Why did Einstein took speed of light unit or constant in his equation of relativity? We can find that no object can have speed more than light from Einstein's equation of relativity because if anything have speed more than light then we get -ve value within square root. But why did Einstein took speed of light as unit or constant?
 A: The speed of light was first measured by

Ole Christensen Rømer (Danish pronunciation: [ˈo(ː)lə ˈʁœːˀmɐ]; 25 September 1644 – 19 September 1710) was a Danish astronomer who in 1676 made the first quantitative measurements of the speed of light.

When Maxwell formed what is the classical electromagnetic theory it was evident that the speed of light would be a constant in the equations.  
The thinking out of the box by Einstein is that he imposed as a limit  the velocity of light  for massive objects also and proposed special relativity. The theory has been validated innumerable times in nuclear physics and particle physics.
A: I think it was partially motivated by the following: With Maxwell's equations, a plane wave is a sinusoidal wave that varies in space in time and moving with speed $c$. These variations are linked by Maxwell's equations. What would happen if you could travel along with a plane wave at the speed c? You would observe fields that would be fixed in space and this would contradict Maxwell's equations. (See http://www.pitt.edu/~jdnorton/Goodies/Chasing_the_light/)
