Road Weather Information System
Environmental Sensor Station
Siting Guidelines
April 2005

Appendix C - Road Weather Observation Requirements

Table C1 contains a list of weather elements for DOT consideration during the analysis of road weather requirements. This list was adapted from Appendices B-1, B-1.1, and B-5 in the report on surface transportation national needs by the Office of the Federal Coordinator for Meteorology - Services and Supporting Research.²⁵ A weather element is a particular weather condition or a consequence of weather conditions affecting road operations or the safety, economic value, or efficiency of transportation or road maintenance activities. Table C1 also indicates the thresholds at which the weather element affects road operations and maintenance activities. In some cases any occurrence of a weather element has impact and may require action. For other elements, the thresholds are variable based on the specific needs of users.

The shaded weather elements in Table C1 deserve primary attention during the requirements analysis and the development of ESS plans. Not all the highlighted weather elements can be measured by an automated ESS; however, they all provide insight into important ESS deployment considerations. For example, an ESS cannot directly measure roadway ice accumulation in inches. If roadway ice accumulation is an important consideration, other sensors discussed in Section 2.1 should be selected that will indicate icing conditions. Identifying the highlighted elements for primary consideration is only a recommendation. The actual elements considered when planning an ESS installation must reflect the DOT's operational requirements for road weather information.

Some weather elements in Table C1 can be determined using the sensors or a combination of ESS sensors. For example:

• Winds associated with blizzards are observations of wind speed and gusts under these high wind conditions.
• Wind chill temperatures are numerically derived from air temperature and wind speeds.
• Heat index is numerically derived from air temperature and relative humidity.
• Air temperature change rate is derived from air temperature.
• Wet bulb temperatures can be derived from dry bulb and dewpoint temperatures.
• Relative humidity can be derived from air temperature and either dewpoint or wet bulb temperatures.

This appendix presents two additional analyses of road weather observation requirements. These analyses supplement the data presented above and are included to help state DOTs evaluate their needs for road weather observations. The first analysis, based on a Strategic Highway Research Program (SHRP) RWIS implementation guide,²⁶ identifies the road weather requirements of highway agencies, as well as those of meteorological organizations and the traveling public. The results are presented in Table C2. A "Y" indicates the users identified a requirement for the associated observation. Requirements are presented for both snow and ice control activities and for other road maintenance activities.

Table C3 presents another list of road weather observation elements adapted from an Ohio University study.²⁷ These data are based on responses from users and administrators from three state DOTs and a European country. Study participants were asked to evaluate the importance of sensors by comparing them in pairs using a scale of 1 to 10, such that the sum of the rankings of the two sensors totaled 10. If two sensors were equally important, they would each be given a score of 5. The evaluator might give a very useful sensor a score of 9 and a sensor not useful to that evaluator a score of 1.

The inputs for all comparisons were used to compute the relative importance of the types of measurements. The results shown in Table C3 provide a prioritized list of the sensor measurements, with the precipitation type considered the most important and wind gusts the least important. While this table may help state DOTs prioritize their sensor requirements, it should be noted that the results were based on a single study with a limited sample size. Table C3 should be considered just another tool in evaluating sensor requirements. The final decision for sensor selections should reflect DOT operational requirements for road weather information.

²⁵ Office of the Federal Coordinator for Meteorology - Services and Supporting Research. Weather Information for Surface Transportation, National Needs Assessment Report. FCM-R18-2002. December 2002.

²⁶ Boselly, 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.

²⁷ Zwahlen, H.T., et al. Evaluation of ODOT Roadway/Weather Sensor Systems for Snow and Ice Removal Operations Part I: RWIS. Human Factors and Ergonomics Laboratory, Ohio University, June 2003.