Road Weather Information System
Environmental Sensor Station
Siting Guidelines
April 2005

2.0 Assessing Road Weather Informational Requirements

Planning for the acquisition and installation of ESSs is best accomplished by a team of road and weather experts. In addition to the DOT team lead, this group should include a meteorologist familiar with road weather requirements and local DOT personnel. The meteorologist can fill the gap between the weather and transportation communities by helping to evaluate road weather information requirements and assess available ESS technologies. The meteorologist can play a key role during ESS site selection by evaluating specific sites for obstructions or weather influences that could compromise the validity of the ESS data. The meteorologist can also consider the broader application of ESS data that may benefit the transportation community and other users. For example, the meteorologist can consider how additional weather observations can be used by high resolution forecast models and can help develop a siting plan that will meet DOT requirements while helping to improve the accuracy of road-specific and general area weather predictions.

The planning team should also include local DOT personnel, especially maintenance personnel. These individuals typically possess a vast knowledge of weather conditions along the road segment they maintain. The maintenance personnel can provide critical input about recurring weather problems such as the locations of frequent slippery pavements, low visibilities, or strong gusty winds that suggest the need for an ESS installation. Additionally, local DOT personnel can often identify areas where an ESS sensor might be vulnerable to large snow drifts, flooding, or pooling water from spring thaws.

Planning the ESS network should include an analysis of the operational requirements for road weather information. This analysis will drive the environmental sensor requirements and lead to decisions regarding sensor selection and siting. Considerations include:

• How will the road weather information be used? For example, will the information be used to monitor roadway conditions as input to winter maintenance decisions or road temperature modeling, or to support weather-responsive traffic management, traveler information systems (e.g., 511 systems) or road construction efforts?
  
• Will the ESS be used to measure a site-specific condition or to provide information that may represent conditions across a general area? For example, installing a sensor to monitor the visibility along a fog-prone road segment may result in completely different siting decisions than if the requirement is to collect wind and temperature information for input to a road weather model.
  
• What needs to be measured at each installation? The information in Section 2.1 and Appendix C will help DOTs evaluate their data collection requirements and the sensors available to meet those needs. System designers should keep in mind that several different sensors may be needed in combination to satisfy observing requirements. For example, if a pavement sensor is to be included in an installation, the DOT may also want to install air temperature, humidity, and precipitation sensors to complement the pavement sensor data. The precipitation sensor can help identify whether pavement sensor readings are indicative of new or continuing precipitation, while the temperature and humidity sensors will indicate whether conditions support the formation of frost.
  
• DOTs may want to create a prioritized list of the road weather elements and sites they need to fulfill their requirements. Such an approach may help in making tradeoffs when data collection needs exceed available funding or when a phased approach to meeting statewide requirements is desired.
  
• DOTs should also consider other sources of weather and pavement data that may be available to meet road weather information requirements. Developing data-sharing partnerships with other agencies may help satisfy RWIS ESS installation requirements while improving the availability of data to all partners. Leveraging existing weather observing networks is discussed further in Section 2.3.

2.1 Environmental Sensors

The RWIS ESS can consist of a relatively few number of sensors providing basic information such as wind speed and direction and air, pavement, and subsurface temperatures, or it can include the measurements of more complex weather elements such as visibility. Table 1 provides a list of the most common ESS sensors. The sensors chosen for a particular site should reflect the results of the requirements analysis, i.e., how the observations will be used and what the minimum required road weather information is at that specific location.

While sensor selection should always reflect operational requirements, a typical ESS installation frequently includes the following:

• A combined sensor to measure both wind speed and direction (e.g., aerovane or sonic anemometer) or individual wind speed and direction sensors (e.g., conventional anemometer).
  
• Sensors to measure air temperature and moisture. Typically two sensors located in a single housing provide air temperature and one of the following: dewpoint temperature, wet bulb temperature, or relative humidity.
  
• Sensors to measure the temperature of the pavement and to indicate whether the surface is dry, wet, or frozen. Active sensors cool and warm surface liquids to determine the freeze point temperature. Passive sensors commonly monitor changes in roadway surface conductivity as surface changes occur. When road treatment chemicals are in use, the surface conductivity can be an indication of the chemical concentration on the roadway. The presence and concentration of chemicals is important, as it will affect the actual freezing temperature of the road surface.⁸ Optical sensors for pavement measurements are also under development.
  
• Sensors to detect the presence, type, and intensity of precipitation. A single, optical, present weather detector can detect the presence of precipitation and measure intensity. By estimating the water content of precipitation and combining this information with optical forward scatter and temperature measurements, these instruments can also identify precipitation type. Optical weather presence sensors capable of differentiating among rain, freezing rain, drizzle, freezing drizzle, mixed rain and snow, snow, and ice pellets are available.
  

Based on roadway operations and maintenance needs, additional sensors can be added from those listed in Table 1 to provide a more capable ESS. For example, visibility sensors can be extremely useful along roadways prone to low visibility due to fog or manmade pollutants (e.g., smoke). Subsurface temperature and solar radiation sensors can provide information to support forecasts of pavement temperatures. Precipitation, snow depth, and video imagery from an ESS camera can provide the DOTs valuable information for managing traffic and planning road maintenance operations.

Appendix C includes a list of weather elements for DOT consideration during the analysis of road weather information requirements. Automated sensors are not yet available to observe all the weather elements in that list, and some sensors, while available, have inherent limitations that restrict their utility as part of an ESS. During the process of analyzing requirements and potential sensor suite solutions, the DOTs may want to consider these sensor limitations.

• Cameras may be an option for determining the presence of snow and/or drifts on a roadway. There are no reliable instruments to remotely measure many roadway conditions such as roadway snow pack depth or roadway snow or ice accumulation.
  
• Observations of thunderstorms, tornadoes, and waterspouts are difficult to automate using sensors deployed as part of an RWIS ESS.
  
• There are no automated sensors for observing sun glare.
  
• While cloud cover information can be of interest to road maintainers as input to road temperature models and to RWIS data users, automated sensors are limited in their ability to measure cloud cover as an individual standing on the ground sees it (the sensor typically only scans directly overhead while an individual can scan the entire celestial dome). These instruments also tend to require more maintenance than most ESS sensors. For these reasons, cloud cover sensors are not normally installed with an ESS and are not included in the ESS sensor list shown previously in Table 1.

2.2 Regional and Local Site Requirements

An ESS installation can be characterized as either "regional" or "local." A regional ESS site would be one that represents the general weather conditions for a large area or road segment. A local site is one where the weather phenomenon of interest (e.g., icy pavements or tidal flooding) is for a specific short segment of roadway, topographic feature, or designated bridge structure. Differences in the siting requirements between regional and local sites result in different philosophies in the selection of the sensors and the siting criteria of the ESS. In general, regional sites include more types of sensors sited in an unobstructed location. Local sites may consist of a tailored suite of sensors (potentially only one or two) that are located close to the road segment or bridge where the targeted weather event is of interest.

A single ESS can satisfy both regional and local requirements for road weather information. For example, a site considered representative of regional road weather conditions may also include one or more sensors focused on conditions of interest within a short roadway or bridge segment. Similarly, road weather information requirements may dictate installing multiple sensors on a local ESS to monitor road conditions (e.g., including a pavement sensor near a visibility monitoring site). Siting a single ESS to satisfy both regional and local road weather information requirements or multiple local weather information requirements requires considerable planning. An agency considering new or relocated ESS locations may need to make tradeoffs in sensor selection and siting or may need to install additional sensors or sites to satisfy all regional and local road weather information requirements.

Regional sites support broad, real-time monitoring of weather and road conditions across a geographic area. Equally important, these sites can also be used to provide data to improve the accuracy of surface transportation specific forecasts (e.g., pavement temperature forecasts). Installing regional sites across an area lacking sensors helps define the initial environmental conditions necessary to run road weather prediction models. These sites can also provide ground truth for comparing surface transportation specific forecasts with real-time observations to evaluate the accuracy of the forecasts. Additionally, locating a regional ESS in an isolated area where no other weather observations are available or in a location upstream of an area of interest can improve the ability to anticipate changes in the road weather environment in a specific area of interest.

Regional sites include many of the same characteristics as weather observing locations satisfying NWS or FAA weather information requirements. That is, they have a generally uniform suite of weather sensors sited to represent a regional area. A key difference between regional RWIS ESS and NWS or FAA weather observing locations is that the ESS sites may include roadway-specific sensors (e.g., pavement and subsurface sensors). ESS regional sites can successfully augment sites used for general weather applications, and the increased data density they offer can improve the accuracy of NWS analyses and storm forecasts.

The size of the area for which road weather observations from a regional ESS site can be considered representative is influenced by a number of factors including topography, climate, and the time and space scale of the weather event under observation. There are no studies that define the optimal separation between regional ESSs to monitor road weather events and to support weather models. Some weather forecasting models include a grid spacing as low as 2.5 miles (4 kilometers (km)). While installing regional ESSs with a 2.5 mile (4 km) separation may be desirable to contribute to more accurate weather forecasts, doing so may be cost prohibitive. A spacing of approximately 20-30 miles (30-50 km) along a road is recommended as a guide.

Local sites are usually located close to the point of interest on the roadway or bridge deck. A point of interest is typically the result of topographic variations, road construction techniques, pavement types, or roadway geometry, e.g., at an intersection that has some sort of managed traffic control such as a traffic signal. Local sites can also provide predictors for conditions at the site. However, the point of interest may also be subsurface characteristics that influence or are influenced by specific weather situations (e.g., high humidity, low solar heating, residual surface moisture, and high water conditions). Because local ESSs are installed to measure specific events of interest to transportation operations and road maintenance personnel, the resulting observations may be pessimistic as compared to observations representative of a larger area.

By definition, a local site is one where the weather conditions of interest are for a specific short segment of roadway, a topographic feature, or designated bridge structure. However, certain general road and bridge deck sites may be considered representative sites for other similar segments or structures within the same general weather area. For example, a pavement temperature sensor on a bridge deck can be considered representative of conditions on other bridge decks over the same body of water or topographic feature, or of other bridges or ramps in the area.

The characteristics of a local ESS site are analogous to specialized weather observing sites supporting other industries, such as agriculture. These specialized sites contain sensors primarily designed to monitor specific elements or situations. For example, some agricultural weather sites only include wind sensors at low heights, such as 6.5 feet (2 meters) above ground level, an array of subsurface sensors, and a solar radiation sensor.

2.3 Leveraging Information Partnerships

An important consideration during planning of a new RWIS ESS is the opportunity to partner with other agencies and share resources. These resources may be other sensor systems belonging to organizations such as the NWS, FAA, USFS, Water Resource Weather Station Networks, and universities. DOTs may want to contact these organizations to determine the availability of real-time weather observations and to assess whether those observations will meet some road weather requirements. While it is unlikely these sources of other weather information will satisfy many road weather information requirements, DOTs may be able to leverage existing observing capabilities to obtain some weather information to supplement weather data collected from the RWIS network. In some situations, partnerships may avoid the costly duplication of sensors that may already exist in the area. The sharing of data with other agencies can be complicated by different data formats, different weather observing frequency requirements, and communications incompatibilities. Still, it is advisable to develop relationships with other agencies to identify areas of mutual interest for future cooperation. Sharing data can leverage assets between organizations and enhance the overall reporting network supporting all users. A key resource in this regard is the NWS, starting at the appropriate regional office.

Additionally, other organizations may be able to provide existing towers, power, and communications to support the installation of an ESS. For example, the NWS partnered with the National Ocean Service (NOS) to increase the availability of weather data along U.S. coastlines and bays. The NWS installed their atmospheric sensors on NOS tide gauge platforms already equipped with power and communications. This partnership saved the NWS significant funds that would have otherwise been required to install NWS observing platforms.

⁸ Keep, Dale. Roadway Weather Information Systems (RWIS), Ice and Snow Technologies, LLC, Report, 2004.