## 1 Introduction  
The airport runway is a complex structure that provides strength, durability, and friction requirements during an aircraft operation [1]. Consequently, runway design, construction, and maintenance are strictly regulated to provide safety for an aircraft. At the same time, the pavement surface is the most important part of a runway, since it is affected by the highest stresses, and it provides the necessary friction [1]. Most of the large commercial airports use either cement concrete or asphalt as the runway surface layer [2]. These are durable materials, but in many cases the friction of the pavement surface must be managed to ensure it does not become insufficient. This is especially the case for densely graded asphalt mixtures because the nature macrotexture is relatively low and usually requires some improvement. For this purpose, pavement grooving is often used [3]. This is important for achieving the overall goal of the runway surface design, which is to make sure that aircraft operations are safe and efficient. Nowadays, however, the runway is facing bigger challenges due to the rapid developments of commercial aircraft [4]. Since their first introduction in the early 1900s, aircraftshave become larger and heavier and particularly since WWII, aircraft wheel loads and tire pressures have increased significantly, as have aircraft length, wingspan, and wheel base/track. The number of large aircrafts has increased as well, due to the economic development around the world. The introduction of more demanding aircrafts sometimes triggers runway, taxiway, and apron pavement upgrades [5], and this affects not only safety, but also economic concerns, since the maintenance of bigger aircraft is more expensive as well as increased average runway occupancy time. The introduction of better runway surface design and maintenance techniques contributes to the necessary safety and economic efficiency of airport pavements for these more demanding aircrafts. New dry surfaces usually provide necessary friction. However, texture deterioration and contamination reduce the friction over time [6]. Furthermore, runway contamination is inevitable, usually due to rubber build-up during the aircraft operation. Hence, it is important to be able to determine if contamination is reducing friction to an unacceptable level. Friction reduction is most critical during wet conditions, which greatly influences the risk of friction-related incidents. The main reason for this is the increased probability of hydroplaning due to a reduction of draining capability of the surface. If hydroplaning occurs, aircrafts lose the ability to decelerate and maneuver. This event can be unexpected during the heavy rain, or wet snow precipitation, and regular runway maintenance, especially rubber removal, helps to reduce this risk. To identify when friction is compromised, international regulators provide standard testing methodologies to determine the actual friction and risk of skid-related accidents. The main methods recommended by the International Civil Aviation Organization (ICAO) are continuous friction measurement (CFME) using a partial-slip wheel fitted to a car, or a car-towed device, and macrotexture measurement [7]. These methods partially correspond to highway friction measurement techniques. However, the runway is affected by different conditions, due to the greater weight and speed of an aircraft, compared to the weight and speed of a cars and trucks. For this reason, the management of runway friction is more critical than for road pavements, and the improvement in runway friction assessment techniques is widely recognized [8]. The most recent and significant attempt to improve the runway friction regulation system was the introduction of the Global Reporting Format (GRF) [9].