Highways England plans to "increase capacity and reduce congestion" between Worthing and Lancing.
But Adur and Worthing councils say the Â£69m proposal is not fit for purpose and the government body should launch a fresh consultation with updated plans.
Highways England project manager Tom Beasley said the improvements were the "best achievable option".
Large-scale options were considered by Highways England, including dual carriageway schemes with flyovers.
But the consultation focuses a mix of new traffic signalling and road widening on six junctions between Durrington Hill and the Lancing Manor roundabout.
In a joint statement, the leader of Adur District Council, Neil Parkin, and the leader of Worthing Borough Council, Dan Humphreys, said the "modest" plans are not good enough.
Mr Parkin said: "Highways England say they want to consult with us but we say this is a sham. By not allowing the public to weigh up options and see full costings how are we to make any kind of decision?
"All I do know is the current scheme on the table is barely worth the disruption and certainly not worth spending Â£69m on."
Mr Humphreys added: "Highways England do not seem to be taking us seriously. Our questions were met with an 'experts know best' response."
But Mr Beasley said Highways England think that the proposal "would make a worthwhile improvement to people's journeys and is the best achievable option within the scope and budget available for the project".
"We welcome all views and promise that we will take all responses to the consultation fully into account."
Consultation into the proposal ends on 12 September.
A consultation into improvements on the A27 has been dubbed "a sham" by council leaders.