ITS Standards Impacting the Maintenance Community
Paul Pisano, Team Leader
Federal Highway Administration (FHWA), Road Weather Management Program
Lynette C. Goodwin, Lead Transportation Engineer
Noblis, ITS Division
Blake P. Christie, Principal ITS Standards Engineer
Noblis, ITS Division
Intelligent Transportation Systems (ITS) apply advanced communications technologies to surface transportation networks in order to improve roadway operations and safety. Standards facilitate the integration of various ITS components to achieve the interoperability required for such systems to function consistently between agencies and across the nation. The U.S. Department of Transportation (DOT) ITS Standards Program promotes extensive use of open, non-proprietary standards. ITS standards, which are developed by industry consensus, define how elements of a system operate within the framework of the National ITS Architecture. These standards ensure that equipment and systems purchased from different vendors are capable of exchanging data automatically, consistently, and reliably. The use of standards also promotes competition among system developers, encourages industry growth by minimizing development costs, and results in cost savings over the life of public agency investments.
This paper provides an overview of ITS standards and the adoption process: highlighting the ways in which standards are important to the maintenance community, identifying relevant standards, and describing resources for technical support. The paper focuses primarily on the Environmental Sensor Station (ESS) standard and related standards that facilitate use of ESS data in Road Weather Information Systems (RWIS) and other systems.
OVERVIEW OF ITS STANDARDS
ITS standards allow different technologies and system components to interact and exchange information. A standards-based approach to deployment and integration allows transportation agencies to more easily accommodate future equipment replacements, systems upgrades, and system expansions. With standards-based systems, agencies can also extend the reach and capabilities of their ITS infrastructure. An example of such integration is an ESS that reports current pavement conditions to a Traffic Management Center (TMC), which sends an advisory to a Dynamic Message Sign (DMS). Various standards for different applications (e.g., maintenance management, traffic management, traveler information) address interfaces in the National ITS Architecture. This section outlines the standards adoption process and introduces the standards architecture.
Standards Adoption Process
ITS standards are developed through cooperative agreements between the U.S. DOT ITS Standards Program and the following Standards Development Organizations (SDOs):
- American Association of State Highway and Transportation Officials (AASHTO)
- American Public Transportation Association (APTA)
- American Society for Testing and Materials (ASTM)
- Institute of Electrical and Electronics Engineers (IEEE)
- Institute of Transportation Engineers (ITE)
- National Electrical Manufacturers Association (NEMA)
- Society of Automotive Engineers (SAE)
The four-step process depicted in Figure 1 includes publication, testing, deployment, and adoption. Publication begins with identification of needs and analysis of technical requirements by an SDO. These requirements are documented in a draft standard, which is then sent to ballot. During balloting, working group members review the technical merits of the draft standard and either reject or approve it. Approved standards are published and can be purchased from the lead SDO. Then the standard is tested in early deployments. Testing measures the operation, correctness, and completeness of a standard under realistic operating conditions. Lessons learned from deployment and testing result in revision to the standard. As standards mature, vendors compete to provide a range of equipment with varying functions. When sufficient standards-compliant products are available and the standard is widely deployed, the U.S. DOT considers rulemaking to promote the production of technically and commercially viable ITS standards and equipment.
Figure 1 - Life Cycle of ITS Standards Adoption
As of June 2007, eighty-eight (88) ITS standards were published and eight (8) of these had been approved. Three (3) standards were in ballot, or being voted upon by a working group, and five (5) were under development.
National ITS Architecture
The National ITS Architecture is the overall structure used to plan, configure, and integrate Intelligent Transportation Systems. The architecture helps transportation agencies define system stakeholders and their relationships in order to facilitate integration of systems locally, regionally, and nationally. As shown in Figure 2, the architecture defines subsystems where functions occur, the data flows that connect subsystems to form an integrated system, and the key interfaces for standardization. Figure 2, also known as the "sausage diagram", is a high-level view of the National ITS Architecture that illustrates where Maintenance and Construction Management fits into the overall ITS operational concept.
As seen in the figure, subsystems are grouped into four interface classes: centers, field, vehicles, and travelers. The lines in Figure 2 represent four types of architecture flows or interfaces between subsystems: wireline communications (i.e., fixed-point to fixed-point), wide area wireless communications (i.e., mobile), dedicated short-range communications, and vehicle-to-vehicle communications. These architecture flows indicate where interoperability is needed and where ITS standards are required.
Figure 2 - High-Level View of the National ITS Architecture
THE IMPORTANCE OF ITS STANDARDS TO THE MAINTENANCE COMMUNITY
ITS standards are valuable because they allow maintenance personnel to deploy equipment and integrate systems that operate consistently across technical, jurisdictional, and institutional boundaries. ITS standards can reduce agency costs and increase competition by prompting vendors to provide implementers with more product and service alternatives. Although using standards may increase initial project costs, total life cycle cost (i.e., procurement, maintenance, and expansion costs) will be minimized. Transportation projects employing ITS standards are also eligible for federal funding through the Highway Trust Fund. The benefits of using ITS standards will increase as more interfaces are standardized for the road maintenance community.
By allowing systems to exchange data in a standard format and syntax, ITS standards facilitate interchangeability and interoperability, reducing dependence on proprietary equipment and software. Interchangeability allows devices of the same type, made by different vendors, to be substituted for one another and interact on the same communication channel. For example, an agency could deploy Environmental Sensor Stations from three manufacturers in a statewide RWIS. With ITS standards, maintenance managers could use one central system to access data from all of the Environmental Sensor Stations. Interoperability is the ability of systems or devices to provide information to, and receive information from, other systems or devices such that they effectively operate as a single system. For example, an interoperable system would allow a maintenance garage to send road weather data to a Traffic Management Center (TMC) and receive traffic flow data from the TMC, regardless of the equipment manufacturer, and without multiple data conversion steps.
Applicable standards for road maintenance cover two types of architecture flows: center-to-field (e.g., between a maintenance garage and an ESS) and center-to-center (e.g., between a maintenance garage and a TMC). Currently, there are no ITS standards available for center-to-vehicle architecture flows. Standards for these flows may be developed in the future. There is one standard for vehicle-to-field architecture flows (i.e., Dedicated Short Range Communications or DSRC). However, no maintenance applications have been developed for this interface.
The following standards have been developed under an umbrella effort entitled National Transportation Communication for ITS Protocol, also known as NTCIP. There are three types of NTCIP standards: data standards, message standards, and communication standards. Data standards include data dictionaries and object standards that define data elements, and groups of data elements, for specific device types.
Message standards consist of objects that are grouped into logical messages, which are comprised of data elements. Message Set Standards define logical groupings of data elements for preset messages used for center-to-center communications. Message set standards are used in conjunction with center-to-center communication standards.
Communication standards provide the means (i.e., protocols) for sending or receiving messages. NTCIP use of a given protocol, or set of protocols, is called a profile standard. Protocols describe how messages are encoded for transmission, how they are transmitted (e.g., dial-up telephone lines, fiber optic cables), and how they are decoded by the receiver. Profiles define how to use or combine protocols to deliver a message or object.
Maintenance managers utilize center-to-field standards to configure, control, and collect data from multiple field devices via a central computer. Interfaces between the Maintenance and Construction Management center that performs environmental monitoring and the Roadway subsystem are covered by center-to-field standards. The architecture flows that are the most relevant for the maintenance community include:
- environmental sensors control
- environmental conditions data
- roadway treatment system control
- roadway treatment system status
- roadway information system data
These are the architecture flows that apply to managing Environmental Sensor Stations, observing weather and pavement conditions, monitoring water levels near roadways, controlling and monitoring automated anti-icing systems, and disseminating road weather information via Dynamic Message Signs (DMS). There are several standards associated with these architecture flows, and multiple standards may apply to any one of these flows.
The center-to-field data standards associated with the architecture flows above are:
- NTCIP 1201 - Global Object Definitions
- NTCIP 1203 - Object Definitions for Dynamic Message Signs
- NTCIP 1204 - Environmental Sensor Station Interface Standard
NTCIP 1201 - Global Object Definitions: Global Object Definitions cover data elements that are common to many different types of ITS devices. At an early stage, the NTCIP SDOs decided that, for the purposes of configuration control, it would be best to put common data elements used by multiple equipment standards in a single "global" standard of elements. This allows commonly used elements to be updated across the entire set of center-to-field standards. The SDOs approved the first version of NTCIP 1201 in 1997 and approved an amendment in 2001. A second version was recommended for ballot in 2002 and approved in 2005. The published version was made available in December 2006.
NTCIP 1203 - Object Definitions for Dynamic Message Signs: Data elements for three types of DMS are defined in the Object Definitions for Dynamic Message Signs standard. NTCIP 1203 was initially approved as a recommended standard in 1997 and amended in 2002 based on user comments from deployment and testing. This is the most mature and widely-deployed ITS standard. The first version of the standard has been deployed in several states including Arizona, Georgia, Illinois, Virginia, and Washington State. Version 2 reflects lessons learned from early deployments and adds new features. The second version includes a concept of operations (i.e., user needs), functional requirements, traceability between the needs and the interface specifications, and expanded functionality (e.g., use of graphics). Version 2 was accepted as a recommended standard in March 2007.
NTCIP 1204 - Environmental Sensor Station Interface Standard: The standard that defines how a central computer interfaces with a field device to control and monitor pavement sensors, weather stations, air quality monitors, and other equipment (e.g., anti-icing systems) is entitled Environmental Sensor Station Interface Standard. The first version of NTCIP 1204 was approved and published by SDOs in 1998 and amended in 2001. This standard has been implemented by DOTs in Alaska, Minnesota, Washington State, and Wisconsin. Case studies of Minnesota DOT and Washington State DOT have documented how this standard has been used in their deployments. The Minnesota DOT used NTCIP 1204 to avoid pitfalls associated with proprietary systems and sole-source vendors. Over 70 NTCIP-compatible ESS and numerous legacy ESS were seamlessly integrated into a single RWIS. The Washington State DOT used the standard to facilitate communication between ESS made by one vendor and a central server from another vendor. This allowed the DOT to manage environmental data from a single platform and reduce procurement costs by nearly 50 percent due to increased competition between vendors.
A second version of NTCIP 1204, which incorporated lessons learned from early deployments and testing, was accepted as a recommended standard in March 2005 and approved in March 2006. Version 2 also included a concept of operations (i.e., user needs) and requirements that explicitly traced from needs to specific interface specifications (i.e., data elements and dialogs). Version 3 of NTCIP 1204, which includes detailed test procedures, was released for review and comment in June 2007. During the user comment period, the Florida DOT carried out the test procedures on two independent device implementations: one with NTCIP 1204v1 and the other with partially implemented NTCIP 1204v2 objects.
For the center-to-field architecture flows identified above, Object Definitions standards would be used in conjunction with the Global Objects standard and selected center-to-field communications profile standards.
Because many events can be related (e.g., increased congestion during rainfall, more frequent crashes on icy pavement), sharing information on weather events and mitigation strategies with managers in neighboring jurisdictions can foster institutional coordination. Center-to-center standards facilitate communication between two or more management centers and allow road weather data to be integrated with Advanced Traveler Information Systems (ATIS) and Advanced Traffic Management Systems (ATMS). These standards allow maintenance managers to interface with traffic operations personnel, inform operators of weather-related conditions affecting traffic flow and road safety, and increase the operational efficiency of all center managers.
Interfaces between the Maintenance and Construction Management center and the Traffic Management center are covered by center-to-center standards. These architecture flows, which can also be used to communicate with another Maintenance and Construction Management center or an Information Service Provider center, include:
- road weather information
- roadway maintenance status
These architecture flows are utilized to send weather data, road condition information, and the status of maintenance fleet operations from a maintenance division to other transportation system operators. They can also be used to send information from a Traffic Management Center to a maintenance division to support efficient management of maintenance activities. The center-to-center standards associated with these flows are:
- ITE Traffic Management Data Dictionary (TMDD)
- SAE J2354 - Message Sets for Advanced Traveler Information Systems (ATIS)
- NTCIP 2304 - Application Profile for DATEX ASN
- NTCIP 2306 - Application Profile for XML in ITS Center to Center Communications
ITE Traffic Management Data Dictionary (TMDD): The center-to-center standard that is most important to maintenance managers is the Traffic Management Data Dictionary (TMDD). All management centers that share information can use this standard, regardless of their function. Version 1 of the TMDD standard is organized into four sections that define elements for the traffic network (i.e., links and nodes), events disrupting the network (i.e., events, incidents, and notification alarms), traffic control devices (e.g., signals, detectors, ramp meters), and information gathering or dissemination devices (i.e., ESS, CCTV, DMS, Highway Advisory Radio).
The TMDD data elements and associated message sets are used together to provide interoperability. The message sets combine TMDD data elements into messages for transmission between management centers. The Version 1 TMDD message sets include six message groups: roadway network, network state, network events, traffic request, traffic device status, and traffic control. The message sets have been implemented by the Texas DOT and in over 30 centers in New York, New Jersey, and Connecticut as part of TRANSCOM—a coalition of transportation and public safety agencies. Version 2 of the TMDD was proposed as a provisional standard in January 2005. In September 2006, a workshop was held to capture user needs that served as input to the Concept of Operations for Version 3 of the TMDD. Version 3 incorporated lessons learned by deployers and addressed additional areas of scope, including extending the standard to include data elements and message sets from the Clarus Initiative (www.clarusinitiative.org) and the Archived Data User Service standards effort.
SAE J2354 – Message Sets for Advanced Traveler Information Systems (ATIS): The Message Sets for Advanced Traveler Information Systems (ATIS) standard provides traveler information messages, including road condition messages, in both Abstract Syntax Notation 1 (ASN.1) and eXtensible Mark-up Language (XML) formats. SAE J2354 was originally published in 1999 and revised in 2004. Version 2 is currently under revision. This message set standard can be used to support data dissemination to the public via ATIS, such as agency web sites and 511—the national traveler information telephone number. The messages contained within SAE J2354 address all stages of travel (i.e., informational, pre-trip, en route), all types of travelers, all categories of traveler information, and all delivery platforms (e.g., in-vehicle devices, portable electronic devices, kiosks).
The SAE J2354 standard includes weather information, either by road segment or by larger geographic area, as transmitted to Information Service Provider centers. Maintenance managers can use the standard to disseminate road weather information (e.g., hazardous winter road conditions) to the public via this type of center. The City of San Francisco and the Gary-Chicago-Milwaukee Corridor have deployed the SAE J2354 standard.
The TMDD and SAE J2354 standards have overlapping content with regards to traveler information. Managers should select one of the standards based upon specific operational needs. The TMDD standard is designed for information exchange between centers, while the SAE J2354 standard is intended primarily for disseminating traveler information to end users, usually via an Information Service Provider.
NTCIP 2304 – Application Profile for DATEX ASN: This application profile includes requirements for the traditional approach to communications between two management subsystems (i.e., data encoded in ASN.1). An example of this is a Maintenance and Construction Management center connected to a Traffic Management center. This application profile lists the requirements for data exchange among systems. The first version of NTCIP 2304 was approved in December 2002 and a revised version (v01.08) was published in February 2006.
NTCIP 2306 – Application Profile for XML in ITS Center to Center Communications: The Application Profile for XML in ITS Center to Center Communications specifies communications interfaces (i.e., message form, message use, and transport) encoded in XML for real-time management of public roads and transit systems. Message content is defined in other standards, such as the TMDD and SAE J2354. NTCIP 2306v01.68 was accepted as a recommended standard in March 2006. It is expected that Version 1 will be approved and published in 2007.
In the future, center-to-vehicle standards will allow maintenance managers to monitor mobile environmental sensors and track the location and operational status (e.g., plow up/down, chemical application rate) of maintenance vehicles that use different technologies. These standards will allow managers to communicate with a single vehicle, a group of vehicles, or all vehicles, independent of the brand of equipment installed, as they travel along their designated routes. The U.S. DOT's Vehicle Infrastructure Integration (VII) Initiative has the potential to enhance center-to-vehicle communications.
Architecture flows between the Maintenance and Construction Management center and Maintenance and Construction Vehicles would be covered by center-to-vehicle standards. These architecture flows, which are candidates for future standardization, include:
- maintenance and construction dispatch information
- maintenance and construction vehicle location data
- maintenance and construction vehicle operational data
- environmental probe data
ITS STANDARDS RESOURCES
Deploying standards can seem overwhelming at first. For this reason, a host of resources have been developed to assist maintenance personnel in implementing ESS and other ITS standards. As a start, the FHWA Road Weather Management Program created a brochure that describes communications standards, siting standards, and calibration standards for RWIS applications. This resource can be accessed on the Road Weather Management Program web site at www.ops.fhwa.dot.gov/weather/publications/RWIS_brochure.pdf. Many other resources provide more detailed assistance, as described below.
An ITS Standards Advisory on ESS highlights standards development, testing, and deployment activities. It also includes guidance on working with ESS vendors, information on the ESS Working Group, and a table of Standards Applicable to ESS Deployments. The advisory is available at www.standards.its.dot.gov/Documents/advisories/ess_advisory.pdf. Specifications for RWIS that reference NTCIP standards can be found on the Aurora Program web site, www.aurora-program.org/matrix.cfm. These specifications have been utilized by several state DOTs.
The NTCIP Guide (i.e., NTCIP 9001v03.02) was accepted as a recommended information report in October 2002. The NTCIP Guide was created to assist decision makers, planners, specification writers, and implementers in understanding the various NTCIP standard publications and how to use them. The guide, which is currently under revision, is organized into 10 chapters including an "Executive Summary" for decision makers, "Understanding NTCIP" for system planners, "Procuring NTCIP" for specification writers, "Designing NTCIP" for communications engineers, and "Implementing NTCIP" for hardware and software developers and system integrators. The other chapters discuss applications for center-to-field and center-to-center communications, as well as other issues such as intellectual property rights, legacy and system migration issues, testing and conformity, and configuration management. A copy of the guide can be downloaded from www.ntcip.org/library/guide.asp.
Training and Software
Training and support tools are available for transportation agencies that are planning, deploying, or upgrading ITS. The FHWA and the National ITS Architecture Team (Iteris, Inc. and Lockheed Martin) created Turbo Architecture, an interactive software tool to help system designers with development of a regional or project Architecture to facilitate usage of the National ITS Architecture. For more information on Turbo Architecture, visit http://mctrans.ce.ufl.edu/featured/turbo/.
Transportation agencies can also receive guidance through the Integrating NTCIP Compliant Hardware (INCH) Project that teaches managers how to specify, purchase, deploy, and test NTCIP-conformant hardware. Information on the INCH Project can be found online at www.enterprise.prog.org under "Completed Projects".
The Institute of Transportation Engineers (ITE) offers free one-day courses including an overview of ITS standards and a course on center-to-center standards. The ITS Standards Overview course is intended for transportation professionals and policy makers involved in ITS deployment. The Center to Center course is an introduction to the use of application profile standards to exchange data among ITS centers. This course also includes an introduction to the TMDD and ATIS standards. Additional information can be found at www.ite.org/standards/courseschedule.asp.
The ITS Standards Field Support Team provides intensive consultation to aid state and local transportation agencies with a wide range issues including system assessment, specification development, contract advice and review, test plan design, compliance evaluation, and other technical issues. This team is comprised of FHWA specialists who are prepared to provide short-term, on-call assistance in the area of ITS Standards. The goal of the team is to support and facilitate the deployment of Intelligent Transportation Systems that advance national and local needs while implementing ITS Standards. More information on the Field Support Team can be found at http://ops.fhwa.dot.gov/int_its_deployment/standards_imp/stdsteam.htm. To request support from the ITS Standards Field Support Team, contact your FHWA Division Office or contact Tom Stout, Office of Transportation Management, at email@example.com or 202-366-6054.
Through the ITS Peer-to-Peer Program, deployers also can request assistance from peers, in the public and private sectors, who have successfully overcome the challenges of designing and implementing various types of Intelligent Transportation Systems. Over 120 ITS professionals lend their technical expertise to the Peer-to-Peer Program. Additional Peer-to-Peer Program information can be obtained by calling 1-888-700-PEER, visiting www.its.dot.gov/peer/peer-t.htm, emailing firstname.lastname@example.org, or contacting Tom Stout, Office of Transportation Management, at email@example.com or 202-366-6054.
Nearly 90 ITS standards have been developed by SDOs for use by transportation agencies that deploy and integrate various technologies to create interoperable management systems. ITS standards are organized in the framework of the National ITS Architecture, which groups subsystems into four categories: centers, field, vehicles, and travelers. Architecture flows denote areas where ITS standards are required to achieve interoperability.
Two types of architecture flows are important to the maintenance community: center-to-field and center-to-center. Emerging communications standards will support center-to-vehicle applications as the VII Initiative advances in the future.
The center-to-field standards that are relevant for maintenance activities include:
- NTCIP 1201 - Global Object Definitions NTCIP 1203 - Object Definitions for Dynamic Message Signs NTCIP 1204 - Environmental Sensor Station Interface Standard
- NTCIP Center-to-Field Standards Group
Architecture flows between Maintenance and Construction Management centers and other centers, such as Traffic Management Centers and Information Service Providers, are used to share road weather data and maintenance fleet status information. Center-to-center standards allow transportation managers to interface and coordinate operational responses to inclement weather and other conditions. The center-to-center standards relevant for maintenance activities include:
- ITE Traffic Management Data Dictionary (TMDD)
- SAE J2354 - Message Sets for Advanced Traveler Information Systems (ATIS)
- NTCIP 2304 - Application Profile for DATEX ASN
- NTCIP 2306 - Application Profile for XML in ITS Center to Center Communications
- NTCIP Center-to-Center Standards Group
Numerous resources are available to assist maintenance personnel with implementation of ITS standards. Documents, training courses, software and other technical assistance can be used to guide state DOTs as they deploy and integrate ITS components. Using ITS standards in RWIS applications will ultimately support a host of road weather management solutions that state DOTs are implementing including weather-responsive traffic management applications, Maintenance Decision Support Systems (MDSS) (www.rap.ucar.edu/projects/rdwx_mdss), mobile data communications for maintenance vehicles, disseminating weather-related traveler information, and utilizing the nationwide road weather observing system known as Clarus (www.clarusinitiative.org). These solutions will benefit transportation agencies and the traveling public by improving safety, mobility, and productivity.
- FHWA, "An Introduction to Standards for Road Weather Information Systems (RWIS)," Road Weather Management Program, July 2002, www.ops.fhwa.dot.gov/weather/publications/rwis_brochure.pdf.
- ITE, "Traffic Management Data Dictionary (TMDD) and Message Sets for External Traffic Management Center Communications (MS/ETMCC)," 2007, www.ite.org/tmdd/.
- NEMA, "Minnesota DOT Statewide R/WIS Project," NTCIP 9008 v01.06, January 2003, www.ntcip.org/library/documents/pdf/9008v01-06.pdf.
- NEMA, "NTCIP Documents Web Site," July 2007, www.ntcip.org/library/documents/.
- NEMA, "Washington State DOT Statewide ESS Procurement," NTCIP 9009 v01.05, January 2003, www.ntcip.org/library/documents/pdf/9009v01-05.pdf.
- U.S. DOT, "ITS Standards," National ITS Architecture web site, Version 5.0, November 2003, http://itsarch.iteris.com/itsarch/html/standard/standard_b.htm.
- U.S. DOT, "NTCIP Center-to-Field Standards Groups," National ITS Architecture web site, Version 5.0, November 2003, http://itsarch.iteris.com/itsarch/html/standard/ntcipc2f.htm.
- U.S. DOT, "NTCIP Market Packages," National ITS Architecture web site, Version 5.0, October 2003, http://itsarch.iteris.com/itsarch/html/mp/mpindex.htm.
- U.S. DOT ITS Standards Program, "Environmental Sensor Station (ESS): ITS Standards Advisory," March 2003, www.standards.its.dot.gov/Documents/ess_advisory.pdf.
- U.S. DOT ITS Standards Program,, "ITS Standards: A Brief Stroll through the Different Document Types," February 1999, www.standards.its.dot.gov/Documents/stddef.pdf.
- U.S. DOT ITS Standards Program, "Leading the Way: Profile of an Early ESS Deployer (Minnesota DOT)," FHWA-OP-02-017, www.standards.its.dot.gov/Documents/Early%20Depl%20FLEEGE.pdf.
- U.S. DOT ITS Standards Program, "Leading the Way: Profile of an Early ESS Deployer (Washington State DOT)," FHWA-OP-02-014, www.its-standards.net/Documents/Early%20Depl-%20SENN.pdf.
- U.S. DOT ITS Standards Program, "Life Cycle of ITS Standards from Initial Development to U.S. DOT Adoption," May 2001, www.standards.its.dot.gov/Documents/lifecycle.pdf.
- U.S. DOT ITS Standards Program, "Published Standards Documents," November 2003, www.standards.its.dot.gov/Documents/publish.pdf.
- U.S. DOT ITS Standards Program, "Test Report Synopsis for NTCIP Environmental Sensor Station (ESS) Standards," November 2003, www.standards.its.dot.gov/testESS.htm.
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