Causeways are how Vice City works as a city. Without them, the strip and the mainland are separate places. With them, the city has the rhythm a coastal Miami-coded city is supposed to have.
We focus here on the causeway crossings specifically. For the wider bridge map, see the iconic bridges piece.
What the causeway frames show
A long, low road across calm water. Multi-lane traffic. Light fixtures along the centre median. Yachts visible to one side. Shoreline visible on the other. Daytime and dusk versions of similar shots in Trailer 2.
The causeway in Trailer 2 reads as a working road, not a sightseeing route - you can imagine commuting it. That's a different feel from a tourist drawbridge and probably what Rockstar is going for.
How many causeways are there?
Trailer aerials suggest at least two, plausibly three - matching the real Miami arrangement of MacArthur, Venetian and Julia Tuttle Causeways. Specific count is unconfirmed.
Why the causeway matters mechanically
It's a choke point. If the strip is on the barrier island and downtown is on the mainland, every drive between them crosses water. Police chases that start on the strip and want to escape will need to cross. Heists that start downtown and want to retreat to the strip will need to cross. Causeways are going to feel meaningful in a way most GTA roads don't.