On the first two afternoons of the Frankston fire danger period, grass fires were lit on the median strip in Cranbourne-Frankston Road, Langwarrin, near Josephine Street.
Detective Senior Constable Chris Nieuwesteeg said the first fire started at 4pm on January 23 and the second began at 6.20pm the following day.
During the second fire, cars travelling towards Cranbourne were forced to move into the outside lane as thick smoke drifted across Cranbourne-Frankston Road.
Information about the fires can be phoned to Frankston's crime investigation unit on 9784 5590 or Crime Stoppers on 1800 333 000.
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