Councillors approved the details of the eastern section of the route from Flansham to Shripney. It is likely to open in 2010/11.

The 2.5km single carriageway route will include four roundabouts – at the A29 and A259 and two in between – cross four public footpaths and an unmade track and take motorists over two bridges across the railway branch line to Bognor and the Lidsey Rife.

It will cost £16.5m which will be funded by the two developers of the 700 homes being built to the south of its route.

But, as councillors backed the road, a warning was given that it is expected to be full within 18 years.

Cllr Ricky Bower told Arun District Council's development control committee: "I still maintain that this road is not of an adequate standard for a long life.

"The last time we did an assessment it showed the capacity will be reached by 2026. Where we go beyond that I don't know. Perhaps, there will be another relief road at some point. We will see."

Committee chairman Cllr Jean Goad said: "One of my concerns is the capacity of the road."

But county council development planning manager Spencer Bryant said driving habits could change in the coming decades. Measures would probably be taken to make other forms of transport more acceptable and motoring less attractive.

The vote by the committee will take effect once Arun planning officer Stephen Cantwell has settled a range of outstanding matters with the developers. These include landscaping, drainage, footpath and cycle path designs with cyclists being able to pedal along the road's southern boundary on a designated route.

He said: "This road has been a long-held aspiration."

His detailed presentation covered matters concerning the road and noise, bridges, footpaths and cyclepaths, junctions, flood risk, drainage, ecology and Hoe Lane.

Among the issues yet to be finalised is the road's speed limit. Originally set at 40mph it could be upped to 50mph if talks between the county council and the developers reach agreement on the design changes needed to enable motorists to go faster.

Cllr Andy Faulkner, of Yapton Parish Council, said his councillors felt that they had not been given sufficient time to consider the proposals under debate. "In view of the scale of this development, it's extremely important we have the opportunity to suggest any amendments we feel will benefit the local community. It's essential we get this right."

Graham Beck, on behalf of developers Barratt Homes and David Wilson Homes, said more than 100 documents about the road plans had been submitted to the district council.

Talks about them had taken place with its officers for nine months as well as discussions with the surrounding parish councils. The chance to build over the railway, especially, could only take place during a specified period with Network Rail.