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36 system. Therefore, the existence of this high-level weather State DOT representatives interviewed recognize that, data source may be preventing state DOTs from investing when loop detectors and communication equipment in the purchase of detailed weather and road condition data are functioning properly, they offer accurate represen- that are now offered. tations of traffic volume and occupancy and also can be used to calculate speeds. At any given time, however, Based on interviews with weather industry experts, the data may be missing for locations where detectors or following observations and insights were offered: communications are inoperable. The accuracy and quality control of real-time weather Nonintrusive Detectors data sources will continue to improve gradually. Federal initiatives, such as Clarus, will advance the Several approaches to nonintrusive traffic data collection state of the art and increase the availability of weather do not require the installation of data collection equipment data. in the roadway. Some examples of nonintrusive detectors A disconnect exists between the detailed weather include wireless radar, Doppler radar technologies, toll tag forecasts that are available and the information dis- readers, and license plate readers. seminated by most state DOT Internet and 511 phone systems. Nonintrusive traffic data collection offers lower cost Although the weather data sources from observations deployment and operation of data collection, and there- and forecasting models deliver quality data, it is still fore some state DOTs have been able to cover expanded a challenge to present these data to travelers as useful areas for their same budget. As an example, a deploy- information, packaged appropriately for the dissemi- ment of Doppler radarbased devices in North Carolina nation medium. combined with wireless communications and solar power provided coverage of many more miles than the budget would have allowed with loop detectors. Traffic Data Availability A challenge with some radar-based traffic detectors is that some only detect the speed of vehicles and not Traffic data refers to speed and volume or occupancy data or vehicle traffic counts, which are often needed by state to travel-time data. Generally, the two categories of sources DOTs for other purposes. In the North Carolina exam- for traffic data are as follows: ple, only speed is measured and not traffic volumes. Toll tag readers provide accurate readings in location Public traffic data collection and assembly by DOTs where a large number of electronic toll tags are located Private sector traffic data collection on vehicles. However, the inherent delay caused by the approach of tracking a vehicle as it crosses two loca- Public Sector Traffic Data Collection tions often causes a delay in readings and can nega- tively affect the accuracy of real-time data. Intrusive Detectors Private Sector Traffic Data Sources Loop detectors were one of the first approaches used for traf- fic data collection, and they still offer a highly accurate and A number of private sector companies offer traffic data to pub- effective source for traffic data when they function properly lic agencies. The traffic data offered may consist of real-time and are properly placed and located. travel time, speed, or volume data, or historic data to be used by DOTs for planning and mobility assessments. The sources Many metropolitan areas operating ramp meters have of traffic data and algorithms for computing travel times and placed loop detectors at 0.5-mile frequency (some even speeds vary and are specific to each product and provider. every 0.25 mile). A network of closely spaced loop However, based on feedback and input from industry experts, detectors provides a solid source of reporting traffic the private sector data collection typically involves a combi- conditions and travel times. nation of feeds from public agencies and data from some form The largest challenge with loop detectors is the cost of of vehicle-based probes (e.g., long-haul commercial vehicles, deployment and maintenance, especially when consid- local delivery vehicles, or cellular phones). ering the need to close a freeway or arterial to replace or repair a loop detector. A number of state agencies are either testing or using pri- Based on the costs of maintenance, existing loop detec- vate sector traffic data sources, and over the next few years, tors may be left inoperable if broken and state DOTs evaluation reports will summarize the accuracy, costs, and are increasingly less likely to install as dense a network ultimately business models of public agencies that purchase of loop detectors as they have in the past. data from private providers.