Transport Minister Doris Leuthard said on Tuesday that the project was needed to ensure the 16.9-kilometre tunnel linking the canton of Uri and Ticino could stay open, while boosting safety at the same time.
The proposal will be decided on by voters in a referendum on February 28th 2016.
At a press conference with cantonal officials in Bern, Leuthard said the new tube would not, as critics fear, open the door to more truck traffic on the route that links central Switzerland with northern Italy.
Once it is completed, the existing road tunnel, the longest in Switzerland and the third longest in the world, would be closed to traffic for renovations, she said, the ATS news agency reported.
After that is finished by around 2030, the two tubes would each carry only one lane of traffic in each direction, as is specified in the Swiss constitution, Leuthard said.
“The fear of seeing a surging avalanche of heavy trucks is unjustified,” she said.
The alternative to building a new tube would be to close the tunnel for three years with vehicles having to be transported by train in a system that would cost from 1.2 million to two billion francs, Leuthard said.
This would have to be done every 30 to 40 years teach time tunnel needed to be renovated, she said.
Leuthard called the second tube option a “lasting solution” for a crucial road access.
Swiss transport policy is based on shifting as much freight as possible from road to rail to protect the country's Alpine environment.
The government said in a release that the opening of the Gotthard Base Tunnel for trains next year will help rail traffic through the Alps.
The Ceneri Base Tunnel is scheduled to open in 2020.
Leuthard said the Gotthard road tunnel project would not put at risk projects to unplug bottlenecks on the national road network.
The government expects to allocate an average of 890 million francs a year between now and 2030 for such improvements.