There's 2 speed things
1) a speed limiter, activated by pressing the lower edge of the cruise control button (marked LIM) then setting the limit using the plus and minus buttons.
2) a speed warning beep, not sure how it's activated but I think it's via the settings accessed via the left side steering wheel buttons (the same buttons you use to scroll through average speed, mpg etc). I think the car needs to be stationary to set it.