In this study of New York City streetscapes, trees made people feel safer than building-related variables, and tall, narrow streetscapes made people feel safer than short, wide streetscapes. More buildings per length of streetscape increased perceived safety, and streetscape design affected perceived safety more than urban form or affluence.
Harvey, Chester, Lisa Aultman-Hall, Stephanie E. Hurley, and Austin Troy. (2015). Effects of skeletal streetscape design on perceived safety. Landscape and Urban Planning, 142, 18-28.