Road Weather Information System
|Water Vapor (Dewpoint or Relative Humidity)||Hygrometer|
|Wind Speed and Direction||Conventional and Sonic Anemometer and Wind Vane or combined sensor (Aerovane)|
|Pavement Temperature, Pavement Freeze Point Temperature, Pavement Condition, Pavement Chemical Concentration||Pavement Sensor|
|Subsurface Temperature||Subsurface Temperature Probe|
|Subsurface Moisture||Subsurface Moisture Probe|
|Precipitation Occurrence||Rain Gauge, Optical Present Weather Detector|
|Precipitation Type||Rain Gauge, Optical Present Weather Detector|
|Precipitation Intensity||Rain Gauge, Optical Present Weather Detector|
|Precipitation Accumulation||Rain Gauge, Optical Present Weather Detector, Hot-Plate Type Precipitation Sensor|
|Snow Depth||Ultrasonic or Infrared Snow Depth Sensor|
|Visibility||Optical Visibility Sensor, Closed Circuit Television Camera|
|Solar Radiation||Solar Radiation Sensor|
|Terrestrial Radiation||Total Radiation Sensor|
|Water Level||Pressure Transducer, Ultrasonic Sensor, Float Gauge, or Conductance Sensor|
While sensor selection should always reflect operational requirements, a typical ESS installation frequently includes the following:
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.
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.
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.