The quality of loop data and the health of California ’ s freeway loop detectors

a) Percentage of missing samples—the fraction of samples that should have been received from field equipment but were not. The vast majority of missing samples are due to loops that reported no samples for the entire month. Missing samples are caused by various factors, analyzed in this report. b) Percentage of bad samples—the fraction of samples that were received but which statistical tests classify as unreliable. Bad samples indicate detector loop malfunction. With the exception of District 4, only a small fraction of received samples are bad. c) Percentage of good samples—the percentage of samples that should have been received and were good. This is the most important indicator of health. A healthy system should indicate percentages above 90. By this standard, California has no healthy district. The percentage of good samples varies: D3—72%, D4—24%, D7—62%, D8—41%, D11—78%, D12—51%.