Quick and Easy - "Random - Visiblility"

Hey guys basicly im making a flash game i have a city scrolling from left to right and theirs a car that looks like its moving but its actually the background now i want a “Stop” sign image to pop up randomly each go, basicly the game is a reactor test so i dont want the user to beable to count how many seconds before it pops up cause that will be cheating im guessing its something to do with

setProperty(myStopImage, _visible, false);

then

random:
setProperty(myStopImage, _visible, true);

please help?