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Accuracy of Traffic Load Monitoring

Goal
Evaluate Alternative Procedures for Estimating Average ESALs/Vehicle at a Given Site

Data
California WIM data for 2000
99 sites
Used data from
19 sites with 12 months of data
36 sites with 10 or 11 months of data
55 sites in all

California
Weigh Stations
by Region

Average ESALs Per Vehicle by Functional Class (Site Average Approach)
Rural Interstate 0.123 0.426 1.090
Rural Other 0.118 0.377 0.958
Urban Interstate 0.147 0.493 0.741
All Sites 0.139 0.470 0.842

Averaging
“All vehicles approach” – simple average of ESALs for all vehicles (in the state, FC, or region)
“Site average approach” –
Get separate averages for each site
Get average of these “site averages”

Average ESALs Per Vehicle by Functional Class (Site Average Approach)
Rural Interstate 0.123 0.426 1.090
Rural Other 0.118 0.377 0.958
Urban Interstate 0.147 0.493 0.741
All Sites 0.139 0.470 0.842
All Sites 0.145 0.464 0.900
(all vehicles appr.)

Average ESALs Per Vehicle by Region
(Site Average Approach)

Using Default Values
Site Average Approach
Statewide Avgs 18.2% 31.4% 25.4%
Functional Class Avgs 17.5% 32.2% 21.6%
Regional Avgs 17.9% 28.0% 21.9%
All Vehicles Approach
Statewide Avgs 20.2% 30.9% 28.7%
Functional Class Avgs 18.4% 30.7% 21.3%
Regional Avgs 19.5% 26.5% 23.6%

Using Site Specific Data (48 Hours)
Unfactored 8.5% 13.2% 8.0%
Factored Using Site Average Approach
Statewide Factors 7.4% 12.9% 7.3%
Functional Class Factors 7.5% 13.8% 7.3%
Regional Factors 7.5% 13.7% 7.5%
Factored Using All Vehicles Approach
Statewide Factors 7.6% 13.0% 7.1%
Functional Class Factors 7.8% 13.8% 7.3%
Regional Factors 7.6% 13.9% 7.4%

Day of Week Load Variation

Seasonal Load Variation

Principal Conclusion

Potential Future Analyses
Use of site-specific data collected over eight hours, 24 hours, or seven days
Effect of lower truck volumes on accuracy of site-specific approach
Use statewide monthly factors to input values for missing months; then repeat analyses using a larger set of sites
Test AASHTO three-step averaging procedure
Evaluate alternative regional and/or functional-class groupings
Explore alternative factoring procedures