Unclear sentence

Hello,

Can you clarify the meaning of the following sentence?

"It was clear that if Jane could build another road, there would be a tidy profit to be made in tolls. Another bridge was not possible—it would deny sailing ships access to the Roin au lemi—and ambitious men turned their thoughts to driving a tunnel instead. This was not an easy idea as it might appear."

I have doubts on "turn their thoughts". Does this phrase mean to change one's opinions ? Is it informal?

Thank you