There are quite a number of threads (and FAQ's) that discuss why the speed of light is the "number" that it is, but I'm having difficulty finding some information on what is causing, or setting, the limit. So, let me ask and answer a different question as an example of the what I am tryinh to find out here. Q: Why is the speed limit on our national roads the number it is? A: This number is caused by goverment / legislation that causes the speed limit to exist. So, without a reference to a measurement or unit, what is it that causes the speed of light to exisit? If anyone can help / direction me to a relevant paper / article on this, it would be greatly appreciated. Regards, Noel.