Estimating critical gap of roundabouts by different methods

Studying Critical gap and headway distribution has vital role in reduction of traffic problems. Critical gap and its distribution are traffic characteristics that are used in determination of capacity, delay and level of service at unsignalized intersection. Many study has been done on critical gap in developed countries under homogeneous traffic and road conditions. This study is aimed to insight available headway distribution and critical gap of driver in urban intersection under heterogeneous traffic condition and weak lane discipline in developing country like Ethiopia. In this paper three unsignalized intersection in Adama city has been selected on the basis of traffic volume and importance of the intersection. The primary data that were used for this study were traffic volume, available headways, waiting time, geometry of road. By using digital Camera, videos data were recorded; later quantitative data were extracted from videos. Two Statistical Packages that were used in analysis of this study. Statistical Package for Social Science Statistic 20 was used to fit best distribution model of headway. Kolmogorov Smirnov and Anderson Darling testing techniques were conducted to check validity of model for headways in different flow ranges. From hypothesized distributions, exponential, gamma, lognormal and normal distributions were selected for different intersection. It has been indicated that, for higher flow rate lognormal distribution model is best fit in estimating cumulative density function of headway. Critical gaps of drivers for three selected intersections were also computed by using maximum likelihood method. Through Comparison of estimated values indicates that, Franko intersection has highest critical gap of 5.17sec.

A gap acceptance study was performed at a T-intersection at which left-turn traffic on the through leg had the right-of-way. A number of methods were used to model the critical gap size at this atypical intersection. The critical lag and gap values varied according to the method used to compute them, and the different movement or conflict combinations exhibited different critical values. On the approach required to stop, representative values ranged from a 7.8-s critical gap with the higher priority traffic stream from the right, to a 2.2-s critical lag with the highest priority traffic from the opposite direction. The drivers involved in certain movement or conflict patterns at this intersection with the atypical right-of-way pattern were quite comfortable with short lags and gaps.

Driver's Critical Gap Calibration at Urban Roundabouts: A Case Study in China

Critical gap through clearing behavior of drivers at unsignalised intersections

Despite an increasing acceptance of modern roundabouts in the United States, unfamiliarity with this type of intersection is still widespread. This unfamiliarity results in drivers making unpredictable decisions about gap acceptance, thus increasing the difficulty of design decisions. The relationship between the spatial and temporal critical gaps accepted by drivers in modern roundabouts is explored with gap acceptance data from more than 1,500 vehicle interactions at the roundabout on the University of Massachusetts, Amherst, campus. The critical gap was determined to be 42 ft spatially and 2.2 s temporally, which correlated directly with the average speed of the circulating flow. A longer critical gap time was observed for heavy vehicles in the traffic stream. Even though the unidirectional flow of the circulating traffic creates a potentially unpredictable environment for the driver, careful and precise analysis to obtain an accurate and reliable value of the critical gap will allow for improved roundabout design.

Roundabout Capacity in Heterogeneous Traffic Condition: Modification of HCM Equation and Calibration

Gap acceptance is the main parameters of intersection capacity and safety research. Although the domestic and foreign scholars and researchers have studied the critical gap deep in the capacity researches, very few paper has been discussed the process of gap acceptance from the view of traffic safety behaviors. The objective of this paper is to analyze the safety behavior of gap acceptance, to examine the relationship between gap acceptance behavior and waiting time, and to make suggestions of safety traffic control design for unsignalized intersections. The authors observed 1216 left-turn gap-acceptance behavior samples from 11 tee type of unsignalized Intersections, which are two ways four lanes in majors and two ways two lanes in minors (i.e. 4 x 2 T Intersections). It is found that some drivers' gap-acceptance behaviors change along with their waiting time. It is also found that the precision of the traditional prediction model for critical gap could be improved if waiting time is considered, which implies that drivers' gap acceptation behaviors have much to do with drivers' waiting time. The waiting time is longer, and the possibility of taking risks to accept the small gap is larger. The quantitative model was also developed to explain the relationship between values of accepted gap and waiting time.The conclusions of paper are expected to be useful in safety control design and discussing safe driving behavior for unsignalized intersections.

In order to evaluate traffic operation status of unsignalized intersection on arterial road in slight snowy weather, it was studied that the influence of slight snow on capacity of unsignalized intersection. Critical gap and following headway data of one unsignalized intersections were observed by video method. And gap accepting theory was adopted to calculate the capacity of unsignalized intersection. An example was given for calculating the capacity of an unsiglized intersection on normal pavement and slight snowy pavement. It showed that capacity of unsignalized intersection in slight snowy weather decreased by 11.26% compared with on sunny weather.

Gap-acceptance method is one of the classical methods used to analyze the capacity of roundabouts. Critical gap has a privileged role in this approach. Different driver behavior and local rules of traffic have key role in implementing gap-acceptance method into the local standard for capacity calculation in each country. Therefore, a reliable method for estimation of critical gap at a certain location can be of great importance. This paper presents an experimental investigation and analysis on whether it is possible to find differences between estimating critical gap using video-based gap acceptance data of roundabouts in Hungary. Three single lane roundabouts of different size were recorded for hours in different locations in Budapest and Érd to assess gap acceptance data. Three different methods or models were used to estimate critical gap and no significant differences were found between their results.

The gap acceptance research is an essential research in the estimation of the values of capacity and delay at priority junction. This study was conducted to estimate and compare the value of critical gap using the equilibrium method, maximum likelihood method and Malaysia Highway Capacity Manual (MHCM), conditioned with and without presence of the motorcycles. The two priority junctions with multilane road were targeted and data were recorded using video camera technique. The equilibrium method estimated the critical gap at priority junction with motorcycles approximately 0.17 s shorter than that of priority junction without motorcycles. Similarly, this study has been carried out for maximum likelihood method, and the magnitudes of critical gap with motorcycles were estimated about 0.20 s less than the situation without motorcycles. In the case of MHCM, the results showed a differentiation around 0.04 s between both situations. These close results show that differences of critical gap between both scenarios are very small. However, considering the importance of critical gap for evaluating the operational conditions at priority junctions, we highly recommend the estimation of that gap in some Asian countries such as Malaysia with many motorcycles.

Analysis of snow load probabilistic models and calculation of reference snow pressure using maximum likelihood method for multiple cities in Liaoning Province, China

For the estimation of critical gaps for unsignalized intersection analysis, the so-called maximum likelihood method currently is treated as the best approach. This paper demonstrates that the maximum likelihood method is a special case of the more general mathematical survival theory for the estimation of interval-censored parameters. The assumption of a special mathematical type for the statistical distribution of critical gaps is an essential part of the method. Test applications with various types of functions show that the results do not significantly depend on the applied type of distribution. Therefore, the traditional lognormal distribution is not an essential part of the theory. Instead, it is reasonable to apply that type of distribution that can be handled more easily. From this standpoint, the logistic distribution is favorable. Because the variance of each kind of distribution of the critical gaps does not contribute to the precision of the result, a simplified method is useful if the standard deviation of the critical gaps can be predefined and if only the scale parameter of the function is adjusted to the measured gap data. The results from this single-parameter method are rather congruent with those from the more complicated two-parameter method. The simplified approach opens the way to apply critical gap estimation in traffic engineering practice to adjust capacity calculations to local driver behavior.

Conference report: Biointeractions 87, Cambridge UK, 6–8 July 1987

Over the past few years there has been increased installation of non-traversable un-signalized median openings in most of the urban regions in India. The motive behind this installation is to eliminate problems associated with illegal U- turns at intersections and other traffic facilities close to the median openings on multi-lane urban roads. Data collected for this study is in the form of video-images of six U-turn median openings on 4-lane and 6-lane roads situated in the cities of Bhubaneshwar, Rourkela and Ranchi located in the eastern part of India. This paper introduces a new concept on merging behavior of U-turn vehicles for evaluation of gaps accepted by drivers at median openings based on the INAFOGA method; which is further compared with the critical gap values obtained by and Macroscopic Probability Equilibrium concept for heterogeneous traffic flow in the urban region of the Indian states. IBM-SPSS 22.0 has been used to perform a paired-sample Hypothesis (t-test) between these two methods which reveal that critical gap values obtained by INAFOGA are 18-31% more than those obtained by Probability Equilibrium method. Radar plots, box-plots, t- statistic, two-tailed significance value coupled with higher critical gap values for different modes of transport (except Sport Utility Vehicles) validates the fact that INAFOGA method is indeed appropriate under mixed traffic conditions. This observation on influence area of gap acceptance at median openings using the merging behavior concept has given sufficient clue to carry out further research on critical gaps at roundabouts or interchanges.

Classical sight-distance problems involve the horizontal and vertical alignment of the road and stationary objects around the roadway that interfere with the motorist’s view. Dynamic sight-distance problems concern the sight-distance situation engendered by “nonstationary” objects such as vehicles using the roadway. This paper discusses a theoretical approach for evaluating the operational effect of a dynamic sight-distance problem that is associated with unprotected left-turning traffic at intersections. Specifically, the problem discussed in this paper concerns the effect of left-turning vehicles on the gap acceptance of the vehicles in the queue. Sample test results indicate that the dynamic sight-distance problem considered here can result in larger critical gaps, and hence significantly reduce the capacity of left-turn movements at unsignalized intersections or signalized intersections with permitted left-turn signal phasing. A sensitivity analysis shows that the severity of dynamic sight-distance problems increases with the traffic intensity (i.e., the degree of saturation). The paper concludes that the critical gap value for unprotected left-turning traffic is not only dependent on the number of conflicting traffic movements, but it is also dependent on the traffic intensity and traffic composition (e.g., percent compact vehicles versus percent noncompact vehicles). These factors should be considered in determining reasonable critical gap values that can be used for capacity analysis of unprotected left-turn movements at traffic intersections.