Common use of PRELIMINARY TESTS Clause in Contracts

PRELIMINARY TESTS. Because a dual-loop detector consists of two single loops (M and S), if a dual- loop detector functions well, the volume data collected by the dual-loop system should be consistent with the volumes collected by the two corresponding single loops. To check this consistency, ten dual-loop detectors were randomly chosen from I-5, I-90, I-405, and SR 520 freeways by the author to perform preliminary tests. Dual-loop volume data (T) and corresponding single-volume loop data (M and S) for a 60-minute period were compared to get general descriptive statistics for these dual-loop systems. As can be seen in Table 2, although the volume difference between each pair of individual M and S loop measurement was not significantly large, the volume difference between the single-loop measurements and the corresponding dual-loop measurement was considerable. Whereas in some cases the difference was small (3 percent or less), in many cases the discrepancies were serious (differences of 60 percent or more). Volume data collected by the dual loop systems were consistently lower than the corresponding volumes measured by the corresponding single loops. To check how accurately dual-loop detectors estimate vehicle lengths and distribute them into corresponding categories (bins), the Washington State Transportation Center (TRAC) performed some tests to compare dual-loop data and video data collected in the HOV Evaluation Project (Ishimaru, 2000). The comparisons indicated that the loop-based truck percentage consistently underestimated the truck percentage observed in the HOV study by degrees varying from 1 percent to 100 percent or more.