Protests are mounting over shock proposals to close the A371 – a major route between Shepton Mallet and Castle Cary – from Monday until Christmas.
Strengthening and repair works are needed at Ansford Bridge near Castle Cary railway station.
The closure means 20 mile detours for drivers and hundreds of commuters will lose access to the station from the south.
Cars will be diverted from Castle Cary westward along the B3153 to the A37 at Lydford-on-Fosse, up to Shepton Mallet and back down the A371 to the station.
The road closure will also affect three major milk and cheese factories.
Nick Haggett, director of Peter Green Haulage Ltd in Evercreech, said they were extremely concerned about the lack of notice and the effect the closure will have on transport costs for companies.
The low bridge on the A37 at Lydford – the alternative suggested route – means that route is shut to large lorries.
From the A303 truckers will have to go via Sparkford, Podimore, Somerton and Keinton Mandeville before getting on the A37 at Lydford to reach Shepton Mallet.
Residents fear drivers will try to find shorter routes on narrow country roads through Bruton, Evercreech and Ditcheat.
Publicans at the Alhampton Inn, the Brook House Inn and the Natterjack Inn say customers will not bother to use the huge detour to get to them and their pre-Christmas trade will be hit.
Network Rail has issued an advance apology for the disruption to Castle Cary and Ansford residents, businesses and station passengers.
Robin Basu, Network Rail's community relations manager for Wales & West, said: "This is obviously going to have a big impact on residents, businesses and passengers, who will have to go on a long diversion in order to reach the station.
"Separate diversions will be in place for car and heavy goods vehicles to ensure minimal disruption and weekly updates will be made."
Mr Basu added: "We will be working hard to minimise disruption and would be grateful if local people, businesses and passengers can bear with us while this work is taking place.
"We are expecting seven-day working where possible and will try to complete the strengthening as quickly as possible."
John Baker from Network Rail Media Relations said they were doing everything they could to minimise disruption for residents and users.
"But the work is vital – we have to close the road, and we cannot apologise enough." he said.
People with concerns can contact Network Rail's 24-hour national helpline on 08457 114141 or email on crwest@network rail.co.uk.