There was a reduction in the number of accidents by around 50 percent when 41 new cameras were installed in the mid 2000s.
However, the research found the number of accidents in the county compared to the number of crashes recorded near camera sites continued to fall despite the switch off.
Director of Road Safety Analysis, Richard Owen, said not covering the cameras was important as many drivers may believe the devices were still working.
The boxes continue to be regularly maintained despite not working which may have created an illusion the devices were still actively in use.
He said there was still “doubt” in many motorists minds that the cameras could still be working before conforming there was not an increase after the devices were turned off.
He told the Northamptonshire Telegraph: “You’d like to think people slow down for the cameras.
“Even though it’s been well-publicised that they are off, there’s still that doubt that they might be on.
“When they were turned off we kept the camera in them to look at data and, actually, there wasn’t really an increase in those caught.”
Mr Adderley defended the use of mobile speed cameras during the coronavirus pandemic after residents complained safety vans were deployed in key areas.
The constable warned motorists were using Northamptonshire’s roads like a race track under the lockdown.
He said: “My stock answer to them all is simple really: Don’t speed, don’t drive recklessly and you won’t need to worry about the camera safety vans or patrolling officers.
“We advertise where the vans are likely to be, it’s not like we’re trying to hide them. Plus, it’s only a money machine if you feed it.”