INTRODUCTIONReducing highway accidents has always been one of the most important tasks for traffic engineers. Estimating the number of accidents resulting from a given highway design is very important in evaluating different design alternatives. Therefore, it is important to understand the relationship between accidents and the characteristics of roadways in order to try to reduce the number of accidents [1]. Accident prediction models have been very useful in estimating the expected number of accidents on intersections and road segments. Essentially, an accident prediction model is a mathematical relationship that expresses the average accident frequency of a site as a function of traffic flow and other site characteristics [2].Road safety modelling has attracted considerable research interest in the past four decades because of its wide variety of applications and important practical implications. Public agencies, such as State Departments of Transportation, are interested in identifying accident-prone areas to promote safety treatments. Similarly, transportation engineers are interested in identifying factors (traffic characteristics, geometric characteristics, etc.) that influence accident frequency and severity to improve roadway design and provide a safer driving environment. High cost of highway accidents paid by societies around the world makes highway safety improvement an important objective of transportation engineering. A significant number of previous studies have indicated that improvements to highway design could produce significant reductions in the number and severity of crashes [3]. Therefore, in the development of feasibility studies it is necessary to properly determine the impact of road and traffic Previous studies utilized different methodologies for accident prediction on highways [4]. Joshua & Garber [5] pointed out that linear regression models do not adequately describe the nature of crash frequency data. Poisson or negative binomial regression models are better suited for defining the random, discrete, and non-negative nature of crash occurrences [6].Accident prediction models have been used elsewhere as a useful tool by road engineers and planners [7]. Fletcher et al. [8] found that due to wide differences in traffic mix, road quality, design and road user behaviour, it would be neither valid nor useful to apply simple multiplicative factors or even devise more complex conversion formulae for models developed elsewhere for another country. The regression models developed for certain conditions, which can vary regionally from country to country, cannot be generalized for all countries, which may have different standards in geometric designs and may create various operational environments for traffic under their jurisdictions.The roadmap of this paper can be summarized in the next few steps. First, the paper provides a comprehensive literature review of the former research of this issue. Special attention is given to several factors which define the quality of similar studies...
Saturation flow is the base rate in the procedure for optimizing traffic signal operation and determining the measure for effectiveness of intersection operation. Different approaches and structures of analytical value models indicate the complexity of the problem of determining the saturation flow value in real conditions. This paper presents the synthesis of the results and conclusions of studying the saturation flow rate phenomenon at signalised intersections in Serbia in the last thirty years, by applying various survey techniques. The surveys relate to straight lane saturation flow value, in the survey conditions mostly resembling the idealised conditions in which saturation flow can be generated. The obtained results indicate that there is a significant trend of changes in the base saturation flow value compared with those first referred in 1963 by Webster and Cobbe, change in the significance of impact factor on saturation flow value, and the necessity to determine them on the local level.
This paper presents the evaluation results of three traffic solutions for the complex grade-separated intersection located in the old part of Belgrade at the junction with the new bridge over the Sava River. The corridor to which the intersection belongs together with the new river bridge are parts of a great urban artery called the Inner Half Semi-Ring Road (IHSRR). The traffic solutions that are evaluated are defined in the preliminary design phase, based on two opposed concepts: a complete grade separation of all intersection legs (the CPV alternative Á 'grade-separated') and a grade separation designed to minimise construction costs (DMC 1 and 2 alternatives Á 'minimise cost'). The evaluation procedure is conducted in three steps: first, the score based on expert assessment of the functionality of the design solutions is determined; second, the alternatives are ranked according to the value of a set of state indicators obtained by microsimulation using PTVÁVISSIM 4.10; and third, the final score is obtained by multi-criteria evaluation using the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) method. The results of the first evaluation step show a small advantage for DMC 2, a sub-alternative of the DMC 1 alternative. The results of the micro-simulation give advantage to the DMC 1 alternative. The multi-criteria evaluation provides a better 'goodness factor' for the CPV alternative against the DMC 1 alternative. At the same time, the least construction cost favours alternative DMC 1.
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