https://www.theguardian.com/politics/2018/may/27/eurotunnel-tory-brexit-customs-union
Below is the telling bit in my view.
He added that technology to prevent delays already existed, but said nothing could be planned or built until the UK and EU had decided on their future trading arrangements. Only then could precise specifications for checking goods be determined, equipment built, staff trained, and people educated about the new situation.
“The important part for industry is having a clear specification of what will be needed – and we don’t expect that to be agreed until the future trading arrangements are settled, during the transition period [from April 2019 to the end of 2020]. But even once industry has a specification, it could still take several years to build it, develop the necessary infrastructure, recruit and train new staff and educate transporters from across Europe in its use.”
