Nothing in the released footage shows a fast travel menu, and Rockstar has not described one. What the showcase does establish is that Leonida is large enough for the question to matter - the drives between the city and the outlying regions are not short.
Transport visible in official footage
- Cars and motorcycles, obviously, with detailed handling and damage
- Boats and jet skis moving between coastal areas and islands
- Airboats in the wetlands, which look like the intended way through that terrain
- Aircraft over both city and swamp
- Taxis operating as part of city traffic
Taxis are the interesting one. In GTA V, hailing a cab and skipping the ride was effectively fast travel with a fare attached, and the same trick worked in GTA IV. If taxis behave the same way here, the feature exists without being labelled.
How Rockstar has handled this before
The studio tends to avoid a bare menu-based teleport and prefers something diegetic instead - a taxi fare, a train ticket, a stagecoach in Red Dead. Red Dead 2 gated its fast travel behind upgrading a camp map, making it a reward rather than a default. Something in that spirit would fit.
Why it may matter more this time
With two playable characters and a state spanning city, keys, wetlands and inland country, moving between story threads could involve long distances. A game that switches you between leads has an obvious reason to move you around the map efficiently. How it does that has simply not been shown.
What we know for certain
The world streams without loading screens between regions, which is a technical detail rather than a travel feature but does mean any transition Rockstar builds can be seamless.
Region breakdowns are on the map page, and the videos are collected on trailers.