Border markings are inconspicuous, in common with many inter-state borders in the European Union. As the two states share a Common Travel Area and () Northern Ireland (the only exception within the UK and only in some respects) and the Republic of Ireland are participants in the European Single Market, the border is essentially an open one, allowing free passage of people since 1923 and of goods since 1993. There are circa 270 public roads that cross the border. Following the United Kingdom's exit from the European Union, this border is also the frontier between the EU and a non-member country. The Brexit withdrawal agreement commits the UK to maintaining an open border in Ireland, so that (in many respects) the "de facto" frontier is the Irish Sea between the two islands.