As representatives of Children’s Hospital Boston at Waltham told the City Council last week that they hope to eventually add 110,000 square feet to the facility on Hope Avenue , councilors and local residents said they are worried about an expansion’s effect on traffic along Highland Street.

The construction, which would not start until the hospital submits a special permit review plan and receives city approval, would bring the facility at the site of the former Waltham Hospital to just under 500,000 square feet.