a Lowercase "d" per here.
Transport NI is the public body responsible for the upkeep and maintenance of highways and roads in Northern Ireland. It is an executive agency of the Department for Infrastructure, which was previously known as the Department for Regional Development (DRD). Transport NI was formerly known as the Roads Service.
The agency has over 2000 staff and as such employs more people than its parent department. In 2010-11 the agency was responsible for just over 25,000 kilometres (15,534 miles) of public roads, approximately 9,500 kilometres (5,903 miles) of footways, 5,800 bridges, 265,000 streetlights and 370 public car parks.
Transport NI was part of the Department of the Environment but was transferred to the then Department for Regional Development (DRD) upon the latter's creation in 1998. In 2016, DRD became the Department for Infrastructure.