## 2 Materials and Methods  
All measurements were made using a specialized CPX trailer with the dimensions of about 3 \(\times\) 5 m, which was designed in 2010 by CDV researchers. In order to suppress unwanted echoes and sounds, the trailer (Figure 1) is designed as uncovered, without fenders, and its body is made of cylindrical profiles without sharp edges, without brakes and with air suspension. It meets all the requirements of ISO 11819-2, which lays down the CPX measurement methodology. The maximum number of microphones was used, i.e., five to six microphones according to year of measurement (see Figure 2). In line with the requirements of ISO 11819-3, a standard reference tyre P225/60R16 SRTT simulating passenger cars and vans, from Uniroyal (Tiger Paw), was used for all measurements. The tyre is recommended in the automotive industry as a standard tyre for ASTM F2493 [58] reference tests as it guarantees unchanging characteristics. The ISO 11819-3 standard describes the use of the tyre in CPX measurements and the method of tyre storage, including maximum wear, so that the tyre hardness correction fits within a defined interval. Sufficient attention should be paid to the change of the hardness of the SRTT rubber [59]. To minimise the impact of wear and degradation, a new tyre was used each year for measurement purposes. Raw data was collected in real time in one third of the octave frequency spectrum at an interval from 20 Hz to 20 kHz at a speed of 80 km\(\cdot\)h\({}^{-1}\). The following devices were used for measurement:  
* UA-9004 RLVBS1 GPS module (vehicle speed);
* PT100 probe (air temperature);
* PC21 MT-0 infrared sensor (road surface temperature);
* PULSE LAN-XI multianalyser--modules 3050-B-060 and 3056-A-040--with PULSE LabShop;
* microphones type 4189 (accuracy class 1, sensitivity 50 mv\(\cdot\)Pa\({}^{-1}\)) + calibrator type N 4231. All measured quantities were synchronised to a common timeline using specific software running on the PULSE LabShop platform. The measurements took place every year between April and October. In each section, non-overlapping segments were measured once a year and the mean value was calculated from the results on a linear scale in order to obtain the noise value for the given section. Measurement segments where the noise level  
Figure 1: The CPX trailer designed by CDV, photo 2021 (source: Transport Research Centre (CDV)). Figure 2: Positioning of the microphones around the reference tyre, photo 2021 (source: CDV). was demonstrably affected by noise from other sources (e.g., emergency services sirens) or anomalies on the road (e.g., random local fault, bridge expansion joints) were excluded from the analysis. The section of the Czech D1 motorway was chosen for the measuring. The D1 motorway is the main trunk road in the Czech Republic as it connects Prague, Brno and Ostrava and continues to the Polish A1 motorway. It is the oldest and longest motorway in the Czech Republic. Its construction began in 1967 and the section of the motorway connecting Prague and Brno was completed in 1980. The paper is focused on measuring the road surface noise on the motorway section between 21 and 182 km (Mirosovice-Kyvalka between Prague and Brno, see Figure 3), because a modernisation of this section has been underway since 2013. According to information from the 2016 National Transport Survey, on average, between 37,000 and 48,000 vehicles per day pass through this section (AADT--annual average daily traffic). Near Prague (up to 21 km), AADT is up to 100 thousand vehicles, in the vicinity of Brno (from 182 km) it is approximately 70 thousand vehicles. However, these sections are not part of the D1 modernisation and are therefore not included in the results presented in this paper. The technology of cement concrete surface, which has joints without any sealing, was used in the Czech Republic until 1993. Since 1995, cement concrete surfaces have been built using double-layered concrete technology with the insertion of dowels and tie bars into joints.