That's alright, in the OP you said
It constantly monitors each of the 4 sensors
I took that to mean all the time including while driving.
I'm happy for it to work only while I'm stationery but I think I can work that out when the time comes.
Like I said, at the lights or something, although there's no reason to believe things will change once the car starts moving (unless your load falls out the back) so I doubt you will have to implement this, although it will make the system automatic, without speed sensing you'll have to press a button or something to make it check.
By "load" are talking about adding the shopping one bag at a time, or dropping a crate with a forklift? The latter will cause a faulty reading if you happen to read just as the crate hits. I'd still average a few readings, not just work with one.
Rob