evidence: 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 (as of 2021[update]) 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.[3][4] 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 all involved parties to maintaining an open border in Ireland, so that (in many respects) the de facto frontier is the Irish Sea between the two islands. [...] Traffic [edit]The border is linked with 268 (often approximated as "up to 275") border crossing points.[3] Every month approximately 177,000 lorries, 208,000 vans, and 1,850,000 cars cross the border.[20] Around 30,000 people cross the border daily to commute.[21] [...] The larger and busier border crossings include national primary (N) roads in the Republic of Ireland which connect to primary route (A) roads in Northern Ireland. As of 2007, the busiest such roads were the N1/A1 route (Dundalk/Newry), N13/A2 route (Letterkenny/Derry) and N15/A38 route (Lifford/Strabane).[39]