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.