Roadside design guidance typically instructs agencies to limit fixed objects, including trees, along roadways to provide a safer recovery area for errant vehicles. On urban streets, trees are amenities that can provide benefits to pedestrians, bicyclists, residents, and others such as shade, potential traffic calming and speed reductions, air and water quality benefits, and aesthetic appeal.  However, trees are fixed objects that can cause serious injury or fatality if struck by an errant vehicle.  Public agencies need more refined data on the effects of trees on safety to inform tree planning and landscaping policies, procedures, and practices that support the safety of all roadway users. 

The objective of this research is to answer key questions on the impact of trees necessary to implement a Safe System approach to the design and maintenance of urban and suburban roadways with posted speed limits of 35 mph to 50 mph. Answers to the following questions are needed, broken down by speed limits ideally in 5 mph increments (i.e., 35, 40, 45, and 50 mph):

• What impact do trees, based on tree characteristics (size, species, placement), have on operating speeds in urban and suburban contexts?  
• What is the safety performance (crash frequency and severity) related to tree impacts?
• What are the trends and risk factors in the crash history (outside of curves, median or outside edge, etc.)?
• What are the roadway context characteristics at tree crash sites resulting in fatalities or injuries (e.g., presence of on-street parking, presence of on-street bike lane, presence of lighting, density of trees, proximity to travel lanes, etc.)?
• What is the frequency and severity of crashes on roadways with trees to similar roadways without trees?
• What are the safety effects of street trees on drivers, motorcyclists, pedestrians, bicyclists, and other users of the roadway?
• How do tree characteristics (size, species, placement/lateral offset) impact the frequency and severity of crashes?
  

The anticipated deliverables for this research project are an abbreviated literature and synthesis report on the effects of trees in urban and suburban contexts and a final report on the research results.