So that you can warn following traffic that you INTEND to slow down, therefore they should let their own speed drop in order to keep further back, before you ACTUALLY start losing any speed.

As more of us want to move about, what's the best way to get the most journeys completed?

Setting non-motorised travellers aside, and assuming a democracy (so, all non-emergency road users have equal urgency), vehicle interconnectedness significantly helps, even when only half of the vehicles share data about their states.

In this scheme, every car in the lane that's ending doesn't move to the adjacent one until the very end, then merging in turn with other traffic that's already there.  This uses all available road space for as long as possible, cutting congestion by 40%, and reducing crashes since all traffic is slow when it manoeuvres.

However, driving habits are 'baked in', so most of us see it as pushy and unfair if others sail past when we've planned ahead and changed from our lane early. As one commentator has said "The zipper-merge is going to be this century's conversion to the metric system in the '70s. Great idea, made perfect sense, and was dead on arrival."

More theory's here.  I'm awaiting pointers to further studies from Prof David Crundall, at NTU.