Traffic jams built up in Hadleigh as more than 150 motorists flocked to test how a new Tesco supermarket might affect roads in the town.
But the result was watered down when police sent in a dozen extra officers to help keep the traffic moving.
Six Pcs, two sergeants and four community support officers policed the event, in addition to the officers doing normal duties in the town.
Even so, organisers the Hadleigh Society believe the experiment showed enough to prove there would be congestion at busy times.
The action, dismissed by a spokesman for Tesco as a PR stunt, came in the same week the society announced a major boost to its fight against the proposed new store close to a riverside beauty spot.
More than 90% of people who have so far responded to a survey do not want the supermarket built there.
On Saturday, volunteers staged the 400 or so car journeys – around 200 in and out – that Tesco estimates for peak shopping periods.
At times, queues of more than 20 vehicles built up at the Calais Street junction, and journeys that would normally have taken around six minutes lasted up to a quarter of an hour. Vehicles were also queuing to get out of the Angel Street junction.
Society member Jan Byrne said the effect of the hour-long trial on Saturday would have been worse, but for the police's efforts to make sure it went smoothly.
"I don't think that if Tesco opened there would be that many officers on duty in the town directing the traffic on a normal Saturday morning.
"I've lived here 46 years and I don't think I've ever seen a policeman on point duty before, except during the Hadleigh show."
The society stationed observers at the major junctions, to count the number of cars and see if queues formed.
Graham Panton, who worked out the plan for the event, said: "Tesco estimated approximately 200 cars in and 200 out in the busiest hour.
"That is what we tried to recreate. There were significant queues, and the fact that the police had to step in at all shows there would have been a problem.”
Mrs Byrne said they had been stunned by how many people wanted to take part in the experiment, which she said was not a demonstration against Tesco, but an illustration of what the effect could be.
They had expected around 50 people to turn up at the Babergh car park in Bridge Street but there were three times that many.
A Tesco spokesman described the traffic experiment as a PR stunt which was not really looking at the details of what was involved.
He said a traffic report compiled by professional transport consultants on behalf of Tesco used data from similar developments around the country.
It concluded that traffic would be measured in the town and appropriate steps, like ensuring no delivery lorries went to the store at peak shopping times, would be put in place to avoid congestion.
“This is not about new traffic but people who are already driving to shop somewhere else.
“It is in our own interest to get it right, because otherwise it would make it difficult for customers to get to the store,” he said.
Meanwhile, the first 1,000 responses to a Hadleigh Society questionnaire – delivered to Babergh District Council on Friday – showed a resounding “no” vote to building the new store near the River Brett.
Four thousand forms were sent out, and Mrs Byrne said 93% of people said no.
“We still have another 3,000 to come in but the message is clear – a Tesco store near the River Brett is not acceptable to the people of this town. How much stronger can the argument against it be?
The Tesco spokesman said the results of the survey did not tie in with their own research.
“It would make no sense to open a store that wasn’t wanted. The site was allocated for a supermarket by Babergh District Council after a decision by a planning inspector.”
He said the new store would have a smaller footprint than the derelict industrial building currently on the site.