Impact Attenuation by Tall Herbaceous Vegetation, Shrub Masses, and Low Branches of Trees

Focus Area

Construction and Maintenance Practices


Environmental Process, Natural Resources







Research Idea Scope

Run cars and trucks containing crash dummies off simulated highways into tall native herbaceous vegetation within the clear zone, masses of native shrubs within the clear zone and into low horizontal branches (uncut, naturally terminus of many small twigs)of trees that extend into the clear zone.  (It might be useful to consider vegetation native to different regions: northeast, mid-Atlantic, and midwest; southeast; northwest; southwest; but probably not essential if cost will be too high.)  Determine vehicle responses and damage to crash dummies as a result of these tests.  Determine if impacts with this type of vegetation is just as safe as impacts with guiderails and sand barrel systems. Measure impact attenuation due to collisions with shrub masses and low horizontal branches…Is it just as good as guiderail and sandbarrel systems?

Urgency and Payoff

This research, if successful, could help stop a planting and vegetation management and maintenance practice that is visually very unattractive and detrimental to the ecosystem.  At least several DOT’s (KY, PA, NY)appear to be mowing the entire clear zone, are refraining from planting any tall herbaceous plants or shrubs in the clear zone, and are systematically removing all low horizontal tree branches from the clear zone, apparently in an effort to make the highways safer.  This results in lines of tree trunks at the edge of the clear zone that have been stripped of all branches reaching toward the highway, and no shrubs or tall herbaceous plants at all within this zone, generally 30 feet from the edge of the nearest travel lane. This is visually very unattractive, and greatly reduces the land area that could be part of a natural native ecosystem.  Trees are healthier when their trunks are shaded by their own branches.  Strong sun on the trunk causes sun scald and frost cracking. Native shrubs at the edge of a wooded area provide food and shelter for many bird and mammal species.  If it can be shown that shubs and low horizontal tree branches slow down vehicles and reduce the chance of occupant deaths (or at least do not increase the chance of occupant deaths, we will have a good argument to eliminate this unappealing planting and maintenance strategy.

Suggested By

Holly Frey, NYSDOT