What is a scan count? How is occupancy calculated?
The data comes from loop detectors embedded in the pavement, each 6 feet square. The loops operate in presence mode (that is, they turn on and stay on as long as a vehicle is over, or occupying, the loop). The field microprocessor (an 8-bit 6808-based machine) checks, or scans, the loop 60 times each second. Every time the scan reports that the loop is occupied, the scan counter is incremented. And, 60 scans-per-second multiplied by 20 seconds-per-period equals 1200. Thus, the scan count divided by 1200 and multiplied by 100 equals the occupancy in percentage. Or, you can just divide scan count by 12 to get percentage directly. (This comes from WSDOT documentation.
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