Iowa DOT stalks roadway weather

RWIS uses pavement and atmospheric sensors to capture data that shape driving

Winter Maintenance Article October 04, 2001
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Roadway weather information systems (RWIS) have been in use in the U

Roadway weather information systems (RWIS) have been in use in the U.S. for many years and have become an important tool for decision makers who call the shots in winter snow and ice removal. There are a number of manufacturers of RWIS systems and configurations available, but this article is a description of the system deployed by the Iowa Department of Transportation.

RWIS sites have been installed at 50 locations in the state of Iowa. All equipment for the system was purchased from Surface Systems Inc. (SSI), and installation of each site was completed by maintenance garage personnel. Initial plans called for installing the sites along a 37.5-mile grid, but that plan was modified in later years to place units in locations that would not only meet our weather needs but also include locations with special winter challenges or high traffic volumes.

RWIS units capture current weather information from a network of sensors positioned either beside or in the roadway. A typical RWIS site consists of a 30-ft tower constructed near the road with several atmospheric sensing devices mounted on the tower along with a number of sensors embedded in the pavement to measure surface and subsurface temperatures.

Pavement sensors

Pavement sensors are embedded flush with the surface of the roadway to measure the pavement temperature. Each site usually has two or three pavement sensors and one or more bridge deck sensors. Each of the pavement sensors is connected to the RWIS tower by Type V cable that is buried either directly in the ground or encased in conduit. A 5 1/2-in. diam. core is drilled 2 in. deep for placement of the pavement sensor and a saw cut is made in the pavement to accommodate the cable. The pavement sensor is placed flush with the surrounding surface and held in place with a quick-drying epoxy. In Iowa the pavement sensors have generally been located just outside the left wheel path of the driving lane to provide roadway surface information from one of the most critical parts of the roadway, but away from areas that might be subject to rutting in the future. The newer pavement sensors also have the ability to determine the depth of the precipitation on the roadway surface and the chemical concentration of the chloride solution on the roadway.

A subsurface probe that constantly measures the soil temperature 18 in. below the surface is normally installed immediately below one pavement sensor at a site. Subsurface temperatures will affect surface temperatures throughout the year but will show little change in temperature during the course of a day. In the fall, warm subsurface temperatures will often keep roadway surfaces warmer than the air temperature and help melt any winter precipitation. Conversely in the spring of the year, a cold subsurface temperature left over from the long winter may keep pavement temperatures colder than the surrounding air.

Atmospheric sensors

Most RWIS sites also include an anemometer to measure the wind speed and direction at the top of the 30-ft, fold-over tower. A thermometer and relative humidity sensor are mounted in a housing that shields the instruments from the weather and thermal radiation. The thermometer and relative humidity sensor are typically located approximately 6 ft above the roadway surface to provide the most meaningful measurements for roadway applications. Precipitation sensors that discern whether precipitation is falling at the site are mounted higher on the tower.

Iowa has 20 optical weather identifiers (OWI), which have the ability to identify the type of precipitation falling (rain, snow, sleet, freezing rain, etc.) and the rate of fall. The OWI measures the number and size of objects that fall between its sensors, looks at other atmospheric readings from the site and calculates the type of precipitation and rate of fall. The remaining sites are equipped with less sophisticated precipitation sensors that simply report the presence or absence of precipitation.

The highway and atmospheric sensors are connected through weatherproof electrical cables to a central control box called a remote processing unit (RPU). The RPU logs a record of all sensor readings whenever a change is detected and holds the information until the central server calls for current information. As temperatures near the freezing point while precipitation is falling the RPU is collecting information constantly, but when temperatures are 70ûF and weather is constant updates at the RPU are less frequent. Most Iowa RWIS sites are powered at the roadside by standard 110-volt electrical service, but two sites located a long distance from a power source are equipped with solar panels.

The Iowa RWIS network currently has one still-frame, black-and-white camera installed in the Des Moines area, and the Iowa DOT is researching whether to install additional cameras in the future. The system also is designed with extra expansion slots to allow either additional pavement sensors or sensors that measure visibility, barometric pressure, water depth, traffic and other environmental factors.

Communication and computing

The Iowa RWIS system communicates the weather data over a local area network (LAN). The servers powering the network send a request for current weather information to personal computers located at garages near the RWIS towers. The garage computer then initiates a call to the tower through a standard telephone or radio modem to collect weather information that has been logged since the previous call to the tower.

The information is then transmitted back to headquarters where it can be made available to internal users through an intranet. The RWIS information also is shipped to the Internet for display on the Weatherview website ( The Weatherview site combines the weather information from 34 automated weather observation stations at local airports with the information from all 50 RWIS sites and makes the information available free to the public.

The use of a LAN helped reduce the number of long distance calls that were used. In the past, a central computer placed a long distance call to each site to collect the most current weather information. The network can now move the data from the roadside system much faster and more efficiently so that weather information is generally no older than 30-40 minutes.

Software and use of the system

The Iowa system uses ScanWeb software developed by SSI and a standard Internet browser for internal users. The software displays all the information from each individual site in a table but also allows the user to view a map of any single parameter (pavement temperatures, precipitation, winds, air temperatures and others) from each site. This provides the user with a quick snapshot of where a precipitation event may be occurring, what the pavement temperatures are doing before and after the front moves through the area and the speed and direction of the winds. This type of weather information can help in determining what strategies will be used to fight a current or approaching storm.

The information from RWIS also is made available to the Iowa DOT weather forecasting contractor through the Internet. The forecaster combines that information with other national weather data and develops a pavement and bridge forecast for different parts of the state along with detailed weather forecasts in three-hour increments for a number of other weather parameters. These forecasts provide Iowa DOT personnel with a look at what will happen in the next 24 hours in their area to help them make critical decisions on winter operations.

Most garages in the state have access to the RWIS information through laptop or desktop computers while at the office or at home. The laptops have been programmed to allow users access to the ScanWeb weather information through a toll-free number for occasions when winter weather strikes on a night or weekend.

Other uses and the future

Although RWIS has been primarily used as a winter tool for maintenance operations, the weather information available on these systems also can be used during the rest of the year to track weather events that affect other maintenance activities. When combined with other weather information such as radar and satellite imagery, it can give decision makers a better picture of the weather and how it will affect their operations.

RWIS can be a valuable tool in winter operations to provide current information on how the weather is shaping roadway conditions but also is becoming more important as a source to help forecast travel weather. Someday weather forecasters may be able to provide travelers with detailed descriptions of the weather they may face in their morning commute or holiday trip. Knowing in the morning that there will be frost on the bridge decks and in sheltered areas may alert motorists to take a little more caution on their commute to work. Accurate forecasts of blizzards accompanied by real-time verifications from weather sensors and cameras around the state may provide the information travelers need to make important travel decisions.

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