LANSING, MI -- Deficient roads and bridges cost Michigan motorists nearly $8 billion a year in aggregate costs, according to a report released Tuesday by Trip, a national transportation nonprofit that advocates for state and national infrastructure funding.

Here in Lansing, driving on deficient roads costs the average motorist $1,032 a year, according to the report, which considers lost time and gas due to congestion, traffic accidents and annual vehicle operations costs including repairs.

The estimated cost is $1,600 per motorist in Detroit and $1,027 in Grand Rapids. The report pegs the total cost for all motorists in Michigan at $7.7 billion a year.

"When you're out there driving on rough roads and feeling all those vibrations, it's putting additional stress on your vehicle," TRIP's Rocky Moretti said at a press conference in Lansing, where he was surrounded by pictures of potholes and cracked bridges. "Those lead to additional costs and shorten the life of your vehicle, meaning you're going to be back in the showroom sooner."

Overall, 29 percent of Michigan roads are in poor or mediocre condition, according to the report, which indicates that major roadways in Detroit, Lansing and Grand Rapids are in even worse shape.

Department of Transportation Director Kirk Steudle said the report makes clear the need for additional infrastructure investment in Michigan. Gov. Rick Snyder has pushed for new funding each of the last two years, but while lawmakers have appropriated one-time money, they have not settled on a long-term solution.

"You may not want to pay more for gas tax or registration fees, but you're already paying these costs," said Steudle. "We can all continue to pay this hidden cost, or we can pay to fix the roads."

MDOT abruptly closed a well-traveled bridge on University Drive over I-75 in Auburn Hills this weekend after an inspection revealed a cracked beam. Crews are installing temporary supports with hopes of re-opening the bridge by Friday, but Steudle predicted that such band-aids will become increasingly common.

"Think of those as crutches," he said of the supports. "That is just the tip of the iceberg. If we don't deal with these bridges, we're going to see more and more closures. What you're going to see is barricades, more and more of them."

Roughly 12 percent of Michigan bridges are structurally deficient, according to the TRIP report, meaning there is significant deterioration to major components. Another 15 percent are considered "functionally obsolete" because they do not meet current design standards.

While much of the state-level debate over infrastructure investment has focused on MDOT funding, nearly three quarters of Michigan roads are maintained by local agencies also struggling under the current funding formula, according to Denise Donohue, director of the County Roads Association of Michigan.

"Our local roads are in very sad shape," Donohue said, pointing to other assessments showing that over 50 percent of local roads are in poor condition, while only 20 percent are in good condition. "The people of Michigan and visitors to our state deserve better than that."

Michigan would need to spend an additional $1.4 billion a year to to keep most roads and bridges in good or fair condition, according to a bipartisan report released in 2012, which noted that costs would continue to grow without action.

Snyder proposed raising $1.2 billion in long-term funding through increased gas taxes and registration fees last year, but the budget he ultimately signed into law included only $350 million in one-time funding.

House Speaker Jase Bolger has said he does not anticipate a "net revenue increase" for roads this year. Instead, House Republicans want to "prioritize state spending to build on last year's additional investment in roads and bridges," according to an action plan released this month.

Snyder discussed road funding again in last week's State of the State address, but he spoke only in general terms and did not propose any specific policy. He is expected to unveil his executive budget recommendations in February.

"We did not achieve comprehensive transportation reform," Snyder said Thursday. "We do need to invest more in our roads, and I think we've seen that. We've entered pothole season now, folks."

TRIP is funded by insurance companies, labor unions and businesses involved in highway engineering and construction, according to the nonprofit's website.

Read the report: Future Mobility in Michigan: The Cost of Meeting the State's Need for Safe and Efficient Mobility

Jonathan Oosting is a Capitol reporter for MLive Media Group. Email him, find him on Google+ or follow him on Twitter.