Engineers have cited safety concerns in advising against tree planting along streets, but recent research shows that the rare tree-lined street may be less dangerous than the much more common street lined with parked cars.
"Traffic and highway engineering textbooks describing the 'clear sight-triangle' concept generally show diagrammatic views of intersections indicating sidewalk trees as objects to be eliminated. In the diagrams, trees are represented as solid circles, implying they are solid cylinders going all the way to the ground. This representation is of course unrealistic because street trees typically are trimmed to branch high. And although the intent of the clear sight-triangle is to eliminate physical obstructions from a driver's cone of vision, which operates in a three-dimensional world, the triangle is conceptualized in two-dimensional terms. In reality, the part of a street tree that would intrude on a driver's central cone of vision is the trunk, a relatively narrow vertical element."
"Engineering policy recommendations inmany cities have resulted in vigorous limitations on street trees near intersections but little regulation of other possibly obstructing elements. This reality is of concern for two reasons. First, restricting street trees may not be solving the intersection visibility problem. Parked cars and blocks of newspaper racks can present more of an obstruction to driver's sight lines than do street trees."