How Do Weather Events Impact Roads?

Weather Events

Mitigating Impacts

Resources

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%			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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