Hammerer: The signs that indicate a slower speed are recommending the safe speed in wet conditions. If you are driving on a dry road then you should be able to go a lot faster. I typically add 50% to the recommended speed when on a dry road in good condition. So if the sign says 70 I can easily drive at the open road limit of 100kmph.
Just for the record the recommended speeds are a safe and comfortable speed for light vehicles in dry conditions.