The concrete blocks were recently tested in Germany and they don't actually stop trucks properly. The test truck was accelerates to 50KPH and it batted away the concrete like it was nothing and kept going for long enough to kill some people. This is what happens when you don't enforce external borders. You end up with internal borders. By the time you have to put up barriers at freaking Christmas markets the terrorist already has the advantage as he is in the country and can freely drive his truck to any unprotected spot.