Points of discussion about limit value
Under the current idea, there are two concerns: the lower limit of Low Mode might be too small, and the upper limit of High Mode might be too loud.
It is necessary to secure the safety under the normal driving environment at the lower limit of sound pressure level because the vehicle cannot control the driver's behavior. Otherwise, the Low Mode will practically function as the Off Switch. On the other hand, it is not appropriate either if the sound pressure level is louder than that of the horn even at High Mode.
When we try to define the 3 modes with the idea that each mode shall have a certain range (ex. 20 dBA) and to include a certain margin (ex. 5dBA) between each mode, it is inevitable that the lower limit value will be quite low while the upper limit will be rather high. It is necessary to discuss whether it is appropriate to divide the modes as requirement. If there shall be 3 modes, it is necessary to consider both the safety when the Low Mode is used under loud ambient sound and the sound environment when the High Mode is used under low ambient sound.
Furthermore, it is possible to have the device which the sound pressure level is manually changed continuously (like changing the volume of audio device). Under the current text, it is necessary to stipulate the requirements for such cases respectively.
As Regulation, Japan suggests to define the lower limit and upper limit only and to clearly mention that the different type of sounds or automatically adjustable device to ambient sound are allowed.