Tree Risk Assessment
The Tree Risk Assessment domain tests whether you can inspect a tree, identify and rate defects, decide whether a target is present, and convert those findings into a defensible risk rating and mitigation recommendation. TRAQ separates the likelihood that a tree or part fails from the likelihood that it strikes something - candidates lose marks when they collapse the two.
The risk equation
Risk = likelihood of failure × likelihood of impact × consequences. A tree with an imminent defect but no target is not a high-risk tree in TRAQ terms; conversely a sound tree over a busy playground can warrant a moderate rating. Rate likelihood of failure as improbable, possible, probable, or imminent; rate likelihood of impact from the target's occupancy (frequent, occasional, rare) and the fall path; combine them to get the likelihood of impacting a target, then combine that with consequence severity (negligible to severe) for the final low/moderate/high rating.
Assessment levels
Level 1 is a limited visual inspection from a drive-by or walk-by; it can only screen. Level 2 is a 360° basic visual assessment from the ground, with a sound root flare, trunk, scaffold limbs, and canopy. Level 3 adds advanced tools - aerial inspection, resistograph, sonic tomography, root excavation - used only when the Level 2 findings justify the cost.
Worked example
A 20 m oak has a codominant union with included bark directly over a car park used daily. The union is cracked and moving: failure probable, possibly imminent. Occupancy is frequent, so impact likelihood is high. Consequence is significant (occupied vehicle, possible injury). The combined rating is high; recommend immediate target exclusion plus cabling and reduction pruning, or removal - with reassessment.
Traps
- Rating a defect without naming a target.
- Calling every hollow "high risk".
- Treating Level 1 as a full assessment.
- Forgetting to record assumptions, limitations, and residual risk after mitigation.