Hmmmm . . . . Methinks it is more of an employee management tool than an engine management tool.
I suspected it in the original post, and with bapos confirming that it is based on road speed, not RPM, I would say that it is there to scare the driver.
If it were my bus, I would possibly install a tach, but would definitely reach under the dash and remove the bulb. Another option would be to install an "Interstate" switch to interrupt the wire to the bulb, so it could keep me honest on secondary roads (NYS 55 mph unless posted), and kill the warning when cruising the big roads.