Many state still have laws on the books that require giving an audible signal when passing another vehicle. Section 316.083(2), Florida Statutes, says "Except when overtaking and passing on the right is permitted, the driver of an overtaken vehicle shall give way to the right in favor of the overtaking vehicle, on audible signal or upon the visible blinking of the headlamps of the overtaking vehicle if such overtaking is being attempted at nighttime, and shall not increase the speed of his or her vehicle until completely passed by the overtaking vehicle." I'm sure there are other references in the statutes to audible signals to other cars.