Road Weather Management Program
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How Do Weather Events Impact Roads?

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Weather acts through visibility impairments, precipitation, high winds, and temperature extremes to affect driver capabilities, vehicle performance (i.e., traction, stability and maneuverability), pavement friction, roadway infrastructure, crash risk, traffic flow, and agency productivity. The table below, summarizes the impacts of various weather events on roadways, traffic flow, and operational decisions.


Table: Weather Impacts on Roads, Traffic and Operational Decisions
Road Weather Variables Roadway Impacts Traffic Flow Impacts Operational Impacts
Air temperature and humidity N/A N/A
  • Road treatment strategy (e.g., snow and ice control)
  • Construction planning        (e.g., paving and striping)
Wind speed
  • Visibility distance (due to blowing snow, dust)
  • Lane obstruction (due to wind-blown snow, debris)
  • Traffic speed
  • Travel time delay
  • Accident risk
  • Vehicle performance (e.g., stability)       
  • Access control (e.g., restrict vehicle type, close road)       
  • Evacuation decision support
Precipitation
(type, rate, start/end times)
  • Visibility distance
  • Pavement friction
  • Lane obstruction
  • Roadway capacity
  • Traffic speed
  • Travel time delay
  • Accident risk
  • Vehicle performance (e.g., traction)
  • Driver capabilities/behavior
  • Road treatment strategy
  • Traffic signal timing
  • Speed limit control
  • Evacuation decision support
  • Institutional coordination
Fog
  • Visibility distance
  • Traffic speed
  • Speed variance
  • Travel time delay
  • Accident risk
  • Driver capabilities/behavior
  • Road treatment strategy
  • Access control
  • Speed limit control
Pavement temperature
  • Infrastructure damage
 N/A
  • Road treatment strategy
Pavement condition
  • Pavement friction
  • Infrastructure damage
  • Roadway capacity
  • Traffic speed
  • Travel time delay
  • Accident risk
  • Vehicle performance
  • Driver capabilities/behavior (e.g., route choice)
  • Road treatment strategy
  • Traffic signal timing
  • Speed limit control
Water level
  • Lane submersion
  • Traffic speed
  • Travel time delay
  • Accident risk
  • Access control
  • Evacuation decision support
  • Institutional coordination

Weather Impacts on Safety

  • On average, there are over 6,400,000 vehicle crashes each year. Twenty-four (24) percent of these crashes—approximately 1,561,000—are weather-related. Weather-related crashes are defined as those crashes that occur in adverse weather (i.e., rain, sleet, snow, and/or fog) or on slick pavement (i.e., wet pavement, snowy/slushy pavement, or icy pavement). Nearly 7,400 people are killed and over 673,000 people are injured in weather-related crashes each year. (Source: Eleven-year averages from 1995 to 2005 analyzed by Noblis, based on NHTSA data).
  • Most weather-related crashes happen on wet pavement and during rainfall. Seventy-five (75) percent of weather-related crashes occur on wet pavement. Fortyseven (47) percent happen during rainfall. Fifteen (15) percent of weather-related crashes happen during snow or sleet. Thirteen (13) percent occur on icy pavement. Eleven (11) percent of weather-related crashes take place on snowy or slushy pavement. Only two (2) percent happen in the presence of fog. (Source: Eleven-year averages from 1995 to 2005 analyzed by Noblis, based on NHTSA data)

    Table: Weather-Related Crash Statistics (Annual Averages)
    Road Weather Conditions
    Weather-Related Crash Statistics
    Annual Rates (Approximately)
    Percentages
    Wet Pavement 1,170,000 crashes 18% of vehicle crashes 75% of weather-related crashes
    544,700 persons injured 17% of crash injuries 81% of weather-related crash injuries
    5,700 persons killed 13% of crash fatalities 77% of weather-related crash fatalities
    Rain 739,200 crashes 12% of vehicle crashes 47% of weather-related crashes
    357,300 persons injured 11% of crash injuries 53% of weather-related crash injuries
    3,400 persons killed 8% of crash fatalities 47% of weather-related crash fatalities
    Snow/Sleet 232,600 crashes 4% of vehicle crashes 15% of weather-related crashes
    75,700 persons injured 2% of crash injuries 11% of weather-related crash injuries
    900 persons killed 2% of crash fatalities 12% of weather-related crash fatalities
    Icy Pavement 197,300 crashes 3% of vehicle crashes 13% of weather-related crashes
    67,300 persons injured 2% of crash injuries 10% of weather-related crash injuries
    700 persons killed 2% of crash fatalities 10% of weather-related crash fatalities
    Snow/Slushy Pavement 168,400 crashes 3% of vehicle crashes 11% of weather-related crashes
    49,500 persons injured 2% of crash injuries 7% of weather-related crash injuries
    600 persons killed 2% of crash fatalities 9% of weather-related crash fatalities
    Fog 38,700 crashes 1% of vehicle crashes 2% of weather-related crashes
    16,300 persons injured 1% of crash injuries 2% of weather-related crash injuries
    600 persons killed 2% of crash fatalities 9% of weather-related crash fatalities
    Weather-Related * 1,561,400 crashes 24% of vehicle crashes
    673,200 persons injured 22% of crash injuries
    7,400 persons killed 17% of crash fatalities

    * "Weather-Related" crashes are those that occur in the presence of adverse weather and/or slick pavement. (Source: Eleven-year averages from 1995 to 2005 analyzed by Noblis, based on NHTSA data).
  • Each year 17 percent of fatal crashes, 22 percent of injury crashes, and 25 percent of property-damage-only crashes occur in the presence of adverse weather and/or slick pavement. That is, nearly 6,600 fatal crashes, nearly 450,000 injury crashes and nearly 1,104,900 property-damage-only crashes in adverse weather or on slick pavement annually. (Source: Eleven-year averages from 1995 to 2005 analyzed by Noblis, based on NHTSA date).

Weather Impacts on Mobilty

  • 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".

    Table: Freeway Traffic Flow Reductions due to Weather
    Weather Conditions
    Freeway Traffic Flow Reductions
    Average Speed Free-Flow Speed Volume Capacity
    Light Rain/Snow 3% - 13% 2% - 13% 5% - 10% 4% - 11%
    Heavy Rain 3% - 16% 6% - 17% 14% 10% - 30%
    Heavy Snow 5% - 40% 5% - 64% 30% - 44% 12% - 27%
    Low Visibility 10% - 12% Empty Cell Empty Cell 12%
  • It has been estimated that 23 percent of the non-recurrent delay on highways across the nation is due to snow, ice, and fog. This amounts to an estimated 544 million vehicle-hours of delay per year. Rain—which occurs more frequently than snow, ice, and fog—leads to greater delay. During adverse weather average travel time delay increases by 14 percent in Washington, D.C. and by 21 percent in Seattle, WA. During peak periods in Washington, D.C. travel time increases by roughly 24 percent in the presence of precipitation. (Sources: " Highway Capacity Manual 2000" Chapter 22, "Temporary Losses of Highway Capacity and Impacts on Performance", " An Investigation into the Impact of Rainfall on Freeway Traffic Flow" and "Analysis of Weather Impacts on Traffic Flow in Metropolitan Washington DC" (PDF 1.4MB))

Weather Impacts on Productivity

  • 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, http://www.fhwa.dot.gov/policy/ohpi/hss/hsspubs.htm)
  • 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).

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