A 40-billion boost to the South West's economy could be lost, if the A303 isn't upgraded.
That's according to Devon County Council - which is working with other authorities to lobby for the changes - saying the plans could create 21,000 new jobs.
The council has delivered evidence at a public examination looking at proposals to improve the A303 by constructing a tunnel at Stonehenge.
There has been concerns improvements might not take place after The National Audit Office suggested the proposed tunnel at Stonehenge may not offer value for money.
It was suggested that without the tunnel then other improvements along the route could see less of an impact.
Councillor Andrea Davis, Cabinet Member for Infrastructure and Development, told the Planning Inspectorate hearing that improving the A303 and the A30 and A358 'swallow-tail' was critical to the future prosperity of the South West.
She said: "The South West desperately needs a safe, reliable alternative strategic route to London and the South East. Poor connectivity is harming our economy and leaves us vulnerable to severe weather disruption.
"The M5 clearly cannot cope but the A303, A30, A358 route isn’t the efficient alternative it should be.
"It is unsafe and unfit for the 20th century let alone the 21st. The notorious hold-ups that plague the route are a brake on business growth. Its sheer unreliability means South West companies have no idea how long it might take to transport their products.
"However, our studies show that improving journey times, making the whole corridor safe and reliable, will deliver a massive boost to the South West economy and a huge return on investment.
"Tackling the tailbacks at Stonehenge in a way that is sensitive to the World Heritage Site is the critical element in achieving that goal."