The number of drivers speeding past Oxfordshire's deactivated speed cameras has increased by up to 88%, a road safety partnership claim.
Thames Valley's Safer Road Partnership monitored drivers on two roads in the county for five days.
The cameras were axed at the end of July after the county council withdrew £600,000 in funding due to budget cuts.
Road safety charity Brake said it was "extremely concerned". The council said it was too early to draw conclusions.
The tests were carried out in Watlington Road, Cowley, and the A44 in Woodstock from Thursday till Monday.
In Cowley, 62 people were clocked speeding, representing a rise of 88%, the partnership revealed.
In Woodstock 110 drivers were caught doing more than the 30mph limit, which is 18% more than the average for 2010.
Although the coalition government ended central funding to pay for the speed cameras, it was the council's decision not to find funding from elsewhere.
Keith Mitchell, leader of Oxfordshire County Council, said the test had only been completed at two sites out of 72 over five days.
He told BBC Radio 4's PM programme: "You need a much longer look at the statistics before we can decide whether there is an increase in danger or not.
"Unless the funding from government comes back, I think there is little chance of us being able to fund this, relative to the other priorities we have."
Those caught speeding during the trial will not receive fines or points.
Insp Paul Winks, from Thames Valley Police, called the results "disappointing".
He said: "It clearly means switching off the camera has given a green light to a small number of people to break the law.
"The consequence is more death and more death is unacceptable."
Ellen Booth, campaign officer for Brake, said: "This is people's lives we are talking about.
"What we would like to see is councils looking at the issue of speed cameras, not only at how effective they are in reducing death and serious injuries, but also how cost effective they are."