Network Rail has apologised to furious motorists and admitted that a 17-mile diversion put in place to repair a bridge is "disruptive".
Despite repairs taking place in 2010 and 2014, the Nazeing New Road bridge beside Broxbourne railway station in Hertfordshire has deteriorated to the point it needs to be replaced.
The existing bridge sits over the West Anglia Main Line railway that carries trains between London and Hertfordshire, Essex and Cambridgeshire.
A route set up that diverts around the closed road stretches for 17 miles, with the reopening not set to happen until 30 April.
The diversion route will send motorists through Hoddesdon, Harlow and Nazeing, in Essex, via the A414.
Following criticism of the four-month road closure, Kate Snowdon, head of communications at Network Rail, said the diversion was "exceptional" but insisted the work was necessary.
She told BBC Essex: "We completely accept it will be disruptive.
"It is an exceptional diversion route and I think what is really difficult here, is if you're going to demolish a bridge, you shouldn't run cars over it – it's just impossible and the bridge really does need to be replaced."
She added: "I understand the disruption it's causing but we do try and give people at least lots of prior notice so they can plan for those journeys."
Snowdon said the unusual length of the diversion was in part because of the awkwardness of the River Lea, and was designed so that it was suitable for HGVs and buses.
Apologising for the disruption, Ellie Burrows, Network Rail’s route director for Anglia, said: “I’m sorry for the disruption that this work will cause to residents and rail users but if we don’t do this work now, it could lead to a much more disruptive closure at short notice."
Rail users will also be affected by the work to the bridge, with replacement services in place.
This section of the West Anglia Main Line, connecting London with parts of Hertfordshire, Essex and Cambridgeshire, is due to close from 13 January until 18 January while the central bridge spans are replaced.
Passengers have been advised to check before they travel.
The new structure is designed to last 120 years when it eventually opens in four months.