Speeding was fingered as a possible cause of what is believed to be Japan's most expensive ever road accident when up to $4 million-worth of supercars ended up in a crumpled heap on a highway.

Eight Ferraris and a Lamborghini -- plus a Toyota Prius -- were among the vehicles involved in the crash, which witnesses said happened when a speeding car slid across a wet road surface.

A pack of about 20 supercars was travelling in convoy on Sunday morning on a stretch of wet highway when the leading Ferrari slid into a guardrail. Those behind slammed on their brakes, but for many of them it was apparently too late.

Those behind slammed on their brakes, but for many of them it was apparently too late.