How do weather conditions impact roads?

JodatJune 6, 2017

How Do Weather Conditions Impact Roads?

Weather conditions impact roads in a variety of ways including visibility impairments, precipitation, high winds, vehicle performance issues, pavement friction, roadway infrastructure, crash risk, traffic flow, and more.

• Weather is a major contributor to vehicle crashes, with over 5,748,000 vehicle crashes annually. About 22% of these crashes – nearly 1,259,000 – are caused by weather. Weather-related crashes are defined as those crashes that occur due to adverse weather (i.e., rain, sleet, snow, fog, severe crosswinds, or blowing snow/sand/debris) or on wet or icy pavement. On average, about 6,000 people are killed and over 445,000 people are injured in weather-related crashes each year. (based on NHTSA data).
• The majority of weather-related crashes happen on wet pavement and during rainfall. 73% on wet pavement and 46% in the rain. There are fewer crashes in winter conditions: 17% during snow or sleet, 13% occur on ice and 14% of crashes due to weather conditions take place on snowy or slushy pavement. Only about 3% occur in fog.
• Capacity reductions can be caused by lane submersion due to flooding and by lane obstruction due to snow accumulation and wind-blown debris. Road closures and access restrictions due to hazardous conditions (e.g., large trucks in high winds) also decrease roadway capacity.
• Weather events can reduce arterial mobility and reduce the effectiveness of traffic signal timing plans. On signalized arterial routes, speed reductions can range from 10 to 25 percent on wet pavement and from 30 to 40 percent with snowy or slushy pavement. Average arterial traffic volumes can decrease by 15 to 30 percent depending on road weather conditions and time of day. Saturation flow rate reductions can range from 2 to 21 percent. Travel time delay on arterials can increase by 11 to 50 percent and start-up delay can increase by 5 to 50 percent depending on severity of the weather event. (Sources: “Weather Impacts on Arterial Traffic Flow (PDF 92KB)” and “Weather-Responsive Traffic Signal Control (DOC 399KB)”)
• On freeways, light rain or snow can reduce average speed by 3 to 13 percent. Heavy rain can decrease average speed by 3 to 16 percent. In heavy snow, average freeway speeds can decline by 5 to 40 percent. Low visibility can cause speed reductions of 10 to 12 percent. Free-flow speed can be reduced by 2 to 13 percent in light rain and by 6 to 17 percent in heavy rain. Snow can cause free-flow speed to decrease by 5 to 64 percent. Speed variance can fall by 25 percent during rain. Light rain can decrease freeway capacity by 4 to 11 and heavy rain can cause capacity reductions of 10 to 30 percent. Capacity can be reduced by 12 to 27 percent in heavy snow and by 12 percent in low visibility. Light snow can decrease flow rates by 5 to 10 percent. Maximum flow rates can decline by 14 percent in heavy rain and by 30 to 44 percent in heavy snow. (Sources: “Highway Capacity Manual 2000” Chapter 22, “Capacity-Reducing Occurrences“, “Driver Response to Rainfall on an Urban Expressway“, “Impact of Weather on Urban Freeway Traffic Flow Characteristics and Facility Capacity“, Empirical Studies on Traffic Flow in Inclement Weather: Summary Report“.
• Adverse weather can increase operating and maintenance costs of winter road maintenance agencies, traffic management agencies, emergency management agencies, law enforcement agencies, and commercial vehicle operators (CVOs).
• Winter road maintenance accounts for roughly 20 percent of state DOT maintenance budgets. Each year, state and local agencies spend more than 2.3 billion dollars on snow and ice control operations. (Sources: “Highway Statistics Publications, Highway Finance Tables SF-4C and LGF-2,” 1997 to 2005, https://www.fhwa.dot.gov/policy/ohpi/hss/hsspubs.cfm)
• Each year trucking companies or CVOs lose an estimated 32.6 billion vehicle hours due to weather-related congestion in 281 of the nation’s metropolitan areas. Nearly 12 percent of total estimated truck delay is due to weather in the 20 cities with the greatest volume of truck traffic. The estimated cost of weather-related delay to trucking companies ranges from 2.2 billion dollars to 3.5 billion dollars annually. (Source: ” Analysis of Weather Incident Effects on Commercial Vehicle Mobility in Large U.S. Cities,” Mitretek Systems).

Source: https://ops.fhwa.dot.gov/weather/q1_roadimpact.htm

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