The Troubles in Northern Ireland required that attempts were made from the early 1970s until the late 1990s to enforce border controls. Many smaller cross-border roads were cratered or blockaded by the British Army with the intention of making them impassable to regular traffic. Bridges were also destroyed to prevent access at unauthorised border crossings (known officially as "unapproved roads"). In particular, the border area in south County Armagh was dominated by British Army surveillance posts. Derry, the second-largest city in Northern Ireland, is close to the border with County Donegal. This meant that there was a heavy security presence around the city, often impeding traffic and general movement between Derry City and County Donegal. Despite these measures, the border was simply too long and had too many minor access roads to enable control of all cross-border movements. In any case, authorised crossing-points on the border remained open to civilian traffic in both directions at all times although vehicles and their occupants were subject to detailed searches while some crossing points were closed to vehicle traffic at night when customs posts were unstaffed.