When traveling close to the speed of light, speeds don't add linearly anymore -please elaborate?

Question

I got this answer from eri, I couldn't really get to the bottom of it .

How does this effect our understanding of special relativity?

Answer 1

as far as I know it has to do with the fact that as you approach the speed of light, time begins to slow down.  So if you think about that you cannot add speed linearily anymore, since time is a factor in speed.  For example if you are travelling at 100 miles/hour, if you change how long it takes an hour to go by you will change your total distance.  If the hour goes by quicker, you will travel a shorter distance, if the hour goes by slower (which is what happens as you approach the speed of light) your distance will increase.

Hope that helps

Answer 2

Assume two cars going at 20 and 30 m/s each in opposite directions. Then the relative speed = 50m/s

Assume two cars going at speed of light in opposite directions. Then according to theory of relativity, the cars although should appear to be moving at 2c = twice speed of light relative that is, it doesn't happen like that. Instead the relative speed is still speed of light...