Research Needs for Weather-Responsive Traffic Management

Paul A. Pisano, Team Leader

Federal Highway Administration (FHWA), Office of Operations

Road Weather Management

400 Seventh Street, S.W.

HOTO-1, Room 3408

Washington, DC 20590

Voice: 202-366-1301

Fax: 202-366-3225

paul.pisano@fhwa.dot.gov

Lynette C. Goodwin, Senior Transportation Engineer

Mitretek Systems, Center for Information and Telecommunications
Technologies

Intelligent Transportation Systems (ITS) Division

600 Maryland Avenue, S.W.

Suite 755

Washington, DC 20024

Voice: 202-488-3033

Fax: 202-863-2988

lynette.goodwin@mitretek.org

Word Count: 3664

Abstract. Weather-responsive traffic management views weather events as
non-recurring incidents that can be predicted, observed, and mitigated.
This paper reports weather impacts on traffic flow and describes an
emerging concept of operations for a system-wide approach to traffic
management in adverse weather. The paper discusses this structured
approach to assess weather impacts and implement operational strategies
that improve safety, mobility, and productivity. Finally, research needs
to advance the state-of-the-practice in weather-responsive traffic
management are enumerated.

# INTRODUCTION

Traffic managers implement a number of strategies such as operating
traffic signals on arterial routes, controlling traffic flow on
freeways, detecting and managing incidents, and disseminating traveler
information to ensure roadway safety and minimize congestion. Weather
events and their impacts can be viewed as predictable, non-recurring
incidents that contribute to congestion by reducing capacity---the
maximum flow rate at which vehicles can traverse a roadway over a given
time period. Every day, weather affects traffic on every road in the
nation by impacting pavement conditions, vehicle performance, and driver
behavior. Consequently, there is a need for a systematic approach to the
significant challenge of managing traffic during inclement weather.

With this in mind, weather-responsive traffic management strategies can
be planned and implemented to mitigate weather effects on traffic flow.
As with any incident, managers must be able to predict or detect a
weather event, assess the nature of the event (e.g., severity, impact
area), and manage traffic under less-than-optimal conditions. To prevent
weather-related congestion, and lessen the impact of unavoidable
congestion, traffic managers need to understand how weather impacts
roads and traffic, as well as the benefits of weather-responsive
strategies. In spite of the ability to prepare for and manage such
incidents, the field of weather-responsive traffic management is in its
infancy. This subject is addressed in this paper, which discusses a
weather-responsive traffic management concept of operations, developed
under the Federal Highway Administration (FHWA) Road Weather Management
Program, and research needs to advance the state-of-the-practice in this
field.

# weather impacts on roads and traffic

Weather events such as precipitation, fog, high winds, high water, and
extreme temperatures reduce roadway capacity. These events can cause
slick pavement, lower traffic speeds, increase speed variability (i.e.,
a measure of speed uniformity), affect traffic volume, increase delay,
and escalate crash risk. Weather impacts can also disrupt access to
roads (i.e., lane obstruction or submersion, pavement buckling), and
damage road infrastructure (e.g., traffic control devices,
communications systems). Large segments of many metropolitan road
networks are currently operating at or near capacity. As congestion
increases in urban areas, weather events will have an even greater
impact on traffic flow (1).

All forms of precipitation can impact traffic operations. Light rainfall
or snowfall after extended periods with no precipitation can cause
slippery pavement when water mixes with oil, and can reduce freeway
speeds by roughly 10 percent. In addition to reducing vehicle traction,
heavy rain or snow can reduce visibility distance and cause drivers to
increase headway (i.e., or spacing between vehicles), decrease
acceleration rates, and reduce freeway speeds by 16 to 40 percent (2,
3). Arterial speeds decline by 10 to 25 percent in rainy, wet pavement
conditions and by 30 to 40 percent with snowfall and snowy/slushy
pavement (3).

In poor road weather conditions, arterial traffic volumes can decline by
six to 30 percent, depending on the time of day (3). A study of Iowa
freeways found that traffic volumes decrease by 22 to 36 percent during
snowstorms. This study also found that volumes can increase during the
early hours of a snowstorm, especially on weekdays (4). A study of
weather impacts on a Texas freeway found that rain reduced capacity by
14 to 19 percent (5). It was estimated that, in 1999, capacity on U.S.
freeways and principle arterials was reduced by more than 11 percent due
to fog, snow and ice (6).

Weather can reduce the effectiveness of traffic signal timing plans
designed for use in clear, dry pavement conditions. On arterials with
coordinated traffic signals, start-up delay is five percent higher on
wet pavement, 23 percent higher in snowy pavement conditions, and 50
percent higher during snowstorms. The number of vehicle stops in adverse
weather increases by 14 percent. Travel time delay on signalized routes
can increase by 11 to 50 percent depending on weather event severity
(3). Analyses of travel time delay on freeways in metropolitan
Washington, DC demonstrated that average off-peak, travel time increased
by 14 percent in the presence of precipitation, high winds, low
visibility, and/or slick pavement. A more detailed analysis using
Doppler radar data indicated that, during peak periods, travel time
increases by roughly 24 percent with precipitation (1). A separate study
estimated that, in 1999, 23 percent of the non-recurrent delay on
freeways and major arterials was due to snow, ice, and fog events (6).

While snow and ice may be perceived as the greatest threats to traffic
operations, national crash statistics indicate that rain and wet
pavement have more significant impacts on road safety (7). In 2001,
nearly 79 percent of weather-related crashes in passenger vehicles
occurred on wet pavement and nearly 49 percent happened during rainfall.
Further, most hurricane-related deaths result from inland flooding on
roads, due to rain after landfall. Over 22 percent of passenger vehicle
crashes in 2001 happened under adverse road weather conditions. Each
year over 450,000 injury crashes and nearly 6,500 fatal crashes in
passenger vehicles take place in rain, sleet, snow, or fog; or when
pavement conditions are wet, snowy/slushy, or icy. In 2001, over 615,000
people were injured and more than 6,900 people were killed in these
weather-related crashes. In that same year, nearly 19 percent of large
truck crashes (i.e., nearly 77,800) occurred on wet, snow-covered,
slushy, or icy pavement. Nearly 12 percent of these crashes (i.e., over
48,500) happened in rain, snow, sleet, or fog.

When fog, drifting snow, or blowing dust reduce visibility some drivers
reduce speed more than others, increasing speed variance and the risk of
overtaking crashes. An analysis of a fog-prone freeway in Utah found
that "overly cautious" or "confused" drivers travel at speeds 20 to 50
percent lower than other drivers (9). Multi-vehicle, chain-reaction
crashes occurring in low-visibility conditions result in fatalities,
multiple injuries, and extended road closures. High winds can blow snow,
dust, or debris (e.g., tree branches) across roadways causing visibility
restrictions and lane obstructions. High winds can also impact vehicle
stability by causing vehicles---particularly high-profile
tractor-trailers---to be blown over or blown off of roads. Such crashes
can cause significant delay due to lane or road closures.

# Weather-rESPONSIVE traffic management CONCEPT OF OPERATIONS

Tackling the impacts of weather on highway operations requires a
structured, systematic approach. This evolving discipline is referred to
in this paper as weather-responsive traffic management. New
weather-responsive traffic management strategies will depend on better
forecasts of weather, pavement, and traffic conditions. In this case
"better" means timely, accurate, route-specific data at high resolutions
comparable in scale to road network databases. As more Intelligent
Transportation Systems (ITS) are deployed, Advanced Traffic Management
Systems (ATMS) are becoming vital tools for managers and operators in
Traffic Management Centers (TMCs). Integration of data from
environmental monitoring systems with ATMS would provide managers with
tailored decision support based on the unique configuration,
characteristics, and microclimatology of an urban road network. The
expectation is that, once equipped with this information, traffic
managers will be able to take targeted actions to respond to or mitigate
weather impacts on traffic flow.

To facilitate the development of such integrated solutions, a concept of
operations for weather-responsive traffic management was drafted (*10*).
This draft report is intended to initiate discussion about the
development, testing, and deployment of effective traffic management
tools for response to weather threats. It addresses weather information
needs of traffic managers, road weather data collection, assessment of
weather impacts, and the effects of various strategies on operational
objectives. Building upon a number of successful tools that have been
implemented to date, the concept of operations presents a systematic
context consistent with broader traffic management strategies.

This framework will enable traffic managers to fully understand weather
impacts and embrace tools to achieve more effective, efficient
operations under weather threats. The concept of operations does not
propose weather-responsive traffic management to be a new set of rules
added to current practices. Rather, it proposes to better incorporate
weather information and decision support into current operational
procedures. With relevant road weather data integrated into the
information stream, traffic managers will be able to better address the
impacts of weather on operational objectives and expand the benefits of
their systems.

## Assessment of Weather Impacts on Basic Operational Objectives 

Under all conditions, independent of weather, traffic managers have
three basic operational objectives that drive their decisions: ensuring
road safety by reducing crash risk, improving mobility by achieving
stable traffic flow, and increasing agency productivity by minimizing
operating costs. Many managers operate from TMCs where ITS components
allow them to centrally monitor road conditions and control traffic
flow, disseminate pre-trip and en-route traveler information, as well as
share data and coordinate with other managers. Based upon prevailing
road weather conditions, managers can make decisions and take actions
that optimize traffic flow on road networks.

Knowledge of prevailing conditions is gained by access to road weather
information---both observations and predictions. The information from
environmental observing systems can be used to track the location and
movement of weather fronts and storm cells. Such dynamic weather
assessments could result in targeted decision making by traffic
managers, leading to more efficient network operation over the life of a
weather event as it traverses a metropolitan area. When one considers
that even minor changes in a single weather variable, such as air
temperature, can cause one storm to produce snow during the morning peak
period and rain in the evening, then it is clear that managers must have
at their disposal many different strategies to achieve their objectives.

The concept of operations proposes that managers utilize road weather
information, combined with traffic flow data, to assess the impact of
various weather events on highways. Such information can then aid in the
design and implementation of weather-responsive strategies that improve
mobility and safety on road segments with increased congestion or more
frequent crashes during adverse weather (e.g., fog-prone bridges, sharp
curves). Transportation agencies can also enhance productivity when
traffic managers coordinate and share resources with maintenance and
emergency managers. The benefits of infrastructure investments can be
maximized by integrating ATMS with Road Weather Information Systems
(RWIS) deployed for winter maintenance or flood monitoring. A
coordinated approach to system design and expansion (e.g., collocating
weather sensors and vehicle detectors) can significantly reduce the
costs of procuring, operating, and maintaining the infrastructure and
information systems that support the basic operational objectives.
Intra-agency coordination facilitates efficient traffic flow across
jurisdictional boundaries, promotes more efficient personnel management
(e.g., scheduling of TMC staff, snowplow drivers, and emergency
responders), and reduces the time to treat routes or repair damaged
infrastructure.

## Operational Strategies and Transportation Outcomes 

To aid in this structured approach to weather-responsive traffic
management, the concept of operations highlights three types of road
weather management strategies---advisory, control, and treatment
strategies (*11*). Advisory strategies provide information on prevailing
and predicted conditions to the traveling public. In a manner similar to
current travel advisories, traffic managers would utilize motorist
warning systems to inform drivers of low visibility, slick pavement,
high winds, and flooding. Strategies include activation of flashing
beacons atop static signs, posting warnings on Dynamic Message Signs
(DMS), and broadcasting messages via Highway Advisory Radio (HAR). The
provision of road weather information is an advisory strategy used to
influence the decisions of travelers (e.g., route choice, departure
time, travel speed). Such advisory strategies are being deployed today
in many areas. By integrating better atmospheric and pavement data with
traffic flow, incident, and construction data, more timely and accurate
messages could be communicated to travelers. Additionally, the
information can serve as the basis of route-specific road condition
reports and travel forecasts through web sites and interactive telephone
systems (e.g., 511---the national traveler information telephone
number).

Advisory strategies are often used in combination with control
strategies, which are another aspect of the concept of operations.
Control strategies alter the state of roadway devices to permit or
restrict traffic flow and regulate roadway capacity. Weather-related
signal timing plans modify cycle lengths, splits, and offsets to
accommodate changes in driver behavior and decrease arterial delay
(e.g., vehicle stops). Speed management involves the determination of
safe freeway speeds based upon visibility, pavement, traffic volume,
and/or vehicle classification data. With an ITS, safe speeds can be
computed and automatically conveyed to motorists via HAR, DMS, or
Variable Speed Limit (VSL) signs. When travel conditions are unsafe,
traffic managers may restrict access to affected bridges, specific
lanes, entire road segments, or designated vehicle types (e.g.,
high-profile vehicles). Ramp gates, lane use control signs, flashing
beacons, HAR, and DMS are typically employed to alert motorists of
weather-related hazards and access restrictions.

Treatment strategies (e.g., anti-icing, fog dispersal) are the third
type of strategy described in the concept of operation. These strategies
supply resources, such as salt and sand, to roadways to minimize or
eliminate weather impacts. Many treatment strategies involve
coordination of traffic, maintenance, and emergency managers. By
communicating and coordinating activities, these managers can quickly
restore capacity lost to snow and ice accumulation, submerged lanes, or
damaged infrastructure.

In the weather-responsive traffic management concept of operations,
advisory, control, and treatment strategies are implemented in response
to weather threats to help travelers make informed decisions and avoid
hazards, reduce crash frequency and severity, decrease weather-related
delay and congestion, as well as minimize operating and maintenance
costs. Currently, these strategies are executed to mitigate localized
weather impacts on relatively short road segments. To achieve
weather-responsive traffic management benefits, as envisioned in the
concept of operations, network-wide solutions are required to complement
the site-specific strategies.

## Weather-Responsive Traffic Management Case Studies

Case studies of successful operational strategies used on an arterial
route, a freeway segment and a bridge are presented to illustrate how
weather-responsive traffic management can improve transportation
outcomes. In the City of Clearwater, Florida traffic signal timing is
changed automatically during frequent afternoon thunderstorms. These
storms cause significant increases in traffic exiting Clearwater Beach
via the Memorial Causeway (i.e., State Route 60) shown in Figure 1. When
an electric rain gauge near the beach senses a predetermined amount of
rainfall, the signal system computer issues a preemption command to 14
signals along the route. Signal controllers execute new timing plans
with longer green times for inbound approaches. When vehicle detector
data indicates that the traffic volume has returned to normal, the
central computer restores normal signal timing plans. By modifying
traffic signal timing in response to rain, the signal system computer
prevents traffic congestion and enhances
mobility.![](media/image1.wmf){width="2.540277777777778in"
height="2.0416666666666665in"}

The Idaho Department of Transportation (DOT) installed a motorist
warning system on a 161-kilometer (100-mile) section of Interstate 84
that was prone to multi-vehicle crashes when visibility was reduced by
blowing snow, blowing dust and/or precipitation. Data from environmental
sensors and vehicle detectors deployed along the freeway are transmitted
to a central computer. The computer sounds an alarm when field data
indicates that visibility has fallen below a predetermined threshold or
that driving conditions are deteriorating. When alerted, traffic
managers decide which advisories to issue and activate DMS. As shown in
Table 2, disseminating advisory information in hazardous conditions
prompts drivers to reduce speed, which lowers crash risk and improves
safety.

**TABLE 2 -- Idaho DOT Safety Impacts**

  --------------------------- --------------- -------------- --------------
  **Road Weather Conditions** **Speed without **Speed with   **Speed
                              Advisories**    Advisories**   Reduction due
                                                             to
                                                             Advisories**

  High Winds (over 32.2 kph   88.1 kph (54.8  68.0 kph (42.3 23 percent
  or 20 mph)                  mph)            mph)           

  High Winds & Precipitation  75.6 kph (47.0  66.2 kph (41.2 12 percent
                              mph)            mph)           

  High Winds & Snow-Covered   87.9 kph (54.7  56.9 kph (35.4 35 percent
  Pavement                    mph)            mph)           
  --------------------------- --------------- -------------- --------------

Following a 193-vehicle crash on the Interstate 10 Bay Bridge in 1995,
the Alabama DOT deployed and began operating a fog warning system in
September 2000. Video from Closed Circuit Television (CCTV) cameras and
data from visibility sensors installed on the seven-mile bridge are
transmitted to a central computer. When fog is observed via CCTV,
operators consult the computer to view visibility distance in six
independently operated zones. As shown in Table 1, warnings are posted
on DMS and speed limits are reduced with VSL signs based upon conditions
in each zone. In very low visibility, Highway Patrol officers are
dispatched to close the bridge. The warning system has improved safety
by reducing average speed and minimizing crash risk in low visibility
conditions.

**TABLE 1 -- Alabama DOT Low Visibility Warning System Strategies**

+------------+-------------------------------+------------------------+
| **         | **Advisories on DMS**         | **Other Strategies**   |
| Visibility |                               |                        |
| Distance** |                               |                        |
+------------+-------------------------------+------------------------+
| Less than  | "FOG WARNING"                 | Speed limit at 65 mph  |
| 274.3 m    |                               | (104.5 kph)            |
| (900 ft.)  |                               |                        |
+------------+-------------------------------+------------------------+
| Less than  | "FOG" alternating with "SLOW, | -   "55 MPH" on VSL    |
| 201.2 m    | USE LOW BEAMS"                |     signs              |
| (660 ft.)  |                               |                        |
|            |                               | -   "TRUCKS KEEP       |
|            |                               |     RIGHT" on DMS      |
+------------+-------------------------------+------------------------+
| Less than  | "FOG" alternating with "SLOW, | -   "45 MPH" on VSL    |
| 137.2 m.   | USE LOW BEAMS"                |     signs              |
| (450 ft.)  |                               |                        |
|            |                               | -   "TRUCKS KEEP       |
|            |                               |     RIGHT" on DMS      |
+------------+-------------------------------+------------------------+
| Less than  | "DENSE FOG" alternating with  | -   "35 MPH" on VSL    |
| 85.3 m.    | "SLOW, USE LOW BEAMS"         |     signs              |
| (280 ft.)  |                               |                        |
|            |                               | -   "TRUCKS KEEP       |
|            |                               |     RIGHT" on DMS      |
|            |                               |                        |
|            |                               | -   Street lighting    |
|            |                               |     extinguished       |
+------------+-------------------------------+------------------------+
| Less than  | I-10 CLOSED, KEEP RIGHT, EXIT | Road Closure by        |
| 53.3 m.    | ½ MILE                        | Highway Patrol         |
| (175 ft.)  |                               |                        |
+------------+-------------------------------+------------------------+

# research needs to SUPPORT THE CONCEPT OF OPERATIONS

Additional work is needed to advance the state-of-the-practice and make
the concept of operations a reality. Better analysis tools, models, and
integrated solutions will lead to more efficient operation of highways
during adverse weather. The concept of operations highlighted the
following key questions.

-   What data, processes, and procedures are needed by traffic managers
    to support weather-responsive traffic management?

-   How should weather-related data, processes, and procedures be
    integrated with other transportation management systems and
    activities?

-   What additional resources are needed to support weather-responsive
    traffic management?

To provide traffic managers with decision support, further research must
be conducted to design, prototype, and test high-resolution predictions
of surface weather, pavement, and traffic conditions. Investigators must
determine how different weather events impact traffic demand, travel
speeds, and roadway capacity to examine and understand weather effects
on intersection, interchange, and overall network operation. To that
end, FHWA recently initiated a study to baseline the use of weather
information in TMCs. This study builds upon a brief FHWA survey,
conducted in 2001, which revealed that most TMCs receive some general
weather information and over 60 percent use customized weather products
(e.g., pavement temperature predictions). Only a few of the centers
integrated weather data into operating systems.

Applied research conducted in TMCs will help system developers
understand user needs, integrate pavement and weather data with ATMS,
and determine the most effective means of presenting environmental
information to traffic operators. Research is also required to provide
additional resources and develop systematic weather-responsive traffic
management strategies. Key issues to be explored include better
understanding of driver behavior in adverse conditions (e.g., headway,
lane-changing, speed) and delivery of road weather information (e.g.,
message sets) to various types of travelers (e.g., commuters, tourists,
commercial vehicle operators).

Given these extensive research needs, it seems reasonable to pursue a
broader road weather research program. Resources to carry out road
weather data collection and research are currently inadequate,
undedicated, and limited by authorization levels. The FHWA Road Weather
Management Program advocates the establishment of a national steering
committee of transportation, public safety, and meteorological
professionals to promote a research agenda including advanced weather
research and applied research to prototype and validate integrated
solutions for transportation managers.

# CONCLUSION

Weather-responsive traffic management is a truly interdisciplinary
endeavor requiring a thorough understanding of the impact of adverse
weather on safety, mobility, and operations. Armed with accurate,
timely, route-specific road weather information, traffic managers can
apply a variety of strategies in a systematic manner to achieve optimal
traffic flow on freeways and arterials in urban road networks. Current
practice has shown that weather-responsive traffic management strategies
are effective when applied to short highway segments. To fully achieve
21^st^ century operations with 21^st^ century technologies, additional
research and development are needed to advance integrated, network-wide
solutions for advising travelers and controlling traffic flow in adverse
weather.

# REFERENCES

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2.  Transportation Research Board, *Highway Capacity Manual*, National
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3.  Goodwin, L., *Weather Impacts on Arterial Traffic Flow*, Mitretek
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4.  Knapp, et al, *Winter Storm Event Volume Impact Analysis Using
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5.  Gordon, R. L., *Traffic Control Systems Handbook*, prepared by Dunn
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6.  Chin, S., et al., *Temporary Losses of Highway Capacity and Impacts
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7.  Goodwin, L., *Analysis of Weather-Related Crashes on U.S. Highways*,
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8.  U.S. DOT, *Motor Carrier Management Information System (MCMIS) Crash
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9.  Perrin, et al., *Testing the Adverse Visibility Information System
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10. Cambridge Systematics, *Weather-Responsive Traffic Management:
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11. FHWA, *Best Practices for Road Weather Management*, Version 2.0,
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