## 4 Experiment Results
### On-the-Road Testing of HR and RR  
Figure 8 shows a waterfall chart of the success rate tested under each of the scenarios (Table 2). An HR measurement from our CVSM system is defined as'successful' if the measurement is within 10% of the HR measured by our reference device, a Polar H10 chest strap. The success rates of HR measurements on highway conditions outperform the success rates on the local road conditions in all cases. Such results are expected since the local road represents a more complex driving scenario. Compared to a highway, local roads have more potholes and busier traffic, which leads to more frequent active and passive motions of the driver. With the adding of more potential sources of motion artifacts in each consecutive scenario, we can see a degradation of HR measurement performance by different amounts. In the case of the highway scenario, the success rate drops from 90% when measured in the lab to 71.9% when the driver is driving the vehicle on the highway, with the most significant performance drop occurring when we add the voluntary head motions from the driver (i.e., the case when driver drives the car him/herself). As a comparison, the success rate drops to a lower 56% when the driver is driving on the local road. However, the most significant decrease in performances for the local road scenarios seems to arise from the road surface and traffic conditions rather than the driver's active motion (the success rate drops most when we started to measure passengers in a moving vehicle). In reality, the highway scenario will be the more important use case for the CVSM system, due to higher speed as well as the longer time of driving, leading to drowsiness and potentially more catastrophic accidents [38]. We will discuss in detail the system performance under each in-vehicle scenario in the following paragraphs. Figure 9 compares the distribution of error rates of the HR measurements with or without depth-based motion compensation when the user is sitting in a parked car with the engine running. In the following analysis, \"motion-compensated\" refers to HR measurements utilizing both grayscale intensity and depth information for motion compensation. The \"uncompensated\" measurements refer to the HR measurement using the same grayscale intensity as the \"compensated\" HR measurements but not using the depth information for motion compensation (as is the case of a normal 2D camera). With the depth-based motion compensation, the success rate is 86% while the success rate drops to 36.5% when the motion compensation is not used. Even though the participant is sitting still in their seat, there still exists involuntary motion, which can obfuscate the HR signal if left uncompensated. Figure 7: Time (**Top**) and frequency (**Bottom**) domain of the motion-compensated HR signal in both bright (**a**) and dark (**b**) environments. Figure 10 shows the Bland-Altman plot of the HR measurements in the parked vehicle with the engine running. With depth-based motion compensation enabled, the mean deviation of the HR measurements from is \(-2.9\) BPM and \(95\%\) of the measurement deviations fall between \(\pm 19.6\) BPM of the mean deviation. When depth-based compensation is not used, we observe a much higher negative bias of \(-16.3\) BPM with \(95\%\) of deviation falling between \(\pm 35\) BPM of the mean. Such larger bias is usually caused by the lower frequency motion artifacts being mistakenly recognized as HR. Figure 8: Waterfall diagram of the success rate of the HR measurements under different testing scenarios on highways and local roads. Figure 10: Bland–Altman plot of HR measurements when participants sit in a parked vehicle with motion compensation (**left**) and without motion compensation (**right**). Figure 9: HR measurement error rates when participants sit in a parked vehicle with motion compensation (**left**) and without motion compensation (**right**). Testing on Highway:  
Once on the road, the car motion from the road bumpiness starts to affect the performance of HR measurements.