 Adjacency of international boundaries may also prohibit ring road completion in other cases.Construction of a true ring road around Detroit is effectively blocked by its location on the border with Canada; although constructing a route mostly or entirely outside city limits is technically feasible, a true ring around Detroit would necessarily pass through Canada, and so Interstate 275 and Interstate 696 together bypass but do not encircle the city.Sometimes, the presence of significant natural or historical areas limits route options, as for the long-proposed Outer Beltway around Washington, D.C., where options for a new western Potomac River crossing are limited by a nearly continuous corridor of heavily visited scenic, natural, and historical landscapes in the Potomac River Gorge and adjacent areas.