Northants County Council says it is working with local hauliers after concerns that lorries are not using the official A6 diversion.
With the A6 Rothwell-Desborough bypass scheduled to be closed until at least the end of April, lorries should be travelling via Corby, before taking the A43 and then the A14.
The county council did put up additional signs for road users, but some lorries are still travelling through Rothwell and Desborough.
A spokesman for Northamptonshire County Council said: “Because of the emergency closure to the A6 at Desborough a diversion route has been put in place which takes in the A14, the A43, the A6003 and the A427.
“We would encourage all motorists to use this official diversion as it is on roads of a comparable standard.
“Additionally we are liaising with the road haulage association and all local hauliers about any lorries reported to us not using the diversion.”
The Northants Telegraph witnessed three lorries travelling along Desborough Road in the space of 10 minutes earlier this week.
People in Rothwell say the failure of lorries to follow the diversion is impacting on traffic in the town.
Paul Kites said: “Rothwell gets so bogged up at peak times because no-one follows the diversion.
“But to be honest I don’t blame them, it’s massive.
“The roads can’t cope with all these vehicles and it’s a nightmare.”
Josie Symons said: “I walked along Bridge Street yesterday and it was so full. I have never seen it like that before this.
“There was a lot of lorries that would have normally gone on the A6. We can’t wait for it to re-open.”