Yes, road signs on turnpikes should be in both miles and kilometers, and speedometers should provide both mph and kph. The reason being safety: having to mentally convert between the systems is an unnecessary distraction for a driver who is attempting to keep a ton or more of lethal machine under control at high speed.
Where immediate safety is not a concern, let the vendor choose between the two systems. Most people these days can pull out their cell phone or pocket calculator and do any conversion they require. The few who cannot do this probably need other help in managing day to day activities so they can just ask someone how many kilograms are in a quart of blueberries.