The driving frames carry more weight than they get credit for. They tell you about traffic AI, road geometry, vehicle physics, and the way the world handles motion.
For the density question, see the traffic density piece.
Vehicle variety
Sedans. Pickup trucks. Sports cars. SUVs. Motorcycles. Service vehicles. The trailer vehicle pool is varied - not the GTA V repeat-stable, but a clean refresh.
Specific vehicle counts are unconfirmed. A larger and more varied pool than GTA V seems likely, partly because the development period has been so long and partly because the city's modern setting demands it.
Driving behaviour
Vehicles in the trailer drive with realistic weight - they squat under acceleration, lean into corners, take time to turn. The handling is physical rather than arcadey.
Whether the released game will tune this toward arcade or simulation is an open question. The trailer trends realistic; players generally prefer arcade. Rockstar will probably split the difference.
Road geometry visible
Highways with real interchanges. Suburban arterials with proper signage. Bridges with proper structural detail. The road network in the trailer reads as designed, not as procedurally placed.
Specific routes shown in detail include the Keys bridge, the causeway, and several urban Vice City streets.