The Federal Roads Office is working on a plan that would see a major widening of the autoroute between Perly and Vernier just after the Bardonnex border.
The route is often jammed at rush hour with thousands of frontaliers heading in and out of Switzerland.
The aim is to build three more lanes, extend two tunnels and dig another.
64,000 m2 of forest would have to be cleared.
The plan has yet to be given the final go-ahead, but the Confederation and the canton are backing the idea.