It has only been in the last ten years that we have learned about proper pruning. Proper pruning equals preventive medicine. In a nutshell, we must not create conditions which predispose to disease caused by pathogens. A foremost example is the creation of cavities in which pathogens thrive.
Cavities result from improper pruning i.e. from cutting flush with the trunk when removing branches.
Originally trees were meant to grow in groups. A single urban tree without symbiotic relationship with others will develop large lower limbs and co-dominant branches at a young age, leaving weak ones that can easily break off. Another means to enhance the safety is the use of bolt rod, round washers, amon nuts and cables in the correct way. Legal aspects are e.g. right of way, drive-ways, powerlines, etc. When calling a professional to work on your trees inquire about credentials, insurance and experience.