1.0 Introduction

1.1 Scope

This document provides guidelines for siting a Road Weather Information System (RWIS) Environmental Sensor Station (ESS) and its associated environmental sensors. The term RWIS has a number of diverse definitions ranging from sensing and processing devices in the field to a composite of all weather and pavement information resources available to highway operations and maintenance personnel. In this document RWIS connotes the hardware, software programs, and communications interfaces necessary to collect and transfer field observations to a display device at the user’s location. While the original purpose of the RWIS was to address winter weather conditions, applications have been developed to detect and monitor a variety of road weather conditions impacting road operations and maintenance. As Figure 1 illustrates, the RWIS collects, transmits, processes, and disseminates weather and road condition information. The RWIS may consist of several meteorological and pavement condition monitoring stations strategically located near a highway that help transportation managers make more informed operational decisions. Specialized equipment and computer programs monitor weather and pavement condition elements that help users observe how adverse weather is currently affecting the highways and assess future impacts. For example, winter road maintenance managers may benefit from such a system during winter storms by making optimal use of materials and staff, selecting appropriate treatment strategies, utilizing anti-icing techniques, and properly timing maintenance activities. Traffic managers may use road weather observations to modify traffic signal timing, reduce speed limits, and close hazardous roads and bridges.

Figure 1. Road Weather Information System (RWIS) Functions

This document focuses primarily on the ESS, the “collection” component of the RWIS shown in Figure 1. These guidelines are intended to help establish uniformity in siting ESSs and to improve the usefulness of road weather information derived from ESS observations. Future revisions to this document may be necessary as environmental sensor technology evolves and research on the characteristics of the roadway environment is completed.

The document is designed to provide siting criteria that satisfy as many road weather monitoring, detection, and prediction requirements as possible. The criteria are based on an analysis of published documents on the siting of weather and pavement sensors, and the results of interviews conducted with nearly two dozen road weather experts representing state Departments of Transportation (DOT), equipment suppliers, and consultants. The individuals interviewed are acknowledged in Section 7 of this document. Roadway and transportation professionals, transportation agencies, researchers, ESS vendors, and meteorologists supporting the transportation community will be able to use these guidelines to aid in siting ESS equipment. The guidelines contained in this document do not represent standards for agencies or vendors to follow, but instead offer a set of recommendations.  A set of RFP’s and specifications used by various State DOTs for ESS procurement and maintenance are included on the Aurora program website:

http://www.aurora-program.org/matrix.cfm/matrix/survey/matrix/survey/matrix/survey/installation.pdf

It should be noted that FHWA does not endorse or recommend any of the approaches taken in these RFP’s but they do provide a useful resources for agencies wishing to procure these services.

Section 1 of this document serves as an introduction, providing the scope and purpose and defining the concept of the ESS as it applies to the guidelines that follow. This section also highlights some of the key benefits of employing RWIS ESSs as part of an Intelligent Transportation System (ITS). Section 2 establishes the ground work for deploying an ESS by helping the DOT planning team assess road weather information requirements. To help decide what sensors to deploy as part of the ESS, Section 2, supplemented by Appendix C, provides an overview of the road weather information elements that may be of interest to road operations and maintenance personnel and how these elements can be measured or detected. Additionally, Section 2 differentiates between regional and local ESS sites and discusses the siting criteria of each. Section 2 also encourages the formation of partnerships to share weather observing and road weather resources. These partnerships offer an opportunity to reduce the number of required ESSs and the cost of gathering road weather information. Section 3 builds on the previous section to recommend guidelines for selecting the ESS site and suggests tools to help make siting decisions. Section 4 provides recommendations for siting the ESS tower and individual sensors. Section 5 addresses some additional considerations to include power, communications, safety, security, and ESS metadata. Appendix A contains Acronyms and Definitions while Appendix B contains a list of References in the document. For easy reference, a checklist based on the recommendations in this document is included in Appendix D.

1.2 Purpose

This document provides a set of guidelines to encourage uniform siting criteria designed to help improve the accuracy and usefulness of road weather observation data. There are several compelling reasons for uniform siting guidelines:

• Provide agencies intending to procure additional RWIS equipment with the appropriate information to select and install ESS equipment and instrumentation in order to maximize the return on investment for both its internal user group and external users.
• Help ensure that the investment in RWIS equipment is not compromised by collection of data that does not adequately support the specified purpose of the observing site.
• Foster a better understanding of the effects of the environment on the acquisition of road and weather data, so procuring agencies can better determine whether potential sites are appropriate locations and will remain so for a number of years.
• Improve the comparison and integration of road weather information with other meteorological data. This integration can significantly expand the coverage of useful information for both roadway applications and other weather data uses. Sharing data will enhance both the road weather and general weather observation networks.

While there are many previously established guidelines for siting weather observing equipment (World Meteorological Organization: Guide to Meteorological Instruments and Methods of Observation, Sixth Edition, WMO-No. 8, 1996, Office of the Federal Coordinator for Meteorology - Services and Supporting Research. Federal Standard for Siting Meteorological Sensors at Airports. FCM-S4-1994 August 1994, US Environmental Protection Agency. Meteorological Monitoring Guidance for Regulatory Modelling Applications. EPA-454/R-99-005 February 2000, US Department of Interior, National Wildfire Coordinating Group.  National Fire Danger Rating System Weather Station Standards. March 2003) there is limited published siting information (Boselly, S.E., J.E. Thornes, and C. Ulburg. Road Weather Information Systems Volume 1, Research Report.Strategic Highway Research Program Publication - SHRP-H-350, National Research Council, Washington D.C., 1993, , S.E., and D.D. Ernst.  Road Weather Information Systems Volume 2, Implementation Guide.Strategic Highway Research Program Publication - SHRP-H-351, National Research Council, Washington D.C., 1993) specifically for the roadway environment. The guidelines in this document are designed to fill that gap and to improve the usefulness of road weather information specifically for the surface transportation community.

1.3 How to Use this Guide

This Guide and the guidelines contained in it may be useful throughout the life cycle of an ESS. In practice, transportation agencies have used the Guide in two primary modes: as a general reference for ESS information within or in parallel with their own guidelines, and more specifically as a basis for the planning and deployment of ESSs. An ESS is an ITS component recognized within the National ITS Architecture and regional architectures. The systems engineering processes used to plan, develop and deploy ITSs applies to ESSs as well. In that context, the major sections of this Guide align well with the major phases of an ITS deployment.

1. Section 1, “Introduction,” provides the basic elements of a Concept of Operations.
2. Section 2, “Assessing Road Weather Informational Requirements,” describes the major classes of requirements to be considered in a Requirements Specification.
3. Section 3, “Site Selection,” provides a basis for design considerations that might appear in a System Architecture or System Design Description.
4. Section 4, “Recommended Siting Criteria,” provides a more detailed basis for ESS tower and sensor design needed in a Design Description and associated plan sheets.
5. Section 5, “Other Considerations,” covers topics related to the power, communication, information, and operational interfaces to the ESS. Aspects of these interfaces may be important throughout the ESS life cycle.

1.4 ESS Description

An ESS consists of one or more sensors measuring atmospheric, pavement, soil, and/or water level conditions. ESSs can be installed in situ within or along a roadway, or on a vehicle. This document only addresses stationary, in situ sensors.

Figure 2. Road Weather Information System (RWIS) Environmental Sensor Station (ESS)

Source: Florida DOT.

Figure 2 is an example of an ESS with multiple sensors located on Route 528, the Beeline Expressway, near Cape Canaveral, Florida.

Figure 3 shows the ESS sensor categories that provide information to identify roadway weather conditions of interest. ESS data are collected in a remote processing unit (RPU) and transmitted to a central processor. Resulting road weather information is used to activate automated warning systems and provide decision support to managers in traffic management centers, road maintenance facilities, and emergency operations centers. By monitoring road conditions using pavement sensors or video, operations and maintenance personnel are able to assess how well their traffic management or winter maintenance strategies are performing, or to determine what additional actions are required. Conditions of interest include pavement condition (e.g., wet, snowy, icy, flooded, plowed), pavement chemical concentration or pavement freeze-point temperature, pavement temperature, soil (sub-surface) temperature, air temperature, wind speed and direction, precipitation (e.g., amount, occurrence, type), humidity, atmospheric pressure, radiation (solar and terrestrial), and visibility. Atmospheric sensors are located above the roadway level and can be used to identify conditions such as strong cross winds or in combination with pavement/subpavement sensors to identify conditions such as icy roads. Some ESSs include water level sensors that are deployed in flood prone areas and on coastal roadways. While not commonly included as part of an RWIS ESS, auxiliary sensors, such as lake webcams and riverbed scouring sensors, can provide opportunities to add sensors that can help monitor and detect events resulting from water-related conditions.

Figure 3. Environmental Sensor Station (ESS) Categories

1.5 Additional Benefits

ESS data provide many benefits, in addition to improving road safety, mobility, and productivity, by supplying information on roadway conditions essential for traffic operations, traveler information, road maintenance, and emergency response. Figure 4 identifies several additional operational applications (Goodwin, L., Best Practices for Road Weather Management, Version 2.0, prepared by Mitretek Systems for the FHWA Road Weather Management Program, May 2003). Benefits derived from these applications include:

• Weather service providers for surface transportation customers use ESS data to develop tailored road weather products (e.g., pavement temperature forecasts).
• Government and university mesonets can include these data to support the development of weather and road weather forecast models.
• National Weather Service (NWS), military, and private weather service providers use these data to develop weather products, short-range forecasts, and forecast verification, and as input to locally run weather forecast models.
• State climatologists can use ESS data for long-term records and climatological analyses.
• Insurance companies can use these data to help determine risks of potential impacts from future weather events.
• Local, state, or federal disaster assessment and response agencies (e.g., Federal Emergency Management Agency and the Department of Homeland Security) may use these data to manage emergencies and related response actions.
• Forensic meteorologists can use ESS data to better understand and reconstruct roadway crashes.
• RWIS ESS data can also be leveraged to support rail, pipeline, and marine operations when such operations are adjacent to or reasonably near the ESS.

Figure 4. Environmental Sensor Station (ESS) Operational Applications

To maximize these benefits, an attempt should be made during the planning process for siting RWIS ESSs to contact other organizations involved in similar data collection that may help both local transportation agencies and other customers (e.g., NWS; Federal Aviation Administration (FAA); U. S. Forest Service (USFS); local TV stations; universities and high schools; and, other city, county, and state agencies). Section 2.3 discusses the potential for establishing information partnerships and/or leveraging the data collected by other organizations. The Siting Checklist in Appendix D provides a reminder to the siting team to consider information partnerships during the siting process.

1.6  Modifications in Version 2.0 of the ESS Siting Guidelines

FHWA sponsored a review of the ESS Guidelines that was conducted in 2007-2008. The project included interviews with a number of State DOT RWIS managers and more detailed reviews with three State DOTs in Idaho, Michigan and New Hampshire. All three states had either recently deployed new ESS or were in the process of doing so. The results of this review are documented in a separate report, “Implementation and Evaluation of RWIS ESS Siting Guide, prepared for FHWA by Cambridge Systematics and Mixon-Hill, November, 2008.”
The report documents a number of issues related to ESS Siting and deployment, but the number of changes recommended for the Guide itself were limited. An important objective of FHWA is to keep the Guide a manageable length so it is used in practical by implementing agencies. The following specific changes have been made to the document:

• In Section 2.2, Regional and Local Site Requirements, language was added to discuss co-location of other traffic management technologies, such as Dynamic Message Signs with ESS.
• In Section 2.3, Leveraging Information Partnerships, language was added discussing the Clarus project, and how it provides an opportunity to leverage information from multiple RWIS.
• In Section 3.0, Site Selection, a specific example was added to illustrate the tradeoffs between meterological requirements and practical siting considerations such as availability of power and communications.
• In Section 4.1, Observation Tower, a reference was added to the U.S. Environmental Protection Agency’s Stormwater Guide.
• In Section, 4.2, Sensor Location, a note is added that manufacturer’s recommendations regarding installation and calibration should be followed.
• In Section 5.4, Aesthetics, Maintenance, Safety and Security, a note was added regarding maintenance of clear zones and discussion was added regarding the importance of maintainability. Discussion was also added regarding geotechnical issues and stormwater runoff.
• The most significant change in the document was made in Section 5.6, Siting Metadata. Metadata recommendations were modified based on work as part of the Clarus project. Because FHWA’s goal is to have all State DOT ESS reporting to Clarus the metadata recommendations should be similar. However, since the Clarus recommendations are extensive, data are divided into Critical (Table 2) and Options (Table 3) categories. Cost considerations may limit the number of data elements that can be collected and reported.