Photo courtesy of www.landscapeonline.com
In order to allow a deviation from DOT standards in certain contexts, a series of studies were conducted to evaluate the impact of adding small trees to a road's median. The results, the entirety of which you can read in this report, show the adding small trees to a median doesn't significantly increase crash rates, crash severity, or injury crash rates.
From the study:
It appears that adding small trees to landscaped medians does not have a detrimental effect on safety. Installation of medians and access control as part of a more general increase in access control generally result in a decrease in midblock crashes, but an increase in crashes occurring at intersections where turning movements are allowed, in large part because turns are concentrated at those locations. These increases are a fraction of the midblock gains, resulting in improved safety overall.In other words, adding small trees to a median doesn't have an effect on roadway safety, but the median itself increases overall safety on the roadway--good news for both pedestrians and drivers.