...you can't get in trouble if it's an app on your phone. You would only get in trouble if you had a separate device meant exclusively for speed camera warnings, which you don't have.
That depends on the country you live in. E.g. in Germany it is forbidden to use any system that gives you the information about speed cameras near by while driving. This includes hints in navigation systems. So you are allowed to have such an app on your smartphone installed, but you are not allowed to use it.