Fears have been raised over the safety of the viaduct which would carry the proposed £251m Norwich Western Link over the River Wensum.
Critics say if a vehicle breaks down on the mooted 500-metre-long bridge, people in the vehicle would not have anywhere safe to go.
But leaders at Norfolk County Council hit back, saying the viaduct would comply with national design standards.
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Safety concerns were raised by Green county councillor Jamie Osborn at a meeting of the Conservative-controlled cabinet.
Mr Osborn said: "Norfolk Constabulary advise that in the case of breakdown, passengers should get out of the car and seek refuge on the verge beyond the safety barrier.
"Yet the Western Link viaduct proposes no verges and no refuge zone behind the safety barrier.
"Does the cabinet member believe there will not be any breakdowns on the viaduct, or does he believe that the advice from Norfolk police is unnecessary?"
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But Graham Plant, cabinet member for highways, infrastructure and transport said the design would be similar to the A47 Southern Bypass viaduct over the River Yare, near Whitlingham.
He said it would be subject to an independent safety audit and the viaduct would be designed to national design standards.
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Mr Plant said: "If, in the very rare likelihood of there being a breakdown on the viaduct, the general advice would be to keep to the left and travel to a place of safety before stopping.
"Beyond the extents of the viaduct there is a 2.5m verge. On the viaduct itself there is
a minimum of a 0.6m hard verge and a 1m hardstrip at the edge of the carriageway
that can be used in an absolute emergency."
The county council is waiting to learn if the government will back the business case and bankroll 85pc of the cost of the road, connecting the Northern Distributor Road to the A47, west of Norwich.
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