Nobody on the panel of experts could precisely predict where more than $2 billion in needed annual funding will come from if state legislators reject Whitmer’s controversial plan to substantially increase the gas tax.

Her proposed tax hike has met stiff resistance from the GOP, as Senate Republicans moved forward this week on a 2020 budget with no tax hike.

Related: Watch video, read quotes from experts at our Detroit roads summit

Further undercutting its chances, a recent statewide poll found 75 percent of residents oppose the suggested tax hike.

Senate Republicans say they won’t release an alternative to Whitmer’s plan until summer. House leadership called Whitmer’s plan a “nonstarter,” and have set no timetable to release a plan of their own.

Jeff Cranson, communications director for the Michigan Department of Transportation, told the audience it’s a tough political environment in which to sell such an ambitious funding plan.