## 4 Results and Discussion
### Sensor Robot Design as Access Point
#### 4.1.9 Vehicle Guidance Method  
The vehicle guidance method includes steps that are processed in time series in the robot device shown in Figures 8-10. The flowchart in Figure 11 is an exemplary view for explaining the method of inducing a vehicle by the robot device. Figure 9: Vehicle induction: information generation from the robot device via access point function. The robot can detect the vehicle (300 in Figure 10(a)) approaching the robot device (S610 in Figure 11), then measure the distance D from the vehicle. For convenience, the distance D between the robot device and the vehicle is expressed perpendicularly to the vehicle while being parallel to the road surface. The robot periodically measures D from approaching vehicles. If a vehicle is detected as approaching, the robot projects an image on to the road surface R of the approaching vehicle (indicated by S620 in Figure 11). Figure 11 includes a warning image with an under construction ('road work ahead') sign. The safety robot device can:  
* Project an image on the road surface R on which the vehicle is approaching, when the distance D with the vehicle is within a preset range E. For example, E can be set to 200 m, and the robot detects an approaching vehicle when D is detected to be within 200 m. The image is then projected on the road surface R.
* Control the projection angle to project an image on R within D from a vehicle and project the image according to the projection angle. * Generate access information when the vehicle approaches the robot device. * Transmit the access information generated by at least one other robot device  
Figure 11: Data flow: vehicle guidance system, from sensor robots to in-vehicle service [27] network. Figure 10: An example view explaining the vehicle guidance system: (**a**) vehicle guidance method between the robot device (200) and vehicle (300). D is the distance between the robot device and vehicle and E is the preset range; (**b**) in normal communication among 200a, 200b and 200c; (**c**) communication between 200a and 200c is available even when the robot 200b is damaged. The first robot (200a) in Figure 10 can detect the approach of the vehicle and project a warning image on the road surface. It also generates access information and transfers the generated access information to at least one other robot device, say 200b. The second robot receives the access information from the first robot and projects the detour path image. Furthermore, when an error occurs in at least one of the robot devices, the third robot device (200c) can replace the robot device in which the error has occurred, as in Figure 10b,c. The third robot device can determine whether an error has occurred based on the state information received from the first or second robot device. Alternatively, the third robot device can extract the unique identification information of the non-communicable robot device to identify the failing robot device. When the second robot device is damaged or felled by a vehicle, the second robot device provides its status information to the first and third robot devices and at least one operator terminal through peer-to-peer communication. The status information includes the unique identification information of the second robot device. Image setting information, which contains the information about the image set that needs to be projected by the fallen second robot, is also transmitted to the first and third robot devices, and the operator terminal. In this case, the operator terminal displays the received status information and the operator checks the status information so that the third robot device can operate to replace the second robot device. For example, when the third robot device receives status information indicating that the third robot device has fallen or moved from the second robot device, the second robot device matches the identification information of the robot device in which the error has occurred.