Transport officials are refusing to identify sites in south east Cornwall where motorists will be stopped for a traffic survey. A similar survey near Plymouth was criticised by motorists for causing traffic jams and delays.
The Highways Agency says there will be a programme of roadside interviews at strategic locations on the A38 in June to assess journey patterns.
The results will be used for the �20m Dobwalls bypass on the A390.
The agency says the reason it will not reveal the locations and dates is to prevent motorists avoiding the area.
It is asking in advance for drivers' co-operation and patience.