When lawmakers passed the current move-over law in 2010, Jacobs said, some believed it would be too complicated to include tow truck operators.
"I think they wanted to take it slowly," said Jacobs, who represents Harford and Cecil counties.
State agencies have tried to remind motorists of the requirement, officials said. The Motor Vehicle Administration, for instance, publicized the law on its website and on the monitors at MVA service centers, spokesman Buel Young said.
But Thelen estimated eight out of 10 drivers he stops for failing to obey the law have never heard of it.
On a recent morning, he stopped a 57-year-old motorist on the interstate near Glen Burnie.
"He thought it was a courtesy thing," Thelen said. "He didn't know it was a law."
As Thelen stood on the shoulder of the interstate to hand the man a warning through the window of his white Acura, more cars and trucks rumbled by in the lane right next to him.
Current Maryland law requires motorists to move over to the next lane when they approach an emergency vehicle stopped on a roadside with its lights on, or to slow down if they are unable to move over. Legislation before the General Assembly would impose the same requirements on drivers when they pass tow trucks.