Proactive TSDD Network Metrics

SCL & PDL

overview

The Surface Cracking Life (SCL) and Pavement Deformation Life (PDL) are TSDD network-level metrics derived from Traffic Speed Deflection Device (TSDD) measurements to estimate the remaining life, in years, for surface cracking and permanent deformation in unbound layers, respectively. These metrics proactively quantify the overall structural capacity of flexible pavements, providing an early indication of potential weaknesses. When distress data is available, it can be incorporated as an additional factor to further prioritize segments for project-level evaluation, combining both proactive and reactive insights. In contrast, existing procedures based on the effective structural number (SNeff) and the required structural number (SNreq) as indicators of structural capacity for in-service pavements primarily focus on protecting the subgrade, assuming that subgrade layer moduli are reliably determined, and do not fully account for the structural capacity of the surface and intermediate layers. The SCL and PDL approach eliminates the need for adjustment or correction factors for subgrade layer moduli and avoids reliance on decision trees or empirical selection procedures. In addition, these metrics provide results expressed directly in years of remaining life, rather than in deflection units or structural-number ratios, offering a simple mechanistic and meaningful framework that can be readily implemented in practice.

mechanistic-empirical metrics

SCL and PDL:

  • Follow the classical mechanistic–empirical pavement design procedure to capture the full picture of pavement behavior by considering surface layer cracking and permanent deformation in unbound layers.
  • SCL detects structural inadequacies in the asphalt concrete layer, the base layer, or a combination of the two, while PDL captures inadequacies in the subgrade or the overall pavement structure when it fails to protect the subgrade layer.
  • Are adjusted to a reference axle load.
  • Can be adjusted to a reference pavement temperature and reference TSDD speed.
  • Can be calibrated based on existing pavement conditions.


In addition, SCL and PDL are considered proactive metrics, as opposed to reactive ones, because they indicate potential structural weaknesses before visible distress—such as cracking or permanent deformation—appears at the pavement surface. By identifying structurally vulnerable locations early, these metrics help agencies plan and implement preventive maintenance strategies at the appropriate time, preserving pavement structural integrity and ride quality. This proactive approach improves user comfort and safety, reduces the risk of accelerated deterioration, and helps avoid more costly rehabilitation or reconstruction later.


Finally, SCL and PDL can be calculated at the smallest TSDD reporting interval, allowing agencies to identify localized areas of potential failure that may require remove-and-replace (R&R) treatments, as well as to define the limits of pavement sections that warrant detailed project-level evaluation.

interactive GIS map | SCL

An interactive Geographic Information System (GIS) color-coded map is created by plotting SCL values using data collected from a case study at 0.01-mile (52.8 ft) intervals. PDL is not shown, as the subgrade was not identified as the critical layer for this highway.


Interact with the map below—either within this page or in full-screen mode. You can:

  • Zoom in and out to explore the color-coded SCL points, and click individual points to view their values.
  • Toggle layers using the layer list in the upper-right corner: turn off the SCL Points layer, turn on the SCL Line layer, and compare it with the SNeff to SNreq ratio, whose values are all greater than 1.
  • Return to the initial view, then measure highway segments that may require rehabilitation by clicking the ruler icon in the upper-left corner and clicking along the path (double-click to complete each measurement).


A review of the latest pavement conditions in the eastbound direction for this highway, based on the September 2025 Google Street View imagery, shows that areas with SCL values below 5 years, shown in red on the map, exhibit significant structural failures in the wheelpaths. In addition, the highway displays extensive thermal cracking along much of its length, which influences the calculated SCL values at 0.01-mile intervals. Using a shorter TSDD reporting interval, together with correspondingly calculated SCL values, would improve the identification of localized problem areas, including those affected by thermal cracking.


In contrast, the SNeff to SNreq ratio calculated for this highway remains greater than one because the subgrade is adequate and well protected by the overlying pavement structure, and therefore does not capture deterioration in the surface or intermediate layers. Refer to the SNeff/SNreq line on the map to see how it fails to reflect the surface-layer damage when compared with the SCL.


Presenting the network metrics on a simple, color-coded map provides an overview of the overall network condition by highlighting sections that may require further investigation.

This interactive map is created in QGIS, an open-source, free, community-supported GIS application, over a Google Terrain Hybrid basemap. It is exported as a web app using the qgis2web plugin and displayed interactively using OpenLayers, one of several available web-mapping libraries, and is hosted on GitHub.