CS, you get startled a lot, probably more often than most of us because you've got three kids. The startle reflex affects us all, but I for instance lead a "quieter" life (no boisterous teenagers to raise) and therefore don't experience it as often.
I nearly fell the other day when I was on the sidewalk in front of our house waiting for my husband to get something out of the car, when there was suddenly a loud screeching noise less than two feet away from me, on my right side. It was two teenage boys who had come running over the lawn that's between our building and a sports club and had come to a screeching halt when they turned the corner of our house and reached the sidewalk, where I stood, deaf on my right ear and totally unaware of their coming. They almost ran me over. I gave such an involuntary jerk (like myoclonus) that they looked quite confused, the poor fellows, and apologized.

When walking on busy sidewalks or otherwise crowded places, I can't look at the poeple coming my way. "Moving targets" make me lose my balance. I have to either look beyond to an unmoving visual reference, like a building, tree or whatever, or else look down on the ground.