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How Do Weather Events Impact Roads?


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

  • Traffic speed
  • Speed variance
  • Travel time delay
  • Accident risk
  • Driver capabilities/behavior
  • Road treatment strategy
  • Access control
  • Speed limit control
Pavement temperature

N/A
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

  • Traffic speed
  • Travel time delay
  • Accident risk
  • Access control
  • Evacuation decision support
  • Institutional coordination

Weather Impacts on Safety

  • On average, there are over 5,891,000 vehicle crashes each year. Approximately 21% of these crashes – nearly 1,235,000 – are weather-related. Weather-related crashes are defined as those crashes that occur in adverse weather (i.e., rain, sleet, snow, fog, severe crosswinds, or blowing snow/sand/debris) or on slick pavement (i.e., wet pavement, snowy/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. (Source: Ten-year averages from 2007 to 2016 analyzed by Booz Allen Hamilton, based on NHTSA data).
  • The vast majority of most weather-related crashes happen on wet pavement and during rainfall: 70% on wet pavement and 46% during rainfall. A much smaller percentage of weather-related crashes occur during winter conditions: 18% during snow or sleet, 13% occur on icy pavement and 16% of weather-related crashes take place on snowy or slushy pavement. Only 3% happen in the presence of fog. (Source: Ten-year averages from 2007 to 2016 analyzed by Booz Allen Hamilton, based on NHTSA data).









    Table: Weather-Related Crash Statistics (Annual Averages)
     

    Weather-Related
    Crash Statistics

    10-year Average (2007-2016)

    10-year Percentages

    Weather-Related* Crashes, Injuries, and Fatalities 1,235,145 crashes 21% of vehicle crashes
    418,005 persons injured 19% of crash injuries
    5,376 persons killed 16% of crash fatalities





















    * “Weather-Related” crashes are those that occur in the presence of adverse weather and/or slick pavement conditions.

    Table: Weather-Related Crash Statistics (Annual Averages)

    Road Weather Conditions

    Weather-Related
    Crash Statistics

    10 Year Average (2007 – 2016)

    10-year Percentages

    Wet Pavement 860,286 crashes 15% of vehicle crashes 70% of weather-related crashes
    324,394 persons injured 15% of crash injuries 78% of weather-related injuries
    4,050 persons killed 12% of crash fatalities 76% of weather-related fatalities
    Rain 556,151 crashes 10% of vehicle crashes 46% of weather-related crashes
    212,647 persons injured 10% of crash injuries 51% of weather-related injuries
    2,473 persons killed 8% of crash fatalities 46% of weather-related fatalities
    Snow/Sleet 219,942 crashes 4% of vehicle crashes 18% of weather-related crashes
    54,839 persons injured 3% of crash injuries 14% of weather-related injuries
    688 persons killed 2% of crash fatalities 13% of weather-related fatalities
    Icy Pavement 156,164 crashes 3% of vehicle crashes 13% of weather-related crashes
    41,860 persons injured 2% of crash injuries 11% of weather-related injuries
    521 persons killed 2% of crash fatalities 10% of weather-related fatalities
    Snow/Slushy Pavement 186,076 crashes 4% of vehicle crashes 16% of weather-related crashes
    42,036 persons injured 2% of crash injuries 11% of weather-related injuries
    496 persons killed 2% of crash fatalities 10% of weather-related fatalities
    Fog 25,451 crashes 1% of vehicle crashes 3% of weather-related crashes
    8,902 persons injured 1% of crash injuries 3% of weather-related injuries
    464 persons killed 2% of crash fatalities 9% of weather-related fatalities
  • By crash type (not shown in above table) for an average year, roughly 15% of fatal crashes, 19% of injury crashes, and 22% of property-damage-only (PDO) crashes occur in the presence of adverse weather and/or slick pavement. That is on an annual basis, nearly 4,900 fatal crashes, over 301,100 injury crashes and nearly 919,700 PDO crashes occur in adverse weather or on slick pavement. (Source: Ten-year averages from 2007 to 2016 analyzed by Booz Allen Hamilton, based on NHTSA data).

Weather Impacts on Mobility

  • 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, 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
    ).

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