Critical Gap Acceptance Research Articles

Implementing the Maximum Likelihood Method for Critical Gap Estimation under Heterogeneous Traffic Conditions

Gap acceptance analysis is crucial for determining capacity and delay at uncontrolled intersections. The probability of a driver accepting an adequate gap changes over time, and in different intersection types and traffic circumstances. The majority of previous studies in this regard have assumed homogeneous traffic conditions, and applying them directly to heterogeneous traffic conditions may produce biased results. Moreover, driver behavior concerning critical gap acceptance or rejection in traffic also varies from one location to another. The current research focused on the estimation of critical gaps considering different vehicle types (cars, and two- and three-wheelers) under heterogenous traffic conditions at uncontrolled crossings in the city of Peshawar, Pakistan. A four-legged uncontrolled intersection in the study area was used to investigate drivers’ gap acceptance behavior. The gaps were investigated for various vehicle types: two-wheelers, three-wheelers, and cars. For data collection, a video recording method was used, and Avidemux video editing software was used for data investigation. The study investigated the applicability of the maximum likelihood (MLM) method to analyzing a vehicle’s critical gap. MLM estimation results indicate that the essential critical gap values for car drivers are in the range from 7.45 to 4.6 s; for two-wheelers, the critical gap was in the range from 6.78 to 4.7 s; and for three-wheelers, the values were in the range from 6.3 to 4.9 s. At an uncontrolled intersection, the proposed method’s results can assist in distinguishing between different road user groups. This study’s findings are intended to be useful to both researchers and practitioners, particularly in developing countries with similar traffic patterns and vehicle adherence patterns at unsignalized intersections.

Investigating the Efficiency and Safety of Signalized Intersections Under Mixed Flow Conditions of Autonomous and Human-Driven Vehicles

Field experiments of autonomous driving systems have already been started in several countries around the world. However, investigations on the performance of signalized intersections under the mixed flow condition of autonomous vehicle (AV) and human-driven vehicle (HDV) are still very limited, especially in terms of considering the interaction between vehicles. This study aims at developing a methodology to evaluate the efficiency and safety performance of signalized intersections under the conditions of AV mixed flows. If AVs are set aggressively such as small critical gap acceptance thresholds, approach capacity of signalized intersections may increase. However, the aggressive driving maneuver and the misjudgment by human drivers to AV may lead to some potential safety risks. On the other hand, if AVs are set conservatively to ensure large safety margins, for example by setting large critical gap thresholds or a longer following headway, a significant drop in the capacity is expected while the safety improvements may not be insured since extra delays will be imposed on HDVs which may induce risky behavior. This study finds that AVs with either excessively aggressive or conservative settings will lead to waste of capacity or potential safety hazard. AVs with aggressive settings will enable them to accept a short gap or lag, while AVs with conservative settings will increase the waiting time of the following HDVs, which may induce aggressive driving behavior. Both types of AV settings may contribute to safety risks represented by short time-to-collision and short post encroachment time.

Effects of Rainfall on Driver Behaviour and Gap Acceptance at Multilane Roundabouts

Background: This paper filled an important gap in the behaviour of driver and gap acceptance model at multilane roundabouts by investigating their traffic performances during rainfall. Introduction: The study carried out an investigation into the impact of rainfall on driver’s behavioural changes and gap acceptance characteristics. Follow-up time headway and gap time were used as proxies. Methodology: Traffic volume, speed, vehicle type and headway data were collected at selected multilane roundabouts in Durban, South Africa. All surveyed roundabouts are within rain gauge catchment area of about 1km. Rainfall intensity was divided into three groups (light, moderate, and heavy). Dry weather data were used as control parameters. Data Analysis: Stepwise data analytical technique was used for ease of explanation and clarity. All model equations were tested for statistical fitness and deemed satisfactory for further analysis. Conclusion: Based on the synthesis of evidence obtained from driver’s behavioural changes and gap acceptance characteristics at multilane roundabouts, the paper concluded that it is correct to suggest that rainfall would have a profound effect on follow-up time headway and critical gap acceptance. However, the effect diminishes gradually as volume to capacity ratio approaches peak traffic operation. Once the threshold value of 0.85 is reached the diminishing effect of rainfall as the sole actuator of anomalous time headway increase is noticeable.

An Approach for the Estimation of Entry Flows on Roundabouts

Roundabouts are commonly used as a means of intersection control for moderate traffic flows and junctions having variations in the intersection geometry. It facilitates an orderly movement of traffic in a circular motion around a central island which is generally circular in shape. The circulating traffic is considered to be the priority stream and entering traffic waits for a suitable gap in the circulating traffic. In this fashion, it reduces the stopped delays as observed on the signalized intersections, as well as reduces the crashes and expenditure required for maintenance of traffic signals. This paper examines the entry capacity of a roundabout under different circulating flows by measuring the field entry flows. Queue formation in the approach is taken as an indicator that the approach is operating at capacity. Relationship between entry flow and circulating flow is found following negative exponential distribution. This is compared with the entry capacity estimations based on HCM, 2010. The field entry flows are found to be higher than that given by the HCM, 2010 equation. This is due to the lower critical gap acceptance behavior shown by the Indian drivers under mixed traffic compared to the homogenous traffic modeled in the HCM, 2010. An adjustment factor, multiplicative to the entry flow estimation based on HCM, 2010 equation, is proposed which can be used by field engineers directly. This will be simpler than the approach based on critical gap computation, which is tedious and difficult.

Time Gap Evaluation due to the Existence of Heavy Vehicles on Urban Expressways

Time gap is important for road user to make decision relative to the lad vehicle at a roadway segment. Theoretically, if the gap is larger than reaction time, drivers would maintain the safe following distance from the vehicle in front or else the probability of vehicle collusion is considerably high. In expressways, gap is important for the purpose of lane changing and overtaking. Due to high allowable speed on expressways, time gap might be affected, especially with the consideration of heavy vehicle existence. This paper attempts to statistically justify any significance correlation between speed and time gap in relative to critical gap acceptance pertaining to the heavy vehicles and cars interaction on urban expressways. Extensive data was collected through video recording before being abstracted and processed by utilizing the TRAIS software. Then, statistical analysis in relative to the speed and time gap for various vehicles interactions are presented. The results showed there is a significant correlation between speed and time gap for all vehicles interaction. When cars following other cars at allowable average speed, the time gap is relatively low leading to a lower critical gap acceptance as compared to the situation with the existence of heavy vehicles.

Capacity Analysis on Multi-Lane Roundabouts: An Evaluation With Highway Capacity Manual 2010 Capacity Model

In this study, capacity estimations with the incorporation of Highway Capacity Manual (HCM) 2010 method are evaluated. Parameter based sensitivity analysis on calculations with the new HCM formula and a comparative evaluation of the new methodology with two most common capacity analysis methods, i.e., the method of critical gap acceptance and the method of regression analysis, are performed. Maximum and minimum headway intervals of follow up time and critical gap parameters are alternated within the sensitivity analysis. The Transport Research Laboratory formula for regression and Australian formula for gap acceptance method are considered in comparison. Relative comparisons of predictions on capacity by HCM2010 method, regression analysis and gap acceptance method are presented considering field data obtained by observations at two roundabouts in Izmir, Turkey. The results of the study show that the HCM2010 formula led to lower capacity estimates than regression analysis and higher estimates than the gap acceptance method. Regarding the real capacity observations under high circulating flow-rates the HCM2010 method yielded to more appropriate results than the regression method. In addition to comparisons, studies on the sensitivity analysis show that entry capacity estimates possess sharper changes as smaller follow up headways are accepted.

현장 임계간격을 이용한 다지 회전교차로 분석

본 연구에서는 한국 운전자의 특성을 고려한 회전교차로 진입 임계간격을 조사하여 이를 기반으로 다양한 교통조건 및 다지 회전교차로에 대하여 그 효과를 분석하였다. 회전교차로의 임계간격은 4지, 5지, 6지, 그리고 7지에 대하여 현장조사를 수행하였으며, Raff의 추정기법을 이용하여 임계간격을 도출하였다. 도출된 임계간격은 모의실험을 위한 정산 및 통계적 검증과정을 거쳤으며, 이를 기반으로 다양한 조건에서 회전교차로 운영분석을 수행하였다. 그 결과 운영 교통량 및 회전비율 등에 따른 다지 회전교차로의 설계요소인 내접원 직경을 서비스수준별로 제시하였다. 이는 향후 회전교차로 설계지침에 반영할 수 있을 것으로 사료된다. In this paper, considering the characteristics of the driver at roundabouts by investigating the critical gap acceptance on various traffic conditions, multi-legged roundabouts were evaluated. The gap acceptance and rejection of 4-legged, 5-legged, 6-legged, and 7-legged roundabout were surveyed on real fields, and the critical gap acceptance was estimated using Raff's methods. Derived the critical gap acceptance was processed calibration and validation for micro-simulation, and then multi-legged roundabouts under variable conditions such as variations of traffic volume, turning ratio, and size of inscribed circle diameter were evaluated to verify operating conditions of roundabouts. As the results, according to the operating traffic volume and turning ratio, the inscribed circle diameters were proposed at each level of service. These inscribed circle diameters were able to reflect the guideline of geometric design for multi-legged roundabouts.

Probability Model for Understanding the Effects of Detector Delays on Right-Turn-on-Red Traffic

Detector delay as a useful detector timing feature is often recommended for minimizing disruptions on major movements caused by one or more minor movements. However, this benefit of detector delay is not always certain because the applicability of detector delay is subject to a combination of gap demand, gap availability, critical gap acceptance, follow-up headways, and other factors. The improper use of detector delay could be costly to traffic on minor streets. With little literature on the subject, the complex relationship between detector delays and different critical factors has not been fully understood. An analytical model that estimates the probability of clearing right-turn-on-red queues within the period of detector delay is presented. This probability is the first step toward an understanding of how this detector feature affects an intersection's operational efficiency. The gap-finding process of multiple right-turn-on-red vehicles is divided into several realistic cases and analyzed in detail. Insights about the process described in each case are applied to derivation of a probability model. Validation of the model is conducted with more than 4,000 simulation runs. A close fit is found between the computed probability and the simulated data with errors no larger than 5.7% of the targeted values. The method for deriving the probability model provides a stepping-stone toward deriving the delay distributions for right-turn-on-red traffic.

Merging Gap Acceptance at Midblock U-Turn Facility (A Case Study on Multilane Divided Highway in Malaysia)

A midblock U-turn facility is often provided on multilane divided highway to manage direct right turn maneuvers from main stream. Mid block U-turn facility provides a safe turn for drivers who want to change their directions. As the number of vehicles in major road increase, the driver who wants to do U-turn faces more difficulties to find a suitable gap. This study is conducted to find a critical gap acceptance and empirical relationship between U-turning delay and volume on near-side traffic lane at midblock U-turn facility in multilane divided highway. Methodology of the study was observation and in order to have a precise data, video recording technique was used. Greenshields gap acceptance method was used to analyse the data. Through the collected data, a total of 916 rejected gaps and 810 accepted gaps were used for analyzing and results show that critical gap acceptance is 3.5 sec. Meanwhile, there is an empirical relationship between delay of U-turning and volume on nearside traffic lane on major road.

COMPARISON OF DIFFERENT CAPACITY MODELS FOR TRAFFIC CIRCLES

Traffic circles have been used in many countries all over the world. Traffic circles can be defined as intersections where traffic circulates around a center island where priority is given to the vehicles entering from branches and are designed considering weaving movements as the basic goal. There are two most common capacity analysis methods for traffic circles: the method of critical gap acceptance and the method of regression analysis. This study explains the methods of gap acceptance and regression analysis. Ashworth and Field method is investigated and the applicability of these capacity models in Turkey is discussed. The obtained results have shown that both methodologies give satisfactory results; however, the existing methods should be improved (modified) considering conditions.