In some jurisdictions, driving faster than the speed limit is only the prima facie evidence of unreasonable speed, where one could argue in court that the speed was safe.
In the United States, according to a summary provided at http://www.mit.edu/~jfc/laws.html#mixed (and meta), out of 52 jurisdictions:
there are 3 where all speed limits are prima facie,
39 where all limits are absolute,
and 10 where there is a mixture of the two.
In Rhode Island, Texas, and Utah driving faster than the speed limit is prima facie evidence of unreasonable speed. One can argue in court that one was exceeding the speed limit but should not be convicted because the speed was safe
