Frequently Asked Questions

Questions

1. What data are available from LTPP?
2. How are LTPP data organized?
3. What is the difference between GPS and SPS?
4. Where are the LTPP test sections located?
5. Where can I find layer thickness information?
6. Does the data go through quality control?
7. How are traffic data collected and stored?

Answers

What data are available from LTPP?

LTPP collects information on pavement performance and the elements that may influence pavement performance. Pavement types include both asphalt and portland cement concrete pavements, with and without various types of overlays and surface treatments. The performance information includes pavement roughness measures, the type and quantity of pavement distress, deflection testing, and skid information. The elements that are considered to have an effect on performance include material characteristics; climatic conditions; pavement loading (traffic); and maintenance and construction activities, including routine maintenance conducted at a site and information on both original construction and rehabilitation activities. Data are generally presented using customary terms and statistics, such as International Roughness Index (IRI), pavement thickness, annual and monthly precipitation totals, and equivalent single-axle loads (ESALs).

Data can be downloaded from the LTPP InfoPave™ Web site or by contacting LTPP Customer Support Service Center. Ancillary data and information not included with the most current Standard Data Release of the Pavement Performance Database may be requested from the LTPP Customer Service Center, telephone: 202 –493–3035, fax: 202–493–3161, or email: ltppinfo@dot.gov. Point-by-point road profile, falling weight deflectometer (FWD) time-history waveforms, distress map data, and raw traffic data are a few examples of this ancillary data and information.

While requests may be filled for free, limitations will exist based on the level of effort required to fulfill the request. These requests will be assessed on a case-by-case basis; please contact the LTPP Customer Service Center.

How are LTPP data organized?

LTPP data are stored in both a database and in a wide variety of original data files that are the supporting information for the database. The database itself is organized into modules by type of data collected. With each module, there are one or more tables for each major data grouping. There are modules for climatic information, materials testing, pavement loading (traffic), original construction activities, maintenance activities, rehabilitation activities, and monitoring data. A complete list of the modules and tables is contained on the Standard Data Release and in the InfoPave™ Online option at www.ltpp-products.com.

What is the difference between GPS and SPS?

Two types of studies are being performed under the Long-Term Pavement Performance (LTPP) program: General Pavement Studies (GPS) and Specific Pavement Studies (SPS).

The GPS experiments focus on commonly used structural designs for pavement. Eight types of existing inservice pavements — in either original or rehabilitated condition — are being monitored throughout North America. The performance of these structural designs is tested against an array of climatic, geologic, maintenance, rehabilitation, traffic, and other service conditions. Each GPS site has a single test section.

In contrast, the SPS test sections have been specially constructed to investigate certain pavement engineering factors. These particular test sections allow critical design factors to be controlled and performance to be monitored from the initial date of construction. Each test site has multiple test sections, each with a different set of design factors. This makes it possible to compare the performance of different design factors, both within and between sites. The results will provide a better understanding of how selected maintenance, rehabilitation, and design factors affect pavement performance.

Where are the LTPP test sections located?

The Long-Term Pavement Performance (LTPP) program has monitored more than 2,500 asphalt and Portland cement concrete pavement test sections throughout the United States and Canada. Several test sections may be at one location. The specific location of a section can be found in the LTPP database information management system (IMS) in the INV_ID or SPS_ID table.

A total of 934 LTPP test sections are in the General Pavement Studies (GPS) category. Each test section is at a different location. When a test section has been taken out of study, data are no longer collected for that section.

There are 1,580 other test sections that fall into the Specific Pavement Studies (SPS) category. These test sections are grouped by projects at 229 different locations. Participating States for each SPS experiment are as follows:

• SPS 1 — Alabama, Arizona, Arkansas, Delaware, Florida, Iowa, Kansas, Louisiana, Michigan, Montana, Nebraska, Nevada, New Mexico, Ohio, Oklahoma, Texas, Virginia, and Wisconsin.
• SPS-2 — Arizona, Arkansas, California, Colorado, Delaware, Iowa, Kansas, Michigan, Nevada, North Carolina, North Dakota, Ohio, Washington, and Wisconsin.
• SPS-3 — Alabama, Arizona, Arkansas, California, Colorado, Florida, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Manitoba, Maryland, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New York, Oklahoma, Ontario, Pennsylvania, Quebec, Saskatchewan, Tennessee, Texas, Utah, Virginia, Washington, and Wyoming.
• SPS-4 — Arizona, Arkansas, California, Colorado, Illinois, Indiana, Iowa, Kansas, Kentucky, Minnesota, Mississippi, Missouri, Nebraska, Nevada, Ohio, Oklahoma, Pennsylvania, Quebec, South Dakota, Texas, and Utah.
• SPS-5 — Alabama, Alberta, Arizona, California, Colorado, Florida, Georgia, Maine, Manitoba, Maryland, Minnesota, Mississippi, Missouri, Montana, New Jersey, New Mexico, Oklahoma, and Texas.
• SPS-6 — Alabama, Arizona, Arkansas, California, Illinois, Indiana, Iowa, Michigan, Missouri, Oklahoma, Pennsylvania, South Dakota, and Tennessee.
• SPS-7 — Iowa, Louisiana, Minnesota, and Missouri.
• SPS-8 — Arkansas, California, Colorado, Mississippi, Missouri, Montana, New Jersey, New Mexico, New York, North Carolina, Ohio, South Dakota, Texas, Utah, Washington, and Wisconsin.
• SPS-9 — Alberta, Arizona, Arkansas, Connecticut, Florida, Indiana, Kansas, Maryland, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, New Jersey, New Mexico, North Carolina, Ohio, Ontario, Quebec, Saskatchewan, Texas, and Wisconsin.

Where can I find layer thickness information?

The LTPP database contains two types of layer thickness information — one from plans and one from the measurement of cores. The plan information found in the INVENTORY module should be considered an estimate since it represents the design thickness or the as-built average thickness from somewhere on the project. The LTPP actual measurement information is found in the materials TESTING module in two tables referred to as TST_L05A and TST_L05B. The A table has individual core and pit measurements. The B table has the representative values for each section. In addition, sites that have been overlaid have thickness information in the REHABILITATION module in RHB_LAYER.

Does the data go through quality control?

Yes. A series of quality control (QC) checks are applied to the data as they are collected in the field and prior to entry into the database. After being loaded into the database and before data are released to the public, a series of QC checks are performed to ensure the integrity of the data. Four types of checks are done within the IMS: (1) mandatory, (2) logic, (3) range, and (4) verification. Mandatory checks review fields that must have data for the database software to function. Logic checks are used to ensure data compatibility across tables, e.g., asphalt testing being done on asphalt layers. Range checks are used to compare data against allowable minimums, maximums, or specific values. Verification is used to ensure that sections exist for which data have been collected. Quality control checks performed on the database are shown in the LTPP IMS Data Quality Checks Manual, which is included with the Standard Data Release, now available through InfoPave™. For more information, contact LTPP Customer Service Center, telephone: 202–493–3035, fax: 202–493–3161, or email: ltppinfo@dot.gov.

How are traffic data collected and stored?

LTPP traffic data are collected by the States/Provinces and submitted to the LTPP Regional Offices. General guidelines for traffic data are provided in LTPP Traffic Directive 55, Issuance of LTPP Traffic Data Collection and Processing Guide, Version 1.3. The extent to which individual agencies have met these guidelines has varied. For information on obtaining these publications, contact LTPP Customer Service Center, telephone: 202–493–3035 or email: ltppinfo@dot.gov.

The Central Traffic Database (CTDB) contains the raw data collected by the States/Provinces as well as summarized data. The Information Management System (IMS) contains traffic data which provide estimates of annual vehicle counts by vehicle classification and distribution of axle weight by axle type. The traffic tables store annual traffic summary statistics. Data are provided for each year since the road was opened to traffic. Equivalent single-axle values for loading may be estimated from prior year information or based on American Association of State Highway and Transportation Officials' (AASHTO) procedures using monitored data.