The nationwide tally of sharp-edged pieces of metal stuck into road guardrails came to 38,000 as of Wednesday, the Ministry of Land, Infrastructure and Transport said.
The ministry held the first meeting of a panel to examine how the sharp protrusions came about and suggest preventive measures.
Hiroaki Taniguchi, the ministry's road bureau chief, said, "Vehicle crashes are believed to be the main cause, but we have not yet reached a conclusion."
He asked the panel to come up with an overall picture by the end of next month.
There have been several reports of cyclists, pedestrians, including joggers, being injured by the metal protrusions.