Gosh, that has actually brought me to tears. I did the same trip one time, all hopes and dreams and excitement. What a horrible, cruel end for the horses. I hope the humans who are involved go easy with themselves.
For many years, I have had cones, flashing lights and a triangle in my lorry. The trouble is, most people don't put them far enough back. They would need to be started at least 100yds, or preferably more, back, and tapering to cover the area of the lorry by about 50 yds, so the driver would hear the thunk as they hit the triangle. From the ground, that set up looks ridiculous, and many people put the triangle just 20m from the vehicle, too late for people to take action.
There would, of course, still be no guarantee that this wold have saved the horses. You can have 200yds of heavyweight cones and signage and someone can still plough through the lot of them.