The laser Traffic Speed Deflectometer (TSD) is a high-tech device for collecting information on the load-bearing condition of pavements. It was created to measure deflections on road networks, but is nevertheless already being used as a validation test of renovated road sections (in Italy). A particular advantage of this device is the efficiency of its operation, allowing it to accurately and quickly identify areas of reduced durability of road pavements – an essential and fundamental thing in road network management. The quality of the data obtained and the new technical solutions make the TSD used by the Institute a unique device not only in the country, but also in Europe and in the world.


The heart of the measurement system is a beam with laser sensors mounted in the truck's trailer so that the main sensor measures deflection in the load axle, i.e. between the wheels of the twin wheel. The next sensors are placed in the line of the wheel track, at fixed distances from each other, in front of and behind the wheel.
The measurement involves determining the speed of vertical displacement and the angle of deflection of the surface caused by the moving truck. The difference in displacement between the loaded and unloaded points is an expression of the current movement of the pavement caused by a vehicle with a known load. Based on these data, the deflection of the pavement at each measurement point is calculated.

The measurement process is automatic and can be carried out by one person. Measurements are usually taken at speeds of 60-70 km/h. The maximum measurement speed is 95 km/h while the minimum is 35 km/h. The typical load size is 10 tons per axle, with the possibility of changing to 7 tons and 13 tons.
In addition, the system is equipped with infrared sensors to measure the temperature of the pavement surface, a camera to record images of the Right-of-Way, a laser sensor to measure the longitudinal profile of the pavement, WiFi and GPS, as well as GPR to identify the pavement structure.
The TSD guarantees high correlation and repeatability of results. The diagram shown illustrates the accuracy with which the measurement system reproduces the deflections recorded on the same section in successive runs. This basic, for advanced measurement systems, property is essential in the correct and reliable evaluation of the road network as regards the current load-bearing capacity of the pavement structure.

TSD rys 6  TSD rys 7  TSD rys 8

The diagram shows a comparison of deflections recorded in the load axle during TSD (in three runs “Run 1-3”) and FWD measurements (green line). The data presented shows that TSD captures changes in pavement deflection as much as FWD. Tests conducted with both devices on a regular basis on test sections allowed to determine the correlation of the obtained values. The correlation diagram of maximum deflections recorded by the TSD and FWD, presented below, testifies to the high compatibility, which allows for verification of road sections subjected to load-bearing capacity assessment, as well as being used in the design of pavement strengthening and rehabilitation.