The map above shows border crossings in real time over the busiest hour of an ordinary Monday, 2 September 2019. The currently invisible frontier is not an arcane footnote of the Brexit process but an everyday part of people's lives and livelihoods

There are 72m road vehicle crossings a year between Northern Ireland and the Republic of Ireland, according to the Northern Ireland Statistics and Research Agency (Nisra). There are also eight trains daily in either direction between Dublin and Belfast.

About 14% of those crossings are consignments of goods, some of which may cross the border several times before they reach a consumer. In border areas it is common for milk which is collected in Ireland to be pasteurised and packaged in Northern Ireland and then sent back over the border for sale in supermarkets.

Overall, 32% of Northern Ireland's exports to Ireland are classified as "food and live animals".

A no-deal Brexit would effectively end this trade. UK government advice says that "to transport animals, products of animal origin (POAO), fish, shellfish, crustaceans, germplasm or fishery products from the UK to the EU in the event of no deal, you’ll need to ensure the trade route for your goods allows for your consignment to be checked at a border inspection post (BIP) at the first EU country you enter for export."

There are two BIPs in Ireland: one at Dublin port, and the other at Shannon airport in the middle of the island. This means that farmers in Fermanagh who wanted to sell livestock at the market in nearby Sligo would have to send their animals to Belfast, across the sea to Scotland, by road through England to Holyhead in Wales, back across the sea, to Dublin, and right across Ireland.

The Democratic Unionist party (DUP) has always insisted its priority is for Northern Ireland not to be cut off from Great Britain, politically or economically. Northern Ireland's export trade with GB in 2017 was worth £11.3bn, nearly three times the £3.9bn it exported to Ireland.

But it's worth considering who the losers would be in the event of a no-deal Brexit and the sudden return of border controls. They are not, typically, employees of large companies who work in Belfast and deal with the rest of the United Kingdom. They are, rather, people working for small businesses in counties Derry, Fermanagh and Tyrone, especially those in the agricultural sector.

It stands to reason that border areas will be worst affected, but even given that, the blow will fall hardest on western rather than southern border areas. With some exceptions, these are more Irish-nationalist areas overall, and least likely to vote for the DUP.

• Main graphic source: Transport Infrastructure Ireland. The simulation shows total traffic reported by under-wheel sensors at 10 border crossings between 17:00 and 18:00 on 2 September 2019. This is a small fraction of the total crossing-points - there are more than 200 - and includes only sensors where most readings can be assumed to indicate a cross-border journey.