The word "improvement," as used in this act, shall mean and include any highways or streets connecting with, or used as direct approaches to, bridges crossing streams or tunnels under streams, which form boundaries between this and any other state, together with viaducts carrying such highways or streets over other highways or streets or over other property, and any ramps and approaches and other structures and superstructures in connection therewith.
53 P.S. § 13801