Maryland Transportation Secretary Pete K. Rahn dangled a $9 billion carrot Wednesday in front of more than 100 companies potentially interested in adding express toll lanes to the Capital Beltway, Interstate 270 and the Baltimore-Washington Parkway.

"It's amazing," Rahn quipped to more than 320 highway engineers, designers and builders in a ballroom at the BWI Airport Marriott in Linthicum. "You can get a lot of attention when you put a nine in front of a 'B.'"

Officially, the Maryland Department of Transportation's industry forum was intended to provide companies more details about the plan to add four toll lanes each to some of the Washington region's most traffic-clogged highways. But as Maryland officials eyed the ballroom filled with business executives tapping on laptops and jotting notes, they also saw potential. It seems Maryland's proposal to build $7.6 billion of the toll lanes via a public-private partnership has prompted a slew of firms to at least consider competing for the job.

State officials say it would be the biggest partnership of its kind in the United States, and several attendees said Maryland has one of the most massive projects up for grabs in the highway construction industry.

"I don't want to just do something," Rahn told the crowd. "I want to do something that delivers a 'wow'" for traffic relief.

One big unknown: the cost of the tolls. After the meeting, Rahn said he was aware variable rates on toll lanes that opened last week on part of Interstate 66 in Northern Virginia had soared to as high as $40.

"Obviously, that caught my attention," Rahn said.

However, he noted the I-66 toll has since peaked at about $14. He said Maryland will know the range of toll rates on its highways just before it chooses a winning proposal. Like Northern Virginia's toll lanes, there will be no limit on how high tolls could go to keep traffic moving.

"Clearly an exorbitant toll means people won't use it," Rahn said after the meeting. " . . . These toll operators understand they need to provide a reasonable toll to have adequate users."

The regular lanes will remain free.

The state is seeking a team of companies to design the lanes, finance their construction, build them and operate them long-term in exchange for collecting the toll revenue. The public-private partnership will be used to add toll lanes to Maryland's 42 miles of the Beltway and 34 miles of I-270 between the Beltway and Frederick.

The state plans to finance and build the 29 miles of toll lanes on the Baltimore-Washington Parkway, which are estimated to cost $1.47 billion, assuming the federal government agrees to transfer it to the state.

The state is not expecting to pay anything on the Beltway or I-270 projects, Rahn said.

State officials said they plan to solicit proposals in 2019 and select a winner in 2020.

Rahn said the state will leave details of the design to bidders in hopes of receiving the most creative ideas. Maryland officials said proposals also will be judged on how the teams plan to build the lanes quickly, keep traffic moving during construction and limit impacts on adjacent homes and businesses.

Between MDOT's presentations, the ballroom buzzed with people exchanging business cards and chatting in small groups. Some attendees said companies were feeling each other out, sizing up possible bid partners and competitors. Maryland officials said the event drew company representatives from across the United States and the world, including Australia, Spain and Denmark.

"Normally at these events, you know everyone in the room. This time, I barely know a quarter," said John Huchrowski, of Prime AE Group, a Baltimore engineering firm. "It's brought out the big guns."

Simon Reuterswärd, a project manager in a local office of the Swedish construction giant Skanska, said his company is considering whether to compete for the project.

Asked to put the potential contract into perspective, he said, "For any company in this room it would be a big deal, absolutely, very big. You can see the attention it's getting. It's quite remarkable."