Think about a typical looping coaster train, say, for instance, an Arrow multi-element car.
Where are you going to attach a lap bar in such a way that the attachment can withstand several thousand pounds of force? How do you get enough clearance for it to work? The questions are not impossible to answer, but those and others have to be answered, and the calculations have to be done to make sure that the resulting system is actually safe and effective. The parts have to be manufactured. It's easy to look at the car and say, "Okay, so you attach here, and come down like so, and you're golden." That's the creative part. It's also the easy part. Once that's done, it becomes an engineering problem to design the retrofit and make it work.
--Dave Althoff, Jr.