This feature is very similar to that of the Need For Speed games. It works in the same way. Press Ctrl once to activate speed breaker and press it again to end it.
It's A Copy... !!
http://www.gtagaming.com/downloads/gta-san-andreas/cleo-mods/7966
I didn't realize there was a mod already for it, i searched for it in gtagarage and couldn't find it. So i made it myself. The mod itself consists of only one primary opcode (the same effect that could be achieved by entering a rockstar cheat). So it ain't a big deal.