If I understand what you're calling 'speed cushions": In my state, they're popularly called "rumble strips". It's a thin strip of serrated concrete that is positioned on the far right side of the road. It is not to be driven on. It's main purpose is to drain rain water off the roadway. It also serves as a warning to you that your car is drifting off the far right-side of the roadway. The noise your tire makes when it's on one of those "rumble strips' will CERTAINLY warn you. The noise can even wake you up, if you go to sleep while you're driving.
If you find yourself driving onto one of them, just keep your right front tire on it, until you can safely pull back onto the roadway. Do NOT try to straddle it with all 4 tires. If you do, your car will then be completely off the roadway. Don 't ever slam on your brakes, if you accidently drift onto one - it can throw you all over the road if you do so. Your tires have next-to-no traction when you're on one of those things.
If a sign says "20 mph", that's the maximum rate of speed you'll allowed to travel in that area. It doesn't mean you HAVE to travel 20 mph - it means you can't go any FASTER than 20 mph.