Does speed limit start at speed limit sign?

If the only thing on the sign is the number then yes, it starts at that sign. Often you'll see a sign like 35 mph limit ahead. That's just a warning to slow down so you're at or below the speed limit when you pass the number sign.

If the speed limit is lower than the speed zone you are in, you must be at or below the new speed limit at the point the sign is posted. You cannot be decelerating after you've passed the posted speed limit sign. If the speed limit is higher than the speed zone you are in, you cannot accelerate to the new speed limit until after you pass the new speed limit sign. You cannot accelerate until you pass the posted speed limit sign. What is legal and what really happens are two different things. Cops hang on the legal requirement - it's their job. Just because everyone else does different doesn't make it right or legal. So be prepared it you are stopped.

Yes. I got a ticket one time because I began to speed up about 100 feet before the sign. It's the same way when you get to a reduced speed sign. Make sure you give yourself an adequate amount of time to slow down before you reach the sign. Once you reach the sign, you must be a the speed posted.

The speed limit changes EXACTLY at the sign, where it says Speed Limit XX. Be warned that in a "speed trap" the cops will be hanging out right at the sign and watching to make sure you don't speed up BEFORE the sign or slow down AFTER the sign.

yes it does. but the original speed limit of a road begins once you enter the road.