In this study; the Texas A&M Transportation Institute (TTI) conducted an independent; follow-up evaluation of StreetLight Data's 2019 traffic count estimates using MnDOT sources of traffic count data. At 442 permanent benchmark locations; TTI found that average annual daily traffic ( AADT) estimation accuracy by StreetLight Data has improved significantly since the 2017 evaluation; especially in moderate- to high-volume categories (i.e.; more than 10;000 AADT). The mean absolute error ranged from 8% to 10% for locations greater than 10;000 AADT and gradually increased to 42% for sites with less than 1;000 AADT. TTI also found significant overestimation bias for low-volume roadways (i.e.; less than 2;500 to 5;000 AADT). This result was present in the permanent benchmark sites and more pronounced in the 265 short-duration count sites. Based on these findings; TTI recommends that MnDOT consider a phased approach to using probe-based traffic count estimates: 1) Continue to maintain MnDOT permanent counter sites; 2) start using probe-based counts for about 90% of the moderate- to high-volume roadways (20;000 or more AADT); 3) continue to use traditional short-duration counts at the remaining 10% of the moderate- to high-volume roadways as a spot check to ensure that probe-based AADT estimates remain within acceptable tolerances in the next five to ten years; 4) periodically monitor the error of AADT estimates on low- to moderate-volume roadways (less than 20;000 AADT); and 5) once acceptable error tolerances for these lower-volume categories are reached; repeat Step 2 for these lower-volume categories.
