The vertical space, horizontal space, or weight available to modes of transport traveling on or through or a structure. Clearance is an important accessibility consideration in routing decisions. The limited vertical clearance of the Howard Street tunnel in Baltimore, for example, has long prevented the travel of double stack intermodal trains to and from the Port of Baltimore. Similarly, ships must take clearance into account under bridges, heavy trucks are forbidden on some roads, and large aircrafts need sufficient runway clearance to take off and land. See: Customs clearance.