With the increasing number of bus accidents in recent years (For example, the Tai Po Road accident in 2018, a bus crashing into a tree in Fanling Highway in late 2019), the public is getting more concerned about the safety of franchised buses. Since most of these accidents are caused by speeding, some relevant safety devices have to be introduced. Now it is a demonstration to regulate the speed of a bus at a bus model. A sensor was installed at the front wheel of the bus model. The sensor will measure the speed of revolution of the wheel and then the measured value will convert into a speed unit by simple programming. When the speed exceeds 50km/h (all these speeds are scaled with the model unit), the safety device will ring for a small while. When the speed exceeds 70km/h the safety device will ring continuously until the speed drops below 70km/h (In Hong Kong, the maximum speed limit of a franchised bus is 70km/h). It is believed that it may remind drivers not to drive too fast, hope this may reduce the risk of traffic accident.