## 1 Introduction  
Regular monitoring of bridges is crucial for bridge safety because it provides an early warning for authorities to take pre-emptive measures to avoid bridge collapse. Aging bridges, which make up a big portion of the current in-use bridges, pose a high risk of collapse because they are more susceptible to deterioration and damage [1]. Furthermore, the increasing traffic loads in the last decades have exceeded the intended design load capacity of those bridges [2]. Therefore, developing damage detection methods has become of a great concern to authorities and decision makers. Vibration-based damage detection methods have been used extensively for bridge health monitoring (BHM). Bridge vibration signals carry useful information about the dynamic properties of the bridge. The change in dynamics properties of the bridge indicates structural damage and deterioration. Direct BHM requires each bridge to be equipped with a sensory system network of many accelerometers and date loggers in order to record the vibration data. Thereafter, the collected data go through signal processing to assess the health state of the bridge [3]. This method is called the direct method because the sensors are installed directly on the bridge. It is obvious how this method is costly and impractical to be used for the bridge network in a country [4]. Therefore, another method called the indirect method has been proposed to overcome the drawbacks of the direct method. The indirect method first emerged when Professor [PERSON] proposed the use of an instrumented vehicle to record the bridge vibration [5]. They proved the feasibility of extracting bridge frequency using the scanning vehicle by theoretical derivation and finite element (FE) modelling. Soon after, [PERSON] and [PERSON] [6] proved the feasibility of extracting the bridge frequency from a moving vehicle in a field test. In the last two decades, the research focus of vibration-based BMH has been shifting towards indirect means that require only an instrumented vehicle to record the vibration signal of the bridge. It has been demonstrated that the indirect method is more effective and less costly than the direct method [7; 8]. Plenty of research has been conducted to investigate the factors and challenges that are encountered when a vehicle is used to scan a bridge vibration. Analytical studies such as FE simulations and experimental studies such as laboratory and field tests have been widely used to study the indirect method. Thorough review papers have been published discussing the accomplished progress of the indirect method [7; 8; 9]. Nevertheless, it is essential to view past research discussing the indirect method and the difficulties encountered when using the scanning vehicle. For example, in a field test experiment, [PERSON], [PERSON] [10] suggested using a heavy vehicle to obtain better visibility of the bridge frequency. [PERSON], [PERSON] [11] proposed an advanced signal processing technique to detect higher modes of frequency of the bridge employing numerical simulation and field test experiments. [PERSON], [PERSON] [12] pointed out to the challenge of road roughness, which corrupts the recorded signal of the scanning vehicle. They proposed the use of data collected from accelerometers fixed on specific vehicle types to estimate the road roughness profile of the road. [PERSON], [PERSON] [13] concluded that a higher vehicle speed would provide larger amplitude frequency peaks, but at the same time, a higher speed would provide low spectral accuracy due to road roughness. In fact, several studies have indicated the negative effects on the recorded signal of the scanning vehicle [8; 9; 14; 15; 16]. [PERSON], [PERSON] [17] proposed the subtraction of the acceleration spectra from two connected vehicles to remove or reduce the effects of road irregularities.