October 2005 Focus - LTPP Traffic Data Study Embarks on Phase II

LTPP Traffic Data Study Embarks on Phase II

The Federal Highway Administration's (FHWA) Long-Term Pavement Performance (LTPP) program has em-barked on the second phase of its Specific Pavements Study (SPS) Traffic Data Collection pooled-fund study. The study is designed to fill in gaps and improve the quality and quantity of monitored traffic data from the LTPP program's SPS-1, -2, -5, -6, and -8 projects. To date, nearly half of the 37 States with SPS sites and a few non-SPS States have contributed approximately $3.4 million to the study.

The SPS experiments were designed to learn how such factors as cumulative traffic loading affect pavements of different compositions, environmental conditions, and layer thicknesses. Answering these questions will help States design and build longer lasting, high-volume pavements.

Phase I of the pooled-fund study involved assessing, evaluating, and calibrating the current weigh-in-motion (WIM) systems used to collect traffic data at the SPS sites across the country. During the recently launched Phase II, new WIM equipment will be installed and maintained as necessary at test sites to ensure high-quality data collection. The WIM sensors collect information on such factors as vehicle and axle weights, vehicle classification, and speed. The quality of the WIM data is highly dependent upon the pavement in which the system is installed, as smoother pavements reduce the amount of suspension/pavement dynamics, which allows the WIM system to better estimate static wheel loads.

Smoothness Index Profile

The smoothness of the pavement surface where weigh-in-motion (WIM) equipment is installed is important in achieving high-quality data, as smoother pavements reduce the amount of suspension/pavement dynamics, allowing the WIM system to make better estimates of static wheel loads. A smoothness index profile developed by FHWA's Long-Term Pavement Performance (LTPP) program allows a State to evaluate a candidate WIM site. This index profile is contained in a provisional specification from the American Association of State Highway and Transportation Officials, "Smoothness of Pavement at the Approaches to Weigh-in-Motion Scales" (No. MP 14-05). The specification requires field collection of profile information for the candidate WIM site. Software developed by the LTPP program is then used to calculate the pavement surface roughness and determine if it meets acceptable index levels.

For more information on the smoothness index profile and the related software, contact Mike Moravec at the FHWA Resource Center, 410-962-5623 (fax: 410-962-4586; email: mike.moravec@fhwa.dot.gov).

The first WIM installation took place on Interstate 57 near Champaign, Illinois, during July 2005. The Phase II contractor, International Road Dynamics, Inc., installed a bending plate WIM system. Prior to the installation, the site location was profiled using the LTPP WIM smoothness index profile (see sidebar). The profile data lets transportation agencies know if a pavement is smooth enough to install a WIM system that will meet the LTPP program's accuracy requirements. The Illinois Department of Transportation (DOT) built a 152 m (500 ft) section of new portland cement concrete pavement and did some grinding to the concrete pad to meet the requirements. After calibration and a self evaluation of the WIM system by the Phase II contractor, follow-up validation was performed by the study's Phase I contractor, MACTEC Engineering and Consulting, Inc. Data collection at the site began in September 2005.

Installation of the frame for the WIM system's leading (right wheel track) bending plate on Interstate 57 near Champaign, IL.

The completed installation of the in-road bending plate components.

"Our goal is to obtain research quality traffic data," says Deborah Walker of FHWA. This is defined to be at least 210 days of data of known calibration. The data collected is downloaded from the site and processed using the LTPP Traffic Analysis Software. It is then entered into the LTPP traffic database and included in the LTPP Standard Data Release. Data collection is scheduled to last for at least 5 years.

The WIM system controller on Interstate 57.

"This is an excellent pooled-fund study that provides a good opportunity to collect high-quality traffic data," says Tom Winkelman of the Illinois DOT. "The study also corresponds well with the new Mechanistic-Empirical Pavement Design Guide, as the guide requires much more traffic data."

The LTPP program expects to perform the majority of the WIM installations in 2006. "Some States will install their own WIM equipment, but they have to maintain it for 5 years, to ensure good data collection," says Walker.

For more information about the pooled-fund study, contact Deborah Walker at FHWA, 202-493-3068 (email: deborah.walker@fhwa.dot.gov), or visit the LTPP Web site at http://www.fhwa.dot.gov/pavement/ltpp/spstraffic/.

		October 2005
		FHWA-HRT-06-019