## 1 Introduction
The third type is the additional Head-Up Display (HUD), which is increasingly being used in vehicles, although it is still not mandatory. A HUD with digital speed presentation reduces distraction time and minimizes eye accommodation [2; 11]. Despite the fact that the speedometer is probably the oldest instrument used since the beginning of the motor vehicle era, it can still be found as a subject of research in recent literature. Vehicle speed monitoring is important and can be treated as a medium to high warning priority. Although it may not always be related to the cause of the accident, inappropriate vehicle speed is one of the major contributors to road safety, which increases both the number of traffic accidents and the severity of their consequences. [PERSON] et al. [12] presented a detailed review of the relationship between inappropriate speed and crashes as well as worldwide statistics of offenses caused by speeding. In addition, they identified traffic congestion as one of the primary causes of drivers' inappropriate speed choices. [PERSON] et al. [13] have investigated the use of speed alerts that minimize the driver's perception time without adding redundant visual clutter. The alert devices were placed at different locations within the vehicle. The experiments were conducted in a simulator with 24 participants. The results showed that younger participants (18-23 years) better responded to the visual alerts and obeyed speed limits compared to the older and more experienced participants. In addition, alerts in the mid-peripheral visual region and alerts that flashed had better response rates for observing speed limits. The driver's reaction is not necessary every time if the speed is appropriate for the current situation. Without a speedometer, the driver's subjective estimation of the vehicle's speed is relatively unreliable. It depends on visual and auditory inputs and his/her experience, and the driver must use visual, quantitative information from the in-vehicle instrument [2; 4]. The importance of speedometers is reflected in the finding that drivers tend to adapt to sustained speed over a period of time. The consequence is a decrease in the perceived speed of the vehicle without looking at the speedometer [4]. According to the study carried out by [PERSON] and [PERSON] (cited in [4], p. 29), the average speed error was more than 20 percent higher when drivers were asked to reduce the speed to the instructed value without the aid of the speedometer. The error was also present when drivers went from a lower speed to a higher one. Re-adaptation from the so-called \"speed creep\" lasted several minutes [4]. Vehicle speed control is also important for fuel-efficient driving. [PERSON] and [PERSON] [14] proposed in-vehicle interfaces designed to reduce fuel consumption and emissions. One example is a dynamic graphical indicator on the speedometer, which suggests the best possible speed for the current location to minimize the likelihood of stopping because of traffic lights or congestion. A similar method was proposed by [PERSON] and [PERSON] [15] to extend the speedometer into an interface for optimal safe speed and distance control. In the study of [PERSON] et al. [16], the speedometer observation pattern was investigated using questionnaires and accident investigations. The subjects were 63 male drivers and 44 female drivers. The authors concluded that about half of the subjects frequently observe the speedometer, but almost half of the drivers think that the speed observation has different degrees of burden. Nearly 80% believe that they are sensible about the speed of a car and they can estimate the speed in many ways. Their results showed that female drivers have a lower sensitivity to speed than male drivers. Female drivers and new drivers usually pay more attention to the speedometer. Drivers who wear glasses have a higher observation frequency and a higher burden. The data about the type of vehicles or design of the instrument cluster were not given in their paper.