The transport secretary, Grant Shapps, has rejected the idea of imposing quarantine rules on people arriving from regions of other countries, as opposed to entire countries. It would be “too difficult”, he said, despite other countries, such as Germany, adopting a more targeted approach. “A week ago Croatia was seeing cases that were at about UK levels. One week later their levels have gone up from just 10.5 cases per 100,000 to 27.5 cases per 100,000,” Shapps told BBC Radio 4’s Today Programme. There were about 11 cases of coronavirus per 100,000 of the population in the UK, Shapps said. The UK could not follow Germany’s lead, he insisted, saying experts at the joint biosecurity centre had told him it was impossible to understand infection patterns overseas “at a completely granular level”, unlike at home. “Lots of other countries don’t have the same level of detail or indeed the 330,000 test capacity that we have, the largest in Europe, to be able to get into detail. So it just isn’t practical to say: ‘You can come from this city but not from this city’. We have to protect the lower levels that we have achieved here,” he said. On Thursday Croatia, Austria and Trinidad and Tobago were removed from the government’s list of travel corridors, meaning travellers will have to quarantine for two weeks if they arrive back after 4am on Saturday. Shapps said it was unlikely that Spain and France would be re-added to the government’s travel corridor list any time soon. He told LBC radio: “At the moment I’m afraid France and Spain have both been going the wrong way. “So just to put numbers on this, we respond when there are about 20 cases per 100,000 of the population measured over a seven-day rolling average. So 20 is the figure to bear in mind.” He added: “I think that the last that I saw of Spain it was up in the 40s and 50s, so a long way off that, and France, who … quarantined from last weekend, I’m afraid to say we were right to do that because we’ve seen the cases continue to carry on up in France as well.” Croatia’s ambassador to the UK said it was “a regret” that the UK government did not impose quarantine rules on regions within countries, rather than entire countries. Igor Pokaz told Today the Croatian government had been lobbying for “a more nuanced approach”. He said cases might be high in Zagreb, the capital, but there were “very, very few cases” in Dubrovnik, a favourite destination for UK tourists. “And I deliberately mention Dubrovnik and the islands as that is where most of the British tourists go. And Dubrovnik has its own international airport and is naturally secluded from the rest of the country,” he said, noting that Germany only required quarantine for visitors to two of Croatia’s 20 counties. Discussing changes to the exemption list, Shapps told Sky News: “This is a very unpredictable virus which unfortunately just doesn’t play ball as far as the way that it can just sometimes take off in a country and I think anyone travelling this year will know that there are risks involved. “Indeed, we’ve added Portugal back on to the list, but you need to go with your eyes open there or anywhere that you travel this year because coronavirus is just a fact of life, we’re having to live with it.” He added: “So to answer your question, it is still rather too difficult to do the kind of regionalisation that you’re talking about because we just don’t have the same control elsewhere.” Asked about possible testing at Heathrow, Shapps said: “I want to see systems in place to do that kind of thing. But you’ve also got to be sure that you’re testing the right person on that second time round because are you going to just send the kit to the house or are you going to require the person to perhaps drive to a test centre? “So the point I’m making is this, it’s a bit more complicated than is sometimes suggested. People say why don’t you just test at the airport? Well, because it wouldn’t provide the results and you’ve then got to make sure the second test goes to the right person.” On Friday, Shapps launched an “acceleration unit” to speed up transport infrastructure projects around the UK. He said he was expecting railway passenger levels to return to normal eventually: “I think we need to think about our railways, our entire infrastructure, over the very long term.” Next year’s rail fare increase would be “probably the lowest that we’ve seen for many years”, he told LBC.