I saw a shuttle launch from the causeway on the Cape too. The final Challenger mission. Five miles away and it was just a crackle. Never heard the explosion at all even though it was nearly directly overhead about nine nautical miles altitude and seven nautical miles downrange.
I'm not sure what causeway you are referring to. I was directed to some grassy area. There was a large lagoon between this grassy area and the launch pad.
ML/NJ