We are looking for a principle that will strengthen the argument.
The argument says that doing X (raising speed limit to average speed) will achieve Y (increased Y), therefore we should do it.
It seems reasonable enough and in everyday speech, this would be enough. But on the LSAT, we need to prove why something that seems reasonable "should" be done. Should is a strong word.
Answer choice C adds a condition that needs to be met in order to strengthen the idea of adopting a uniform speed.
We don't know that ALL roadways have roughly equal average speeds, we just know the average speed.
E tells us, if something helps us increase safety, then we should. This would link our reasoning.