This can happen with a cigarette flicked out a window, or with other ignition sources like "deposits that can come from the exhausts of diesel vehicles", Mr Howell said.
"But cigarettes are the big one."
A ditch full of water will not prevent fire spreading, Mr Howell warns, as fire will travel through the top of the grass and through a boundary fence on to a farmer's property.
The long grass can be either sprayed, mown or grazed by the farmer.
"Some farmers often fence and graze it," Mr Howell said.
"I try and promote that people get out and reduce that risk along the roads."
The build-up of long grass, roadside and otherwise, was a fire hazard that had eased somewhat after three days of rain from December 30 to January 1, Mr Howell said.
However, a forecast northwesterly wind this week will dry out the grass - now even longer - all over again.
"Surface fuels like that grass dry out extremely quickly," Mr Howell said.
Wairarapa is still in an open fire season but Mr Howell said those lighting outdoor fires should take precautions.
"Have a fire break and a supply of water handy," he said.
"If you have some questions phone us and we can help you, or even come out."