Dumbaugh 2005 - "Safe Streets, Livable Streets"
While conventional wisdom recommends limiting roadside hazards, such as trees, and increasing lane and shoulder width will reduce the number and severity of crashes where the driver leaves the roadway, this author posits that trees and narrow lanes encourage drivers to drive more slowly and carefully, reducing the total number of crashes.
The author cites a few studies, including on in Washington on an urban/rural arterial (HWY 99?), where the presence of trees, sign supports, and other fixed objects is associated with fewer total crashes while wider lanes and shoulders are associated with more crashes. The author then conducts his own study comparing different sections of the same roadway, finding similar results.
I'd say that the numerical results in this study are a little squishy, the they and the theory are nonetheless very compelling.