Over the past couple of construction seasons, the Minnesota Department of Transportation has rebuilt portions of Interstate 35 between Forest Lake and Duluth and repaved others.

It took a lot of time and cost a lot of money, but the result is a smooth ride for motorists traveling between the Twin Ports and the Twin Cities and should serve them well for years to come.

Unfortunately, many motorists on other state roads find them in need of a similar makeover. MnDOT will get $18 billion over the next 20 years to do that. While that sounds like a whole lot of cashola, a recent report found that the agency will need $30 ­billion “to keep pace with Minnesota’s ­growing population and aging infrastructure.”

Minnesota ranks 38th in pavement quality, and more than half of the state’s highways and 35 percent of its bridges are more than 50 years old. At that age, things need some TLC.

Without the extra $12 billion, the “Assessing Return on Investment in Minnesota’s State Highway Program” report put out by the Transportation Finance Advisory Committee said there will be serious ramifications. Over the next two decades, the amount of pavement in poor condition will rise by 13 percent and more than 200 bridges will be in poor condition. For motorists, that translates into slower travel times, increased safety issues and higher vehicle operating costs. The report also said congestion will worsen in the Twin Cities and “MnDOT will have little ability to address local ­concerns or add capacity.”

Transit advocates are seeking $20 billion to help build out a ­transportation network.

That money would be used to pay for such projects as extending the Blue Line from downtown Minneapolis to Brooklyn Park, adding a Bus Rapid Transit (BRT) Line on I-35W and additional BRT or streetcar lines on such thoroughfares as Lake Street, Chicago Avenue, W. Broadway in Minneapolis and on W. 7th and Robert streets in St. Paul.

In recent weeks, MnDOT Transportation Commissioner Charlie Zelle and Met Council Chairwoman Susan Haigh have been touring the state together to make their case for more funding to and ask residents and business owners what they want to see in a transportation network.

In town hall meetings, including one from 4 to 6 p.m. Tuesday at Burnsville High School, their message in a nutshell is pay now or pay later.

The money for roads could come from raising the gas tax (last done in 2008) and for transit by adding a metro-wide half-cent sales tax. Both would need legislative approval.

The metro area’s population is expected to grow by 900,000 people by 2040, and that will further strain existing infrastructure. With the additional funding you could see more MnPass lanes and upgrades to key freeways such as I-94 between the Twin Cities and North Dakota.

Without more cash, Zelle said the state will struggle just to keep roads structurally sound. Transit helps improve mobility and stretch infrastructure further, Haigh said, noting that transit riders make 250,000 trips a day, relieving congestion and helping to extend pavement life cycles.

“If we don’t invest, this [the current roads and mass transit system] is what we will have,” Haigh said.

Follow news about traffic and commuting at The Drive on startribune.com. Got traffic or transportation questions, or story ideas? E-mail drive@startrib une.com, tweet @stribdrive or call Tim Harlow at 612-673-7768.