Publication Number: FHWA-HRT-05-001

Weathering the Storm

by Roemer M. Alfelor

Guidance is now available for siting sensor stations that collect data on weather conditions on or near road surfaces.

(Above) Foul weather like the rain that this truck is driving through calls for traffic countermeasures, such as variable message signs that reduce speed limits on slick roads.

According to the Federal Highway Administration (FHWA), more than 1.4 million highway crashes occur under adverse weather and road conditions each year. Maybe nothing can be done to change the weather, but something can be done about measuring and alleviating its effect on road conditions. By observing and predicting the impacts of weather on highways, transportation experts who operate the Nation's roadways can determine appropriate management strategies, such as applying anti-icing chemicals, reducing speed limits, or closing hazardous areas, to make driving safer during and after inclement weather.

Transportation managers use anemometers (wind speed and direction sensors) and other meteorological and pavement monitoring equipment to provide real-time observations and data that can help them prepare for, or respond to, a variety of emergency conditions, such as flooding, roadway icing, and strong winds, caused by adverse weather. To make driving safer during extreme events, these managers need to know how the weather is affecting the vehicles, the drivers, and the road surfaces right now in real time.

To collect real-time weather observations along highways, transportation managers use environmental sensor stations (ESS) that are strategically located to help them identify appropriate maintenance and traffic management strategies. These sensor stations are the building blocks of a road weather information system (RWIS)--which includes the hardware, software, and communications equipment used to collect and transfer road weather data to a central location.

Although established guidelines are available to help determine appropriate locations for installing atmospheric weather observing equipment, siting information geared specifically to roadways, or surface transportation in general, is limited. To fill that gap, FHWA, the American Association of State Highway and Transportation Officials (AASHTO), and the Aurora RWIS Pooled Fund Program partnered to publish Road Weather Information System Environmental Sensor Station Siting Guidelines (FHWA-HOP-05-026). The new publication, available at www.ops.fhwa.dot.gov/publications/ess05/index.htm, provides guidelines to help State and local departments of transportation (DOTs) site sensor stations and thus improve real-time data about weather conditions on the roads.

"State and local DOTs are the only agencies collecting environmental sensor information about the road surface," says Dan Roosevelt, research scientist with the Virginia Department of Transportation (VDOT). "These unique data are used to determine road conditions and forecast changes, but a forecast is only as accurate as the data backing it. Proper siting of weather sensors is an important first step in the data-gathering process."

Types of Sensors

Environmental sensors such as anemometers and wind vanes can help determine the direction that the wind is blowing. Other equipment can help traffic managers ascertain the rate at which the rain or snow is falling, the speed of the freeze cycle, and myriad other weather and pavement observations.

The environmental sensor tower on the cover of the siting guidelines (shown here) is located on Route 528, the Beeline Expressway, near Cape Canaveral, FL.

Road weather information systems, originally developed to address winter weather, now operate year-round and monitor a variety of weather and pavement conditions. An ESS may have multiple sensors that detect atmospheric conditions. For example, it can provide data for informing motorists about strong crosswinds. Pavement sensors, on the other hand, monitor conditions such as wet, snowy, icy, or flooded surfaces. Still other sensors provide data on subsurface conditions, such as soil temperatures.

Other road weather data of interest include precipitation, humidity, and visibility; atmospheric pressure; the concentrations of chemicals on pavements from de-icing treatments; and solar and terrestrial radiation to determine the potential for nighttime cooling. Water-level sensors may be needed in flood-prone areas and along coastal roadways.

A typical ESS installation includes a thermometer to measure air temperature, a hygrometer for water vapor (dewpoint or relative humidity), an anemometer and wind vane, and a pavement sensor to monitor temperature, freeze point, and chemical concentration. A rain gauge and infrared sensor can measure precipitation occurrence, type, and intensity. By using sensors and video cameras mounted on a tower or next to it, operations and maintenance personnel can determine appropriate strategies and evaluate the outcome of those strategies.

But sensors do have limitations. There are no reliable instruments to remotely measure roadway conditions such as snowpack depth. Also, there are no automated sensors to provide observations of thunderstorms, tornadoes, waterspouts, and sun glare.

Types of Data

The sites that provide data on local weather phenomena have different requirements than the regional sites that support broad, real-time monitoring across a large geographic area or road segment. Local sites are selected to detect road weather conditions of interest for a specific road segment, bridge, or other transportation-related feature. Conditions of interest are typically the result of topographic variations, road construction techniques, pavement types, roadway geometry, or subsurface characteristics.

Regional sites support broad, real-time monitoring across an area or region. They can provide empirical verification (ground-truthing) for comparing specific forecasts for surface transportation with real-time observations to evaluate the accuracy of road weather prediction models. A key difference between regional sites and weather-observing locations that satisfy the requirements of the National Oceanic and Atmospheric Administration's National Weather Service or the U.S. Department of Transportation's (USDOT) Federal Aviation Administration is that the ESS sites may include roadway-specific pavement and subsurface sensors.

This illustration shows a typical ESS tower and sensor configuration, including a wind sensor and camera mounted near or at the top of the tower, precipitation and radiation sensors several feet above ground level, and a snow depth sensor just above the roadway surface. The siting guidelines provide recommendations for placement of the sensors shown in this diagram.

The FHWA siting guidelines recommend a spacing of 30 to 50 kilometers (20 to 30 miles) for regional ESS sites. For local sites, the sensors are placed close to the point of interest on the roadway or bridge deck.

Some sensor stations can satisfy both local and regional requirements for road weather information. When multiple sensors are installed on a regional ESS, for example, they might include one or more instruments focused on conditions of interest on a specific road or bridge segment. Siting a single ESS to satisfy both information requirements requires considerable planning.

Selecting ESS Sites And Sensors

An ESS installed at a poorly chosen location can result in servicing difficulties, sensor readings that are not representative of conditions, and possibly damage to the sensors from water runoff and ponding in low-lying areas. The site selection team also needs to minimize nonweather influences that can result from nearby buildings, billboards, tall vegetation, bridges, topography, or elevated portions of the highway.

Site conditions can change significantly from summer to winter when sun angles are low and trees lose their foliage. The ideal ESS site will rarely be found, given narrow rights-of-way and even the traffic itself. The planners will most often be in the position of needing to make tradeoffs.

"There is no mythical perfect siting location for an RWIS-ESS," says Ralph Patterson, weather operations and RWIS manager at the Utah Department of Transportation (UDOT), "just the best spot you can find to meet the intended purpose as best you can."

However, there are some better locations for sites than others. For example, a regional ESS should be sited on relatively flat, open terrain. Also, a regional ESS should be on the upwind side of the road, based on predominant wind directions.

For local sites, the circumstances that may require a sensor station include surface conditions such as a historically cold spot that creates slippery pavement; a location where significant drifting of snow or flooding occurs; local environmental conditions such as fog, smoke, or dust that reduce visibility; crosswinds along a confined valley or ridgetop; and roadway segments abnormally susceptible to ice or frost.

In areas prone to road frost, DOTs may consider mounting a dewpoint sensor close to the pavement. For segments prone to low visibility, DOTs might consider providing safety warnings via dynamic message signs. Visibility sensors should be installed 2 to 3 meters (6.5 to 10 feet) above the roadway so they are high enough to avoid frequent maintenance because of salt spray from snow- and ice-control operations.

For dangerous crosswind conditions, sensors can take wind measurements at 10 meters (33 feet), and an additional sensor at 3 to 5 meters (10 to 16.5 feet) can measure the winds most likely to affect high-profile vehicles. To monitor flooding conditions, DOTs can consider pressure transducers in standing bodies of water, ultrasonic sensors in fast-moving streams, and float gauges installed in standpipes (vertical pipes) to monitor precipitation and runoff. For bridges, sensors used to measure scouring can provide warnings of danger to the integrity of the foundation.

Thermal mapping--the use of vehicle-mounted infrared radiometers to map warm and cold spots along a roadway--can be a useful tool in selecting local ESS sites. Analysis of the data from thermal mapping can determine locations where frost and ice tend to form and thus suggest the need for an ESS and other locations where an ESS may not be needed. Where thermal mapping reduces the number of ESS installations, it can pay for itself.

Criteria for Siting Towers And Sensors

Several circumstances may affect tower siting, including access requirements for power, communications, and maintenance; geographical terrain, water bodies, and neighboring structures; aesthetic considerations; security concerns; and city, county, or State codes. Where a limited right-of-way precludes the installation of a tower and requirements for road weather information rules out selection of another site, DOTs may find other options, such as installing anemometers on utility poles.

If a tower is used, it should be sturdy, such as the open matrix type, and anchored to a concrete pad. At this time, there are no studies about the minimum distance that transportation personnel should place the tower from the roadway to avoid the effects of traffic on the accuracy of the sensors. According to the research performed for the siting guidelines publication, towers are most frequently installed 9 to 15 meters (30 to 50 feet) from the edge of the paved surface. Sites near steep roadcuts, swampy areas, and bedrock (an impediment to trenching for power cables) should be avoided.

Uses and Benefits of ESS Data "ESS data enable us to monitor and react to changing road conditions in order to maintain safe driving conditions as best as possible," says Mike Adams, RWIS program manager with the Wisconsin Department of Transportation. In addition to the key benefit of improving road safety during and after storms, ESS data supply information that traffic and weather response managers need to maintain vehicle flow and mobility during adverse weather, and thereby help increase the productivity of the Nation's transportation system. Other key benefits of ESS data include supplying the information that traffic managers need to provide traveler information. Local, State, and Federal disaster response agencies also may use the information to assess and manage emergencies and response actions. Transportation managers use ESS data to support road maintenance activities. According to Ralph Patterson, weather operations/RWIS manager and traffic operations manager at the Utah Department of Transportation (UDOT), "The use of ESS data here at UDOT crosses over into almost all aspects of operations, that is, winter maintenance, road rehab projects, construction, risk management, traveler information, planning, and, last but not least, our meteorology staff uses some of the data to produce and verify forecasts." Additional consumers of ESS data include the National Weather Service and military and private weather service providers, who use the information for developing short-range forecasts. Other applications are forecast models produced by government and university environmental monitoring networks. State climatologists can benefit from ESS data for developing long-term records and conducting climatological analyses. Insurance companies can use these data to help determine the risks of potential impacts from future weather events. And forensic meteorologists can use ESS data to reconstruct roadway crashes. Users of ESS data, as shown in this diagram, include traffic and emergency managers, who may use the information to deploy dynamic message signs and other roadside devices that warn motorists of adverse conditions; maintenance managers; information service providers (public or private entities that distribute road weather information to the public); public and private weather service providers; and environmental monitoring networks (collections of weather observation systems and/or equipment that is linked together).

DOTs should maintain complete documentation on the positioning of the tower and the height of the sensors. This metadata (data about data) file should be made available at a central location for data customers. Metadata provide users with a better understanding of what the information collected by sensors really represents.

"One of the most important uses for the guidelines," says UDOT's Patterson, "is to emphasize the importance of metadata. If you know the strengths and weaknesses of a particular site, you can turn the acquired data into more useful information. There are no bad data; it's just that some are more useful than others."

Air temperature and dewpoint sensors should be mounted 1.5 to 2 meters (5 to 6.5 feet) above ground level on a boom extended at least 1 meter (3 feet) from the tower toward the predominant wind direction. For anemometers, a general rule is that they should be positioned at a distance of 10 times the height of the nearest large obstruction. For example, if the obstruction is 6.1 meters (20 feet) tall, the wind sensor should be positioned 61 meters (200 feet) away.

Pavement sensors should be sited in unshaded areas to represent the surrounding road segment under maximum cooling conditions, except in the case of road segments that are predominantly shaded. In an urban environment, if only one sensor is to be installed, the typical location is the travel lane (the rightmost lane in a multilane roadway). Consideration should be given to siting the pavement sensor in the travel lane of the morning outbound traffic to reduce the influence of heavy vehicle traffic on pavement observations. In general, pavement sensors should be installed near the edge of the inside wheel track.

The guidelines provide similar advice on the location and selection of other types of sensors. For example, on location, subsurface sensors should be installed at a depth of 30.5 or 45.5 centimeters (12 or 18 inches), depending on the manufacturer's guidelines. For selection, in the case of precipitation accumulation sensors, one type--the tipping bucket--often underreports rainfall totals during heavy precipitation. The other type--the weighing gauge--can measure both solid and liquid precipitation and is more sensitive to light rainfalls. Both sensors require a heating device in freezing climates. A new technology for determining precipitation amounts, the hot-plate rain gauge measures the power needed to evaporate precipitation falling on a sensor plate. All precipitation sensors should be placed in an open area and away from the roadway to avoid splashing.

This open matrix sensor tower is located in Wisconsin.

The recommendations for selecting sensors and locating towers reflect a range of values because of the complexity of the roadway environment and the need for additional research. In any case, mounting sensors on a tower requires careful planning so that they do not interfere with one another.

Planning an ESS Network

The siting guidelines also discuss the methods for selecting a team of road and weather experts to plan for the acquisition and installation of sensor stations. In addition to a DOT team lead, the group should include a meteorologist who can help assess information requirements and ESS technologies. The meteorologist can evaluate specific sites for weather influences that could affect the validity of the ESS data, such as the influence of solar radiation on road surface temperature.

Other team members should include maintenance personnel because of their familiarity with weather conditions along their road segments. They may know, for example, the locations of pavements that are frequently slippery, areas with low visibility, or road segments with strong gusty winds that suggest the need for an ESS installation.

Once selected, the team needs to determine the uses of the weather information, including input to winter maintenance operations or support for weather-responsive traffic management or 511 traveler information systems. Another crucial decision is whether the ESS will be used to measure a site-specific condition, such as visibility along a fog-prone road segment, or to provide information that may represent conditions across a general area.

The next questions include what should be measured at each installation and thus which sensors are needed. The DOT may want to create a prioritized list to help in making tradeoffs when data collection needs exceed available funding. A phased approach may be the answer.

Finally, DOTs should consider developing data-sharing partnerships to leverage the information collected by other organizations. Possibilities include the National Weather Service, Federal Aviation Administration, U.S. Department of Agriculture (USDA) Forest Service, local television stations, universities, water resource weather station networks, and other city, county, and State agencies. Partnerships may avoid the costly duplication of sensors, although it is necessary to recognize that data sharing can be complicated by different data formats and communications incompatibilities.

"In the past, VDOT has partnered with the city of Richmond and the National Park Service to share data from stations installed by all three jurisdictions," says Roosevelt. "This has increased the coverage we have on the road system in Virginia and reduced the number of stations each jurisdiction needs."

Similarly, Utah is partnering with two agencies, the USDA Forest Service and the Tooele County Emergency Operations Center (TEOC). "With the Forest Service, we offer manpower and expertise for maintenance of a site owned by the Wasatch-Cache National Forest," says Patterson, "and with the TEOC, we are in the process of putting some of our hardware on their weather stations to gather road and precipitation conditions so their existing mesonet of stations can do double duty."

Additional Considerations

Planning should address data requirements first and then address how to satisfy power and communications requirements. Power options include commercial connections, wind power, or solar with batteries. Commercial power is usually the most economical and reliable. Solar power can support nominal loads but typically is incapable of sustaining heavy power consumption for heated sensors. North Dakota has successfully used wind power for a number of ESS installations.

In some cases, the sensors can be located near other intelligent transportation system (ITS) devices such as traffic counters, dynamic message signs, and traffic signal controllers to share power and communications costs. For critical sites, backup sources of power or communications may be needed.

The diagram shows the desired tower location relative to a roadway.

This diagram shows the typical siting for pavement sensors, near the edge of the wheel track.

Communications options include hardwired telephone, cellular, copper wire, fiber-optic cable, wireless, radio, microwave, or satellite. Important factors in the selection of the communication method and equipment are how much data are included in each observation (bandwidth) and the frequency of transmittal of observations. For sites with low bandwidth requirements (that is, no video camera or infrequent reporting), telephone lines or some type of wireless communication may be more economical than hardwired options. For high data volumes, a hardwired system (wire or fiber optic) appears more appropriate.

For historical polling (data retrieval), road weather data are stored in the remote processing unit (RPU) and retrieved at set times, such as the top of the hour and every 15 minutes thereafter. This process differs from polling the RPU to obtain only the current road weather observations.

Regarding aesthetic considerations, following the criteria related to maintaining adequate distances from obstructions can result in a sensor tower that is very obvious. Pre-siting discussions with the surrounding stakeholders may forestall any aesthetic problems.

Siting too close or too far from the roadway may seriously complicate maintenance procedures or unnecessarily jeopardize the safety of maintenance personnel. Likewise, extra security measures may be needed in areas where the threat of vandalism is present. Possibilities include a security fence around the tower, anti-climb panels, or even security cameras.

Over the years, periodic reevaluation is needed to ensure that the data from the site are still valid and that the metadata are still correct. Construction projects and vegetation growth may change the representativeness and usefulness of ESS locations. This reevaluation can be part of an annual preventive maintenance program for sensor calibration.

"One key point is that these are precision instruments and thus need to be looked after and cared for," says UDOT's Patterson. "A lesson learned for us is to make sure that your agency's infrastructure [servers, communication backbone, technical support, financial support] keeps up or preferably slightly ahead of your field device deployment. Have a deployment plan but understand the need to be flexible, because keeping abreast of advances in technology, especially communications, continues to be a moving target."

Metadata about the ESS should include the platform owner, station name and identifier, station coordinates and elevation, sensor types and manufacturers, location of the sensors, sampling interval, reporting frequency, and the history of any changes in the metadata. The final chapter in the guidelines includes a comprehensive table with a recommended ESS metadata set.

At the end of the publication, an appendix includes a list of metadata references, and another contains an exhaustive list of weather conditions for a DOT's consideration during analysis of requirements for road weather observations. A final appendix consists of a detailed checklist that serves as a synopsis of the siting criteria.

This sensor tower, protected by a security fence, is located in Utah on I-15 near Alpine.

The recommendations in the siting guidelines are designed to satisfy as many road weather monitoring, detecting, and prediction requirements as possible. Weather conditions and their consequences affect road operations and the safety, economic value, and efficiency of transportation and road maintenance activities.

"The guidelines give agencies a place to start--a baseline, if you will," says UDOT's Patterson. "An RWIS-ESS can be and is a very useful tool in myriad disciplines."

Roemer M. Alfelor is a transportation specialist with the FHWA Office of Operations Road Weather Management Program. Alfelor has been with FHWA for 5 years, having worked in the Office of Infrastructure before joining the Office of Operations in 2004. Prior to that, he held various positions at a number of transportation consulting firms for 9 years. He holds a master of science degree in transportation from the Massachusetts Institute of Technology and a Ph.D. in civil engineering from Carnegie Mellon University.

For more information, contact Roemer Alfelor at roemer.alfelor@fhwa.dot.gov or 202-366-9242.