Gibraltar has looked at a lot of proposals to alleviate its runway problem one was a vehicle underpass but I guess it will never be sorted coz of 1/ Space restrictions. 2 Its Ministry of Defence property and 3/ The land that the airport is on, is disputed ownership with Spain as they say it used to be no-mans land and was not cited in the Treaty of Utrect which ceded the Rock to the UK in Perpetuity.