That's the kind of observation that any driver notices! diminution of space in front, driver ahead isn't maintaining speed uphill etc. Or - car ahead , limiting speed downhill, by selection of lower-gear and using engine-braking (per highway code, road signs etc) -it'll slow at an arbitrary rate, even abruptly - with no external signal.
13s 60-0 deceleration rate is .. really bloody slow ... a vehicle ahead coasting uphill to a stop without brakes, also , long, perfectly normal; say a motorway off-ramp. You might see a flash in the last second or two as vehicle ahead brought to a positive stop/held.
TBH - as a driver, one in the shoal - you watch for the relative movement of the Shoal around you, to avoid incursion into the stand-off space you are managing - ahead, to the side, behind. Vehicle signal lights - brake, indicators - are not to be relied-upon, ever.
If you wait for, let alone believe, signal of intention, you've failed.
Easier to demonstrate, than discuss - ATB : )