That’s actually the original use of an umbrella funnily enough.

I imagine they’d just be updated via a USB drive or something similar, no need for a wi-fi adapter.

Seriously, driving slow and clogging the passing lane is irritating and dangerous, but tailgating is just making a dangerous situation even worse. Plus, if a person is already absentminded enough to be cruising in the left lane they’re likely not going to be rational enough to interpret that as “move over.” Instead they’ll just think “some crazy asshole is on my ass” and either keep as-is, panic, or try some spiteful move like slowing down in the left lane and further compound things.