Traffic Data For Pavement 2012 FHWA Highway Information Seminar, Arlington, VA Tianjia Tang, PE, Ph.D. Chief, Travel Monitoring and Surveys Division 421

Objective To enhance the knowledge of the traffic data collection and processing professionals on the needs of traffic data for pavement design 422

Pavement Basics Surface Base Sub-base Sub-grade 423

Stress and Strain tension compression 424

Pavement Basics Pavement is designed and constructed to meet future needs. Past Future Past Present Future 425

Pavement Design A set of plans covering types of materials to be used, depths of various layers, treatments of various layers, and ways to construct various layers to meet a set of performance goals (e.g., service life…) meaning to withstand cumulative deteriorations occurred during the entire design life (e.g., 25 years) 426

Traffic Data For Existing Roadways Past and present traffic data are obtained from field observations through collection. Future traffic data are obtained through traffic demand modeling. Past and present data are used for trend analysis and calibrate and validate traffic demand models. 427

Traffic Data For New Roadways Past and present traffic data do not exist. Future traffic data are obtained through traffic demand modeling. Past and present data are used for trend analysis and calibrate and validate traffic demand models. 428

Concept of ESAL 429

Equivalent Single Axle Load (ESAL) One ESAL is known to cause a quantifiable and standardized amount of damage to the pavement structure equivalent to one pass of a single 18,000-pound, dual-tire axle with all four tires inflated to 110 psi. 4210

Compute Number of ESAL AASHTO Guide for Design of Pavement of Structures The ESAL method is the so called “empirical” method. 4211

Compute ESAL W 18 = D D x D L X ẇ 18 Where W 18 is the ESAL for the design lane. W 18 represents the ultimate traffic data need for pavement design D D is the traffic directional factor for a two way roadway. D L is a lane traffic splitting factor for a roadway having more than one lane in each direction ẇ 18 is cumulative two way ESAL projected for a roadway segment. 4212

Compute ẇ 18 Information needed: 1.The number of axles for various axle configurations under various axle loads from Traffic Demand Modeling 2.Axle load equivalence factors - to be obtained from AASHTO’s Guide Appendix D 4213

Compute ẇ

Compute ẇ

Compute ẇ

Critical Traffic Data 4217

Mechanistic-Empirical Method 4218

Traffic Data Needed for Input 1: Opening Year Two Way Annual Average Daily Truck Traffic 4219

Traffic Data Needed for Input 2: Percent of Truck Traffic in Design Direction % 4220

Traffic Data Needed for Input 3: Percent of Truck Traffic in Design Lane % 4221

Traffic Data Needed for Input 4: Truck Monthly Adjustment Factor The Truck Monthly Adjustment Factor (MAF) reflects truck travel patterns throughout the year 4222

Traffic Data Needed for Input MAF 4223

Traffic Data Needed for Input MAF 4224

Traffic Data Needed for Input -MAF MAF for month i and class j truck 4225

Traffic Data Needed for Input 5: Vehicle Class Distribution Vehicle class distribution (VCD) refers to AADTT distribution among the 10 vehicle types 4226

Traffic Data Needed for Input 6: Truck Hourly Distribution Factor (THDF) Truck hourly distribution factor refers to the percentage of hourly AADTT among a 24 hour time period starting at midnight. There are 24 THDFs 4227

Traffic Data Needed for Input 4228

Traffic Data Needed for Input Axle Load Factor FHWA vehicles in class 4 to 13 can have a variety of axle configurations including single axle, tandem axle, tridem axle, and quad axle 4229

Traffic Data Needed for Input Single Axle Configuration Axle Load Factor There are 39 axle weight groups for single axle configuration vehicles. The axle weight group ranges from 3,000 lbs to 41,000 lbs with increments of 1,000 lbs. The computation of the ALF data is based on MAADT for a particular vehicle class and axle weight group 4230

Traffic Data Needed for Input Tandem, Tridem, and Quad Axle Vehicles For tandem axle vehicles, the axle weight group starts at 6,000 lbs and ends at 82,000 lbs with increment s of 2000 lbs. For both tridem and quad axle vehicles, the axle weight group start at 12,000 lbs and ends at 102,000 lbs with increments of 3000 lbs. 4231

Traffic Data Needed for Input – Single Axle 4232

Traffic Data Needed for Input 7: Number of Axle Types per Truck Class The number of axles per vehicle class for a given axle configuration is an annual average number of axles per vehicle category (per vehicle class and vehicle axle configuration). 4233

Traffic Data Needed for Input 4234

Traffic Data Needed for Input 8: Axle Spacing Axle spacing data are only applicable to tandem, tridem and quad vehicles. It is the distance between two consecutive tandem, tridem, and quad axles. 4235

Traffic Data Needed for Input 9: Average Axle Width: The distance between the two outside edges of an axle is defined as axle width 4236

Traffic Data Needed for Input 10: Wheelbase The distance between the steering and the first device axle of a tractor or a heavy single unit 4237

Traffic Data Needed for Input 4238

Traffic Data Needed for Input 4239

Summary Time frame Pavement layers ESAL Mechanistic and Empirical 4240

Q/A 4241

Thanks you! Traffic Data For Pavement 2012 FHWA Highway Information Seminar, Arlington, VA Tianjia Tang, PE, Ph.D. Chief, Travel Monitoring and Surveys Division 42