Yesterday, in two separate crashes, two women drivers slid on the new gravel on the 30km/h section of road, he said.
Fire, ambulance and police were called to the later crash at about 9am, when a middle-aged Masterton woman lost control of her Suzuki Swift and had to be towed out of the culvert.
The woman was unhurt but was taken to Wairarapa Hospital for a check-up.
There had also been another crash on that section of the road last Thursday, Mr Ashby said.
It was likely the new gravel and speed had contributed to the crashes, he said.
"There was a lot of loose metal and that has been the cause of these three crashes. I suspect all the vehicles were going greater than 30km/h... but there are issues at the corner."
Mr Ashby, who lives on the corner, said there were usually about five or six crashes there every year.
"It's a problem bit of road because people do generally go through there quite quickly."
The recent upgrade had cost $185,700 to date.
Carterton mayor John Booth said roadsweepers were being used on the gravel and the council's roading consultants were investigating the tar problem.
"Our roading consultants have been looking at what's happened and what can be done to lessen or alleviate it. Hopefully they can come up with something before the heat of summer."
It was expected consultants would get back to council by the end of the week, Mr Booth said.