Designers at Columbia University have a plan to convert part of the Cross Bronx Expressway into a sprawling green park.

It involves placing an elevated park above more than 2 miles of one of the city's busiest highways.

"There's a whole cascade of events that happen when you do something like this," says Peter Muennig, a professor of health policy and management.

Benefits would include filtering vehicle exhaust and noise from the air. The park also would include areas for outdoor exercise.

Muennig says the proposed park would cost more than $100 million -- which he says is economically viable.

"It's actually relatively cheap to put a cap on top of a piece of freeway that's already underground," he says. "This is basically like building a bridge, and even easier than that, because it has support on both sides."