Assessing Winter StormRelated NonRecurring Congestion in the Salt Lake Valley

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Assessing Winter Storm-Related Non-Recurring
Congestion in the Salt Lake Valley

• Ralph Patterson1, Randy Graham2, Kevin
  Barjenbruch2, Glen Merrill2, Glenn Blackwelder1,
  Brett Hansen3, Mike Holts3, Julie Miller1, Mike
  Seaman2, Jeff Williams3, Audra Yocom1, Carol
  Werner4
• 1Utah Dept of Transportation, Traffic Operation
  Center, Salt Lake City
• 2National Weather Service, Salt Lake City
• 3NorthWest Weathernet, Salt Lake City
• 4University of Utah, Salt Lake City

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Outline

• Partners
• Weather Impacts on Roads in the Salt Lake City
  metro
• Comparison of travel impacts associated with two
  winter weather events
• Future work

Photo courtesy UDOT
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Unique Collaboration

• Federal (NWS), state (UDOT), and private
  meteorologists (Northwest WeatherNet), academia
  (University of Utah)
• Meteorologists embedded at the UDOT Traffic
  Operations Center (TOC)
• Meteorologists with UDOT and Northwest WeatherNet
• Direct involvement of UDOT Traffic Engineer

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Increased DemandsIncreased Commute Times

• Rapid population growth
• 23.5 increase between 2000 and 2008
• Increased demand on existing infrastructure
• Congestion results in annual cost of 250 million
  in Utah (UDOT)
• Recurring (i.e., AM/PM peak commute times)
• Non-recurring congestion (weather, accidents)
• Inclement weather plays a significant role in
  non-recurring congestion
• Delays, mobility, productivity, and safety
• Large weather events cause trips to take 40-50
  longer (UDOT report)

Photo courtesy UDOT
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So

• Which event will produce the biggest
  impact?
• A. 1-3 between 200 PM - 600 PM
• B. 4-6 between 1000 PM - 400 AM
• C. 6-8 between Noon - 1000 PM

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Factors in Event Impact

• Time of day
• Day of week
• Snowfall rates
• Snowfall amounts
• Road temperatures
• Mitigation strategies
• Holidays
• Public perception/preparedness

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Weather Impact on Commute
This is a striking finding but it seemed
intuitive on further consideration. The only
things that affect the entire road system around
Salt Lake City are snowstorms, or major holidays.
Other sources of delay (crashes, thunderstorms,
congestion) tend to be more localized. (UDOT)

• Travel Time Index (TTI) Measure of the increase
  in travel time associated with congestion,
  incidents and weather.

Example commute of 20 minutes TTI of 1 20
minutes TTI of 1.3 20 x 1.3 26 minutes

• High TTI (congestion and delays) dominated by
  weather

Image courtesy Glenn Blackwelder
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DataDataData

• 20 events from the winter 2008-09
• Collection of weather and traffic data
• Radar and satellite data
• Mesonet observations
• Road temperatures
• Traffic information
• Plow driver reports
• Accident information
• Investigation of NWS and UDOT statements
• Discussion of societal impacts
• Does lead time impact volume?

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Tracking Impact

• Performance Management System (PeMS)
• 240 sensors in the SLC Metro
• High temporal (5 min) and spatial resolution (1/2
  mile)
• Vehicle Miles Traveled (VMT)
• Vehicle Hours Traveled (VHT)
• Flow
• Speed
• Delays
• Accident Information
• Plow Driver Reports

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5 Jan 2009 Well forecast.but

• 1-3 forecast for Salt Lake Valley prior to onset
  
• Verified well
• Winter Weather Advisory issued near onset
• Based on potential impacts on the afternoon
  commute
• Minimal discussion of impact in days leading up
  to event
• Several significant storms had preceded this
  event

Photo courtesy KSL-TV
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Road Considerations and Impacts

• Road temperatures in the low to mid 20s F
• Snow stuck to roads at onset
• ??Limited mitigation prior to event??
• Over 180 accidents in Salt Lake and Utah Counties
  alone
• Worst commute of the season

5 Jan 2009 - Road temperatures 420
PM MST
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19 December 2009

• More ramp-up to the event
• Watch issued 36 hours before onset
• Winter Storm Warning issued 21 hours before
  onset
• Area Forecast Discussion 442 PM 17 Dec 2008 -
• With very cold temps expect snow to rapidly
  accumulate on area roadways
• Winter Storm Warning 302 PM 18 Dec 2008
• Snow expected to rapidly accumulate along area
  roadways and result in treacherous driving
  conditions
• ..Much of this snow may be occurring during the
  late afternoon and early hours which would have a
  very significant impact on the evening commute

Add image
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Road Considerations and Impacts

• Roads temperatures remained near 30 degrees
  through the event
• More effective mitigation
• Level of service remained higher throughout the
  event
• Aggressive mitigation prior to and during event
• 3-7 storm total

19 Dec 2008 - Road temperatures 400 PM
MST
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Speed and Flow

• January 5th Event
• Speed trough on the 5th was lower
• Bigger event
• Reduced flow due to congestionreduced capacity
  of system due to slow travel?
• December 19th Event
• Increase in flow (above normal) prior to snowfall
  onset
• Well forecast onset?
• Dip right at peak traditional max flow time due
  to increase in snowfall intensity

• A lot of similarities between the two events
• Drop in discretionary travel
• Travel is not displaced in time (i.e., flow
  doesnt recover)
• Speeds and flow drop off rapidly right after
  onset

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Level of Service (LOS)

• Freeway level of service is defined by the
  density of vehicles on the road (vehicles per
  mile per lane), and is a measure of a driver's
  freedom to maneuver
• LOS A - Free flow - the ability to maneuver is
  unimpeded
• LOS B - Reasonably free flow - slight restriction
  in maneuverability
• LOS C - Ability to pass or change lanes is not
  always assured, but posted speed is maintained,
  more driver vigilance required
• LOS D - Slight reduction in speeds, ability to
  pass or change lanes reduced, traffic stream has
  little space to avoid disruption, driver comfort
  reduced
• LOS E - Ability to pass or change lanes is
  constrained, flow is unstable, speed varies, road
  is at capacity, drivers are uncomfortable
• LOS F - A breakdown in vehicular flow - each
  vehicle follows another at minimum safe spacing
  due to demand exceeding capacity.

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So, how was the commute?

• January 5th event
• Note the significant difference between Mon and
  Tue
• As flow goes down there is high density of
  vehicles on the roadway
• No space, very difficult to switch lanes,
  vehicles move in lock step, frequent stops

• December 19th event
• Higher level of service indicates that people
  are moving (albeit at a reduced speed, in this
  case)
• Not completely packed on the roads there is
  spacing
• Difference between Thu and Fri is not that
  significant

• Friday afternoon is typically a heavier commute
  than Monday afternoon
• Flow and speed showed similar trends, but driver
  perception of 19 December commute likely very
  different than 5 January commute
• Better prepared? More effective mitigation?

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Uncertainty and Impact

• This is also an encouraging finding because it
  suggests that the highest overall delay may be
  predictable (but perhaps not avoidable) with
  accurate weather forecasts.
  UDOT 2007 Congestion Report

• It is important to note that one of the most cost
  effective solutions for mitigation of the
  congestion problem is driver habit modification.
  UDOT

Impact
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Future Work

• Human Behavior
• With confidence in forecast and road impact, can
  we influence response?
• Effective communication strategies?
• Increased penalty for false alarms?
• Project with NWS/UDOT/NWN and University of Utah
  - Winter of 09-10
• Two to three events
• 400 professionally administered driver surveys
• Levels of awareness
• Sources of information
• Responses to road/weather info (behavior
  modification)
• Detailed correlation of weather to travel impacts
• Weather Enterprise impact statements

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