A target means people, vehicles and structures. Therefore, a defective tree in the woods or an open field, or away from paths in an arboretum need not necessarily be considered a hazard. To look for hazardous conditions, inspect each tree systematically. Start by scanning the top, using binoculars if necessary. After reviewing the crown, look downward along the trunk, then carefully examine the root zone.
Some species are simply more brittle than others.
This is one reason why city ordinances sometimes prohibit or discourage trees such as willows, box elders and silver maple. Plant these trees only in open areas. If they already exist on your property, a minimum precaution would be to avoid locating play areas or patios beneath these trees. Sometimes past events warn of potential trouble. For example, previous topping will almost invariably result in weakly attached regrowth.