane-shaped Slip Lane concept and an adjustment of intervals between intersections.

PURPOSES : Because elderly drivers are more prone to becoming confused when approaching an urban intersection and thus may yield prolong judgment and decision times than non-elderly drivers, to increase the comfort and safety of the intersection environment for elderly drivers, this study applied autonomous driving tests at an urban intersection to examine their driving characteristics. METHODS: To obtain a more comprehensive understanding of driving features, this study collected drive data of non-elderly drivers and elderly drivers via an autonomous experiment using OBD2 and an eye-tracker, in addition to performing a literature review on the measured visibility range of elderly drivers at intersections. This literature review was conducted considering the general knowledge of elderly drivers having relatively reduced visibility. Additionally, as they are commonly more vulnerable, this study analyzes characteristics of elderly drivers as compared to those of non-elderly drivers. CONCLUSIONS: The results of this study can be summarized as follows: 1) the peripheral visible distance of elderly drivers is reduced as compared to that of non-elderly drivers; 2) elderly drivers approach and proceed through intersections at slower speeds than non-elderly drivers; and 3) elderly drivers yield increased driving distances when performing a right or left turn as compared to non-elderly drivers as a result of their reduced speed and acceleration and larger turning radii relative to non-elderly drivers.