Extended Abstract
Introduction
 Road signs not only provide drivers with the necessary information and guidance, but also inform them of related rules and probable risks along roads. Safety of roads, and thus minimum delay and discomfort for drivers depends on traffic order. This order is only achieved if road signs can accurately guide drivers. Design of road signs have been evaluated in different fields of traffic engineering and urban design. Based on these evaluations, parameters like proper distance (distance in which a sign is legible for those driving in different speeds), and proper height (the height in which light reflection from the surface of the sign is minimized) have been introduced. Lack of a generalized method for designing and positioning of road signs, along with inadequate attention to their proper installation can cause a serious risk for drivers.
Systematic positioning of road signs on highways and urban pathways with an especial attention to different criteria of sights has a significant impact on drivers’ ability to find the best route on time, and thus minimizes probable confusion and heavy traffic. Visibility in three-dimensional space refers to three-dimensional characteristic of different barriers along the roads. In most analytical studies, extruded objects and a perspective of the three-dimensional model are simulated. In this approach, three-dimensional analysis is usually performed based on an analysis in two-dimensional space. As an instance, the concept of spatial openness index (SOI) was introduced in 3D space. This concept refers to the volume of space observable for an observer. SOI is measured by defining a cone in the observers’ position based on which simulation is performed. In this way, the volume of observable space will be reduced in the presence of obstacles.
3D visibility analysis is closely related to human perception. When human eyes observe a scene, distant objects appear smaller than closer ones. Thus, if this difference in distance is considered, the final simulation will be closer to reality. Distance index shows the space width scale by calculating the distance between the observer and the target. In this method, a decrease in distance results in a more comprehensive perception, while increased distance decreases observers’ ability to perceive the environment. Based on the distance to target and observer’s view angle, three-dimensional projection simulates observers’ view and illustrates 3D obstacles on a 2D plane. The present study seeks to provide an approach based on spatial analysis in 3D space to evaluate the visibility of road signs.
 
Materials & Methods
Indices like height and direction of road signs, perceivable distance and horizontal angle between signs and the observer (driver), and finally perceivable area of the signs effect the visibility of signs. In the proposed method, total area of each sign perceivable for drivers driving in different situations is calculated using projective geometry.
In order to evaluate visibility of road signs for vehicles (driver) in different positions, spatial indices such as overlap area (area resulted from the reflection of barriers on the sign face), distance between the center of road signs and the center of overlap area, and a combination of overlap area and distance are presented. Then, different simulation scenarios are designed for the vehicle’s motion on a simulated roadway and the performance of each indicator are evaluated. Index of combination (combination of overlap area and distance) was selected as final visibility measure. With an increase in distance from the center of the sign, the overlap area decreases and visibility increases. In order to determine visibility, visual status of the vehicle (driver) is evaluated based on four categories: poor, good, medium and excellent.
 
Results & Discussion
In order to simulate drivers’ vision, model spatial objects along the route and find optimal position for road signs, an appropriate analytical model is required. Results indicate that the proposed method can be used as an appropriate tool for optimal positioning of road signs along a route.