Traffic will be brought to a grinding halt on key Melbourne roads if Opposition Leader Matthew Guy's "congestion-busting project" materialises, top infrastructure experts warn.

Former directors at Victorian infrastructure and planning departments say Mr Guy's plan to rip up congested intersections in middle and outer Melbourne suburbs will lead to hundreds of property acquisitions, create a "physical barrier" within neighbourhoods, and double congestion on arterial roads.

One month ago, Mr Guy promised if the Coalition wins government next year it will replace 55 of Melbourne's busiest road intersections and roundabouts with an underpass or overpass at a cost of up to $5.3 billion.

But the approach is "fundamentally flawed", according to 23 of Melbourne's most senior private sector traffic engineers, planners and architects. The group includes advisors to the Office of the Victorian Government Architect, a former Planning Panels Victoria member, and the Victorian president of the Planning Institute.