Determining the thickness of pavement layers is traditionally carried out by drilling cores in situ. As well as removing part of the existing pavement, this method obviously only gives a spot measurement and does not provide all the information contained in the radar images obtained with Ground Penetrating Radar (GPR) technology. With the GPR system, the radar antenna is not in contact with the surface, which makes it possible to operate at higher speeds and hence avoid the motorway lane closures necessary for core drilling. The radar survey system enables the thickness of the layers to be measured. Determining the thickness of the pavement layers can contribute towards an objective assessment of the condition of pavements. In the Netherlands, four companies have performed a practical GPR test. The radar data was later analyzed on personal computers, using software specially developed for analyzing these radar signals. Special attention was given to the repeatability (comparing the results of several identical measurements) and reproducibility (comparing the results of the different companies) of the measurements. The results from the four companies differ from each other. In general, the asphalt thickness measurements are far better than the results of the road base layer thickness where the penetration depth was insufficient.