The people of Brisbane should demand the Queensland government widen the Western Freeway in next month's state budget to make the $1.5 billion Legacy Way tunnel at Toowong work more efficiently, Queensland's biggest engineering firms say.

Queensland Major Contractors Association president Iain Ward said widening the Western Freeway could be easily brought forward and would deliver immediate benefits for Brisbane's western suburbs.

"Look at Legacy Way; you can get onto it easily from the Inner City Bypass and yet it puts you out into the Western Freeway, which in the afternoon is a very congested piece of roadway," Mr Ward said.

"Legacy Way is a very efficient piece of infrastructure, but you get dropped off in at the Western Freeway and you come to a grinding halt," he said.