نشریه مهندسی ترافیک
پیاپی 46 (تابستان 1390)

Urban highways are one of the infrastructural facilities that have an important role in daily transportation. Proper management of traffic using the highway controllers would lead to a uniform flow that is very effective in reducing the delays and increasing the flow capacity. Studies have been conducted on the effectiveness of these controllers on flow and this is becoming one of the important debates in the field of traffic management. Hence, in this paper, the procedures and methodologies used for designing and coordinating ramp metering and variable speed limit are generally classified and presented. Then by evaluating the results obtained from the implementation of each method, a qualitative and comprehensive comparison between them is presented. As a comparison result, it is understood that ramp metering is effective only when there is little congestion on the main road. Influence of ramp metering decreases when congestion increases. In this case, applying the variable speed limit at the upstream of the onramp would help to make the traffic flow uniform. Using RM and VSL simultaneously, RM reaches maximum efficiency on the main road traffic flow.

IIn this research, the problems and shortcomings of the traditional freeway capacity evaluation based have been studied. At first, the most common methods for freeway capacity evaluation are introduced and their possible problems and shortcomings are discussed. As the LOS measure only presents a rough estimation of the real maximum flow, a replacement has been proposed. A case study, based on the gathered data from a selected section of Hakim Highway has been studied. The results of the study on two consecutive hours, 6 am to 8 am, on the section show that although the V/C ratio is higher in the first hour, the reliability is higher in the first hour, proved by more detailed analysis, which is a good reason to express the problems of the traditional evaluation methods.

Traffic safety evaluation based on traffic conflict is a new method noticeable for traffic safety researchers. Traffic conflict is an observable interaction between two or more vehicles and can be calculated in various methods. In this paper, time-to-collision (TTC) as a microscopic traffic safety index is used in traffic conflict technique. Microscopic traffic data of I-80 freeway in NGSIM project has been used for analytical calculations. For different groups of traffic, parameters such as headway, spacing, leading and following vehicle velocities and leading and following vehicle types, TTCs have been calculated and then compared. Results indicate that traffic safety in lane 1, as HOV lane, is the highest. Also, vehicle following heavy vehicles have the smallest TTCs in percentage. It seems that traffic conflict technique makes new opportunities for traffic safety engineers to evaluate freeway safety without the need for freeway crash data.

Increasing number of vehicles has caused the arterial intersections as the main elements of urban transportation network to face difficulties such as congestion, longer hindrance (delay) for vehicles, etc. Increasing efficiency and improving intersections operation lead to promoted performance of the urban transportation network. Studying left- turn behavior as a main factor in reducing intersections efficiency is of great importance. Bearing in mind that the vehicle turning, particularly left- turn movement, can cause traffic jam, create delay and such other problems at intersections, current traffic management strategies have failed to resolve the problems for long term. This research introduces a new, effective strategy called quadrant roadway intersection in the form of unconventional intersection design and development. Also, the paper compares the operation of jughandle intersection with quadrant roadway intersection in order to increase the intersection efficiency by removing the negative impact of the left-turn movements.

The process of route choice by road users (Trip Assignment), as the last phase of the four-step travel demand forecasting and its complement, plays a major role in transportation studies. In order to forecast the requirements of this process and assign traffic to the network lines, various models have so far been proposed. However, due to their theoretical foundation, most of those models cannot take qualitative parameters into account as well as quantitative ones in the method of route choice by users. Such deficiency can be attributed to lack of appropriate tools for investigating qualitative and quantitative parameters and identification of the most effective parameters affecting the process of trip assignment for future modeling. In this research, there has been a great attempt to introduce Rough Sets Theory as a precise and powerful mathematical tool for identifying effective parameters affecting one phenomenon among many substantial factors. Questionnaires were prepared and given to the road users in the city of Semnan and the extracted data were analyzed using Rough Sets Theory and the Linear Stepwise Regression method. The comparison of the results has been presented in this paper. The analogy indicates a high level of accuracy in Rough Sets Theory compared with Linear Stepwise Regression method. Also, the research shows that many factors may affect trip assignment process among which Travel Time, Route Length, Pavement Condition and Familiarity with Route (as a qualitative factor) are identified as the most dominant and influential within the study domain.

Several factors influence positioning vehicle maintenance centers in cities. Factors such as population, type of access route, land use and value, coverage level, etc., may directly or indirectly affect positioning the centers.The main discussion is which of these factors are more important and how much. Providing an answer for the inquiry has engaged the minds of the policy makers and experts. Moreover, due to financial restrictions, it is of vital importance to develop a process for resource distribution and assignment further to prioritizing the establishment of such centers in the cities and towns. In this paper, an approach has been suggested to prioritize the cities and towns in positioning vehicle maintenance center. Then, the cities of the country have been ranked based on the collected data and using the suggested model.

Multimodality is an issue that needs to be considered in the design of bus transit network, particularly in large metropolises. This necessitates the integrated design of bus transit system and other modes. One way to tackle the problem is to locate the transit hubs efficiently, the problem which has been ignored in previous researches. In such networks, the bus transit route network should be designed according to the location of the hubs considering simultaneously that locations of the hubs are also affected by the bus route in the network. In this paper, a hybrid Ant Colony Optimization-Genetic Algorithm (GA-ACO) has been proposed for joint location and design of hubs and bus routes within a network. Some urban planning issues have been also considered in locating the hubs. An illustrative example network, based on a network similar to that of central Hamedan (a city in Iran), has also been solved using the proposed algorithm.