At what speed can cruise control be used?

Can be safe are two diferent things.

Some can be used as slow as 50kmph (30mph) most will not work below 60Kmph (40 mph)

Some will set as high as 160 kmph (100mph)

It is not recomended that you use cruse control in urban trafic, under heavy freeway trafic conditions, poor traction and poor visiblity conditions.

Be safe.