Impact of Weather Events of Road and Traffic Safety

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Road traffic safety is the result of a complicated interaction of factors such as driver, vehicle, infrastructure or road conditions as well as weather condition. Usually the weather condition is not the major cause of traffic accidents compared to the human factors, but it still has a clear impact on traffic accidents. For example, the foggy winters and heavy rain impair or weaken the road and driver conditions by reducing the pavement friction between tires and surface of road in case of a rain, and by declining the visibility level in case of a fog. In the best scenario, when there is no traffic accident, the weather condition still influences the road mobility. Surely it decrease the traffic flow by reducing the speed limit of a road as well as Capacity reductions which can be caused by lane submersion due to flooding. In addition to that, it financially consumes country’s economy through road crashes that drain billions of US dollars from the health care system of a country, and property-damage-only (PDO) crashes occur in the presence of adverse weather which drain billions of dollars as well. Recently, engineers and designers take in consideration the weather condition, environmental and weather-related factors when designing roads. They design infrastructures to be safely drivable as well as economically valuable in which for the infrastructures to last to the period they designed for. In this paper we investigate the impact of weather condition on traffic road safety.


Adverse weather conditions are the major causes of an elevated risk of traffic accidents and traffic flow. Adverse weather conditions (e.g. precipitation, fog, winds, sand storms, etc.) contribute to serious hazard to roadway safety through increasing travel time, decreasing speeds, increasing speed variance and decreasing roadway capacity. Weather and climate factors directly affect the planning, design, construction and maintenance of transportation infrastructure in several ways. They also indirectly affect the demand for transportation services [1]. (Road transport sensitivities) Weather conditions are associated with considerable impacts on road safety through their influence on the behavior of road users. The interaction between weather effects and the effects of other road safety factors (e.g. human factors, vehicle conditions, road conditions, etc.) is surely a complex phenomenon that attracts increasing attention by researchers. Stipdonk (2008) emphasizes that weather effects need to be controlled for in any multivariate analysis aiming to explain changes in road safety outcomes. Koetse and Rietveld (2009) further confirm this need within the climate change context [2]. (Weather Effects on Daily Traffic Accidents)

Worldwide, an estimated 1.3 million people are died in traffic road crashes each year and as many as 50 million are injured. The human as well as material damage caused by road accidents is estimated on 1 to 2% of the gross national product [3]. The relationship between weather conditions and crashes has been the subject of a number of studies. It is estimated that weather conditions can illustrate 5% of the variation in injury crashes. This may seem small but one has to take into account that traffic intensity explains over 70% of the variation and another 5% is determined by randomness [4]. (The Impact of Weather Conditions)

The paper is organized as follows: in the literature review section we give an overview of the most relevant literature concerning weather impact studies. Second, the methodology will be amplified, followed by the data presentation and an elucidation of the data processing. After that, the results will be presented and discussed. Finally, this report is completed by a summary of the conclusions and topics for further research.

Whether the weather be fine

Or whether the weather be not,

Whether the weather be cold

Or whether the weather be hot,

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Literature Review

Several studies included or focused on the impacts of weather conditions on road accidents incidence and severity, trying to capture these often complex effects. Eisenberg (2004) is considered to be one of the best source of a thorough review of mostly earlier studies on weather effects [5]. (Weather Effects on Daily Traffic Accidents) Some studies dealing with weather conditions, road system quality and speeding and speed limit enforcement [6]. An important difference between strategies is formed by the prime direction of the analysis, i.e. the road user, the vehicle, a combination of driver and vehicle, the infrastructure (and its immediate surrounding), accident hotspots, weather events, and injury vs. accident [7]. Furthermore, studies assessing the effectiveness of road transport safety policies need to be cross-cutting in order to avoid risks of erroneous attribution of effects to trends or measures [8]. (Weather Conditions)

In USA on average, there are over 5,891,000 vehicle crashes each year. Approximately 21% of these crashes – which means over one million – are weather-related. Weather-related accidents are defined as those crashes that happen in adverse weather (i.e., rain, sleet, snow, fog, severe crosswinds, or blowing snow/sand/debris) or on slippy pavement (i.e., wet pavement, slushy pavement, or icy pavement). On average, nearly 5,000 people are killed and over 418,000 people are injured in weather-related crashes each year [9]. For further information, the giant majority of most weather-related crashes happen on wet pavement (70% of the time) and during rainfall (46% of the time). A much smaller percentage of weather-related crashes occur during winter conditions: 18% during snow, 13% occur on icy pavement and 16% of weather-related crashes take place on slushy pavement. Only 3% happen in the presence of fog [9]. (How Do Weather Events Impact Roads?)

Studies in Norway and Finland find that weather conditions are the main cause in approximately 10% of crashes. According to Andrey’s report, 18.5% of crashes involving injuries and 16% of crashes leading to fatalities. The same study investigated the effects of rainfall and snowfall on relative crash risk levels in selected Canadian cities, finding that snowfall has appreciably stronger effects than rainfall [10]. In conjunction with other factors, extreme weather can have a secondary contributing role. For instance, in the case of driver fatigue. Jaroszweski finded a significant relationship between heavy rain and crash incidence in selected urban areas in the UK [11]. (Weather Conditions)

The demand of road traffic decreases during weather, which weakens the probability of rise in accidents, while the range of the decrease depends on the purpose of the trip (work, emergency case, etc.). The structure of road traffic, the composition of drivers (by skill level) in the traffic, and the average degree of time pressure are different during the week [12]. In addition to that, Cools finded that inferred deterrence effects of a given level of weather hazard depend also on the season and on the time scale (hours to days) at which responsiveness was assessed, as well as the kind of forecast weather and conformity between forecast and experienced weather [13]. (Weather Conditions)

Weather events can reduce mobility of road and reduce the efficiency of traffic signal timing plans. The reduction in speed could range from (10 – 25) percent on wet pavement and from (30 – 40) percent with snowy or slushy pavement [14]. Average arterial traffic volumes can fall by (15 – 30) percent depending on two variables: road weather conditions and time of day. Saturation flow rate reductions can range from (2 – 21) percent. Travel time delay on arterials can rise by (11 – 50) percent depending on severity of the weather event [15]. On freeways, light rain or snow can reduce average speed by (3 – 13) percent. Heavy rain can decrease average speed by (3 – 16) percent. In heavy snow, average freeway speeds can decline by (5 – 40) percent. Low visibility can cause speed reductions of (10 – 12) percent. Free-flow speed can be reduced by (2 – 13) percent in light rain and by (6 – 17) percent in heavy rain. Snow can cause free-flow speed to decrease by (5 – 64) percent [16]. Speed variance can fall by 25 percent during rain. Light rain can decrease freeway capacity by (4 – 11) and heavy rain can cause capacity reductions of (10 – 30) percent. Capacity can be reduced by (12 – 27) percent in heavy snow and by 12 percent in low visibility. Light snow can decrease flow rates by (5 – 10) percent. Maximum flow rates can decline by 14 percent in heavy rain and by (30 – 44) percent in heavy snow [17].

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