How to give a number describing a "kilometer per hour speed limit"?
Firstly, for the example you provided, you may say 那条路限速50英里 then I will know it is 50 miles/hour

secondly, in China the default unit is kilometers per hour for speed limit, so if you want to say "that road has a 50 km/h speed limit" you can just say 那条路限速50