Decay is common cause
Regular visual inspections, even by trained arborists are not foolproof, said an arborist with over a decade of experience.
"It might be difficult to assess from the ground if branches of a tree are decaying, even by an experienced arborist, as not all checks are done at the branch level," said the tree expert, who is in her 40s. She declined to be named.
She said that the most common reason for falling tree branches is decay, which could be caused by water-logging, insect infestation or tree disease.
However, inspections are still useful because there might be tell-tale signs, such as falling leaves or crack lines, that might indicate a weak branch.
Mr Goh Mia Chun, 36, a consulting arborist with CSK Landscape Services with more than 10 years' experience agreed.
He added that poor species selection could be a cause for tree failure.
"The angsana tree is one of the species that has a high rate of branch failure," he said, although he maintained that the angsana is still one of the better species to be planted by the roadside.
This is because of its sturdiness, ease of maintenance and generous shade it provides.
Mr Goh was engaged by JTC Corporation in 2011 to assess the impact of the construction of the Hyflux building on the surrounding trees. He remembered the angsana trees being in good condition then.