fripou, You underestimate the maginitude of the event :) There will be traffic restriction (not just cars, but pedestrians too) from 6:00 to 9:30, so you want to get to your viewing spot far, far before 6 o'clock. On the bridges across the river, there will be traffic controllers to keep you walking only in one direction, and you are not allowed to stop to watch...
http://sumidagawa-hanabi.com/road_map.html
On this map, where you see two red circles are where they shoot up the fireworks, and the areas colored orange are off limits from 16:30 - 21:30. The area marked by pink line is area with traffic restriction from 18:00 - 21:30. The areas shaded in with green, along the river for example, are parks and pedestrian walkways you can see fireworks from. Black arrows indicate the one-way direction where pedestrians are allowed to walk.
It's really a huge event - people book seats on river boats that offer meals on board, or hotel rooms nearby far in advance!