Guide to Urban Traffic Volume Counting September 1981 This report presents methods by which urbanized areas can develop and implement integrated traffic counting programs to serve the volume data needs of all agencies. Methods for estimating volume at a single location, volume across a cordonline or cutline, VMT within a corridor or other small area, and regional VMT are presented. Sound statistical sampling concepts permit the collection of volume data at predetermined levels of precision and in a cost ‐ effective manner.
Acronym Quiz VMT Vehicle Miles of Travel ADT Average Daily Traffic AADT Annual Average Daily Traffic AAWDT Annual Average Weekday Traffic AADTT Annual Average Daily Truck Traffic AWB Average White Band ATR Automatic Traffic Recorder HPMS Highway Performance Monitoring System AAWET Annual Average Weekend Traffic PTR Portable Traffic Recorder SADT Seasonal Average Daily Traffic TMG Traffic Monitoring Guide
Reasons for Estimating VMT Air Quality Model Validation / Calibration Performance Measures Demand on Highway System Crash Data ‐ Rate Evaluate Active Transportation VMT per Capita
Development of an Integrated Traffic Counting Program Consideration of Five Tasks Define program objectives Select data collection locations for regional estimates Select data collection locations for focused studies Develop a data collection plan Identify supplementary study needs
Define Program Objectives Determine the type of volume counts required Intersection Midblock volume/classification Cordonline/Screenline counts Peak Period – Weekday ‐ Weekend Define the sample population Metropolitan Planning Area Urban Area Subarea(s)
Define Program Objectives Determine the desired level of precision Confidence Interval/Confidence Limits (tolerance) Acceptable difference between the obtained/estimated measure and the true value Confidence Level/Confidence Coefficient Probability the sample estimate will fall within this range
Select Data Collection Locations for Regional Estimate Three Approaches for Obtaining a Regional Sample Rotational Count every location at some minimal time interval Randomly select locations to yield an unbiased VMT estimate Three year rotation, randomly select 1/3 of locations for each year New Sample Every Year Same Sample Every Year
Program Objective Trade ‐ Offs Higher precision levels ‐ larger sample ‐ higher cost Value of estimating VMT for local roads Investigate integration of multiple objectives A rotational sampling process for VMT estimate can be integrated into an existing coverage count program Cordonline/Screenline counts Subarea analyses Other special counts
NIRCC desired Level of Confidence and Confidence Intervals Level of Confidence ‐ 95% Confidence Intervals Freeways/Interstates ± 10% Expressways ± 10% Arterials ± 10% Collectors ± 15% Locals ± 25%
Define Sample ‐ Stratification Two Types of Stratification Reporting Strata Roadway Classification Sample Strata Volume Groups
Define Sample ‐ Stratification Two Types of Stratification Reporting Strata Interstates/Freeways Expressways Arterials Collectors Local Streets Sample Strata Volume Groups
Arterial Example Sample Link Average Multi ‐ Axle Stratum Mileage Population Volume VMT Vehicles 0-4,999 40 80 2,500 100,000 2% 5,000-9,999 70 140 7,500 525,000 2% 10,000-14,999 50 100 12,500 625,000 2.5% 15,000-19,999 40 80 17,500 700,000 3% 20,000-24,999 30 60 22,500 675,000 5% 25,000-29,999 20 40 27,500 550,000 7% 30,000-39,999 15 30 35,000 525,000 10% 40,000-49,999 10 20 45,000 450,000 10% 50,000+ 10 20 55,000 550,000 10%
Estimating Sample Size for Regional VMT Define Sample Strata Reporting Strata Sample Strata Compute Regional VMT estimate Known Mileage for Sample Stratum “h” Average Volume for Sample Stratum “h” VMTh = Mh x VOLh
Estimating Sample Size for Regional VMT Estimate Sample Parameters ‐ Uncertainty Spatial Uncertainty ‐ variability of volume between locations Standard deviation of volume across locations Temporal Uncertainty ‐ variability of volume over time Standard deviation of volume across days within a season Standard deviation of volume across seasons of the year Standard error of seasonal adjustment factors Measurement Uncertainty ‐ variability due to traffic counter error Standard error of correction factors (axle)
Sample Size Recap Sample Size Dependent on Desired level of precision Acceptable difference between VMT estimate and true value (±) Stratification and Stratum Variation Sample Parameters ‐ error and introduced variation
SEASONAL ADJUSTMENT FACTORS Report Stratum Period of Data Fse SADJe SEADJe June, September – Freeway 1.0105 0.0249 0.0124 December Expressway March – November 0.9816 0.0185 0.0093 Arterial March – December 0.9816 0.0185 0.0093 Collector March – December 0.9735 0.0127 0.0064 Local March – December 0.9735 0.0127 0.0064 Fse – Seasonal Adjustment Factor for Sample Stratum “e” SADJe ‐ Standard Deviation of the Seasonal Adjustment Factor for Sample Stratum “e” SEADJe – Standard Error of the Seasonal Adjustment Factor Sample Stratum “e”
Interstate Original Sample Volume Volume Number Sample Link Group Range of Links Size Mileage 1 0-19,999 3 3 11.95 2 20,000-29,999 5 2 9.43 3 30,000-39,999 2 2 4.18 Current Sample Volume Volume Number Sample Link Group Range of Links Size Mileage 1 0-19,999 7 7 17.58 2 20,000-39,999 8 8 19.06 3 40,000-59,999 4 4 9.63 4 60,000-79,000 5 5 10.54
Expressway Original Sample Volume Volume Number Sample Link Group Range of Links Size Mileage 1 0-9,999 3 3 3.36 2 10,000-14,999 1 1 0.80 3 15,000-19,999 1 1 0.45 Current Sample Volume Volume Number Sample Link Group Range of Links Size Mileage 1 0-19,999 19 19 12.65 2 20,000-29,999 5 5 3.02 3 30,000-39,999 2 2 0.86 4 40,000-49,000 1 1 0.65
Arterial Original Sample Volume Volume Number Sample Link Group Range of Links Size Mileage 1 0-4,999 294 11 146.48 2 5,000-9,999 344 9 111.53 3 10,000-14,999 197 6 59.06 4 15,000-19,999 137 4 34.68 5 20,000-24,999 53 2 13.67 6 25,000-29,999 55 2 12.72 7 30,000-34,999 11 2 2.87 8 35,000-39,000 7 2 2.08 9 40,000-45,000 5 2 0.81 10 45,000-50,000 1 1 0.54
Arterial Current Sample Volume Volume Number Sample Link Group Range of Links Size Mileage 1 0-4,999 260 138 163.87 2 5,000-9,999 268 126 88.05 3 10,000-14,999 238 128 74.54 4 15,000-19,999 149 73 45.16 5 20,000-24,999 92 46 25.86 6 25,000-29,999 43 28 12.86 7 30,000-34,999 21 12 7.30 8 35,000-39,000 15 9 4.70 9 40,000+ 14 8 3.33
Arterial Comparison of Samples Volume Original Current Original Current Group Links Links Sample Sample 1 294 260 11 138 2 344 268 9 126 3 197 238 6 128 4 137 149 4 73 5 53 92 2 46 6 55 43 2 28 7 11 21 2 12 8 7 15 2 9 9 6 14 3 8
Collector Original Sample Volume Volume Number Sample Link Group Range of Links Size Mileage 1 0-999 179 14 121.0 2 1,000-1,999 86 6 48.61 3 2,000-3,999 115 9 46.75 4 4,000-5,999 39 3 12.81 5 6,000-7,999 29 2 8.11 6 8,000-9,999 9 2 0.61 7 10,000+ 8 2 2.18
Collector Current Sample Volume Volume Number Sample Link Group Range of Links Size Mileage 1 0-999 73 37 68.18 2 1,000-1,999 104 53 48.61 3 2,000-3,999 184 100 60.46 4 4,000-5,999 104 55 35.81 5 6,000-7,999 69 34 23.14 6 8,000-9,999 39 17 14.09 7 10,000+ 22 12 7.68
Collector Comparison of Samples Volume Original Current Original Current Group Links Links Sample Sample 1 179 73 14 37 2 86 104 6 53 3 115 184 9 100 4 39 104 3 55 5 29 69 2 34 6 9 39 2 17 7 8 22 2 12
2015 VMT by Street Classification Level of Confidence ‐ 95% Maximum Difference Average Annual Average Between AAWD Report Stratum Weekday Weekday VMT VMT and True VMT Value 2,172,031 2,114,343 ±108,591 Freeway 280,583 277,248 ± 8,190 Expressway Arterial 4,299,774 4,237,826 ± 85,264 Collector 840,000 816,311 ± 59,944 7,445,728 ± 261,989 Total VMT 1,471,578 1,430,078 ± 334,367 Local 8,875,806 ±596,356 Grand Total VMT
2015 VMT by Street Classification Level of Confidence – 95% Desired Achieved AAWD Confidence Confidence Report Stratum VMT Interval Interval 2,114,343 ±10.0% ±5.14% Freeway 277,248 ±10.0% ± 2.95% Expressway Arterial 4,237,826 ±10.0% ± 2.01% Collector 816,311 ±15.0% ± 7.34% 7,445,728 ± 3.52% Total VMT 1,430,078 ± 25.0% ± 23.38% Local 8,875,806 ±6.72% Grand Total VMT
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