## 2 Background  
### Factors Influencing Road Traffic Collisions  
Many factors affect road traffic collisions. Understanding how much each factor contributes to collisions is an important area in road safety research. A thorough understanding of such factors is key to laying out the theoretical landscape in which collision data have been collected and analyzed. Such a review not only elicits the multidimensional nature of such factors and the extent to which those dimensions intersect, it also reveals the descriptive nature of past studies, which are mainly focused on statistical correlations. Often, driver behavior, vehicle design, and conditions, as well as road conditions are among the main factors influencing road safety [7]. Statistical analysis shows that speed and age along with gender are the major factors of accidents on rural areas, while drivers' age and gender, running speed, road conditions, lighting conditions are found to be the main factors of collisions on urban roads [8]. Using a combined GIS-empirical Bayesian approach, reference [9] showed the relationship between collisions and traffic density, road width, the density of intersections per segment, flow direction, and land use in the urban context. A field data analysis using a logistic regression model shows that speed, pedestrian age, vertical height from the ground level to the front/top transition point, along with the vehicle's centerline and the average height of the top and bottom of the bumper are statistically significant predictors in the injury risks of vehicle-to-pedestrian frontal crashes [10]. Based on the literature, influencing factors can be divided into four main categories including: (a) road characteristics; (b) vehicle characteristics; (c) driver characteristics; and (d) environmental attributes (Table 1). The probability and severity of collision vary in different road types. For example, in 2004 in the province of Ontario, Canada, collisions occurring on 400 series highways represent 14% ($2 billion) of social costs, 12% of fatalities, and 11% of all collisions [11]. Lane width, flow per lane, curving parameters, vertical curves, super elevation, transition curves, avoiding surprises and confusion in road design and traffic signs, etc., are the factors included in the design stage of road construction and in determining the speed limits. Speed is known as an important factor in the severity of collisions [8]. The road surface can be dry, wet, snowy, or icy depending on the weather conditions. This factor is reported to be important and examined by a significant number of studies [12; 13; 14; 15; 16]. Road configuration and high-risk (critical) segments include intersection, non-intersection, type of intersections, tunnel inlet/outlet, near bridges, and culverts, which are reported to be important [12]. Any restriction on visibility such as sharp turns and blind corners, or lanes covered with shrubs and trees may lead to a collision. Other potential threats include billboards located near streets [17; 18; 19], reaction to control devices such as yield signs, warning signs, traffic signals, stop signs [20], the presence of surveillance cameras [13], lighting conditions [14; 20; 21], as well as vehicle types and conditions [22]. Individual and demographic factors such as drivers' age [12; 13; 20; 23], sensory-perceptual deficits could affect driving performance even in the absence of overt disease [23], as well as gender and race [12; 13; 16; 20]. Furthermore, environmental factors such as weather conditions and temporal characteristics in accident analysis [14; 16; 21] have been revealed in previous investigations. Weather conditions are usually categorized as snowy, sunny, foggy, rainy, stormy, and heavy rain [14; 16; 21]. Studies such as [16] and [20] showed that time of day had a significant effect on the injury risk of emergency vehicle occupants. The month of a year has also been revealed as an important parameter affecting the number of collisions [15; 24].