The department for transport wants to help the county council make the 12 mile stretch of road, between Petworth and Chichester, safer.
The investment is to be spent in the coming 2017/18 financial year.
A spokesman for the county council said it would be used to widen the road from Halnaker to Upwaltham by half a metre on either side.
Engineers will also install improved audible lining, which makes a vibrating noise to warn drivers they are at the edge of the road if they drive over it.
The changes are aimed at reducing the likelihood of vehicles leaving the road and falling into the verge, lessening the chance of serious injury if an accident occurs.
Work on the scheme is due to start later in the year.
West Sussex County Council’s cabinet member for highways and transport, Bob Lanzer, said: “We are trying to make that road as forgiving as we possibly can.
“By adding half a metre either side and improving the lining, it gives a motorist that critical extra moment to correct themselves and avoid trouble if they make a mistake.”
He added: “This £1 million investment is the first phase of a programme of works to improve safety along the A285.
“We are also pleased to announce we have the opportunity to make further bids to the DfT, as we seek to install these measures along the rest of the road.
“We want to make other safety improvements to some junctions and bends along the A285, and welcome the opportunity to work with the DfT and the Road Safety Foundation to make this road as safe as we possibly can.”
Work has already begun on improving safety at the Eartham Lane junction in Upwaltham.
Vegetation is being cleared and the kerb realigned to make it safer.
The improvements at Eartham Lane are expected to be completed by the end of March.
In November 2014 the stretch of road was named the UK’s ‘persistently highest-risk’ road in a survey by the Road Safety Foundation.
And in a new report last November the foundation said it was still Britain’s most dangerous road.
According to the 2014 report it had seen a 16 per cent increase in fatal and serious crashes over time at junctions, running off the road and even head-on collisions.
Safety measures taken so far had not been not enough to tackle the issues, said the foundation and ‘more far reaching intervention’ was needed.
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