This paper reports on the GPR-based assessment of railway ballast which was progressively "polluted" with a fine-grained silty soil material. It is known how the proper operation of a ballast track bed may be undermined by the presence of fine-grained material which can fill progressively the voids between the ballast aggregates and affect the original strength mechanisms. This occurrence is typically defined as "fouling". To this effect, a square-based methacrylate tank was filled with ballast aggregates in the laboratory environment and then silty soil (pollutant) was added in different quantities. In order to simulate a real-life scenario within the context of railway structures, a total of four different ballast/pollutant mixes were introduced from 100% ballast (clean) to highly-fouled (24 %). Ground-penetrating radar (GPR) systems equipped with different air-coupled antennas and central frequencies of 1000 MHz and 2000 MHz were used for testing purposes. Several processing methods were applied in order to obtain the dielectric permittivity of the ballast system under investigation. The results were validated using the "volumetric mixing approach" (available within the literature) as well as by performing a numerical simulation on the physical models used in the laboratory. It is important to emphasize the significance of the random-sequential absorption (RSA) paradigm coupled with the finite-difference time-domain (FDTD) technique used during the data processing. This was proved to be crucial and effective for the simulation of the GPR signal as well as in generating synthetic GPR responses close to the experimental data.
Several studies have indicated that road crashes are more likely to occur on horizontal curves than on straight roadway segments for a good number of reasons, the most important of which is associated with the driver's behaviour along the curve depending on his perception of the road geometry. However, the evaluation of the effects of curve features on driving performance still remains a critical issue for road safety and design.\ud
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The main objective of this study is to investigate driver's behaviour and his perception of road curves, which is directly related to road safety. Specifically, the effects of some curve features (radius, transition curve, visibility, cross section) on driving performance are investigated through a multi-factorial experiment based on driving simulation. The driving speeds and trajectories of a sample of 34 drivers were statistically processed over 72 different curves distributed along three test scenarios. The main and interaction effects of the independent variables are described and discussed in the results section of this paper providing a significant improvement of the actual knowledge on in this field of research.\ud
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In general, the results confirm that driving simulation can disclose the relationships between road design features and driver behavioural aspects that are crucial issues in creating a safer road infrastructure
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