evidence: Perhaps the most discernible aspect of the border between the Republic of Ireland and Northern Ireland is just how indiscernible it is. With few signposts marking the end of one jurisdiction and the beginning of another, travelers going between the two rely on subtle clues indicating they’ve crossed from one country into another—the changing color of road signs, speed limits switching from kilometers (the Republic) to miles (Northern Ireland), or the more obvious shift between sightings of the Irish tricolor and the Union Jack. [...] "Now if you travel between the Republic of Ireland and Northern Ireland, it’s like driving from New York to Connecticut—you just keep going," Senator George Mitchell, the primary architect of the Good Friday Agreement, told me. "In fact, it’s hard to even know it now." [...] The challenge facing British and European negotiators is how to reconcile the U.K.’s decision to leave the EU with the bloc’s customs union and single market, without implementing a hard border. This is particularly important when it comes to trade between the EU and the U.K. and, more specifically, between Ireland and Northern Ireland. According to a December 2016 report by the U.K. House of Lords, approximately 60 billion euros are traded between the U.K. and Ireland each year, and as many as 30,000 people are estimated to commute across the border between Ireland and Northern Ireland each weekday for work. Between members of the EU, such trade is not subject to tariffs, nor are the individuals crossing the border subject to passport checks.