Full piece of the developer's diary:

The city’s complex urban environment presented significant technical and game balancing challenges as well. The sheer density of terrain - particularly when combined with large numbers of active vehicles and characters – complicated level flow and put a heavy strain on our framerate. A balance had to be found for both gameplay and technical reasons. Too few vehicles, props and characters and the environment felt unrealistic and uninteresting. Too many and the scenes became crowded, snarling the player’s path and creating problem areas where a single mistake would bring him to a dead stop. Thankfully, early focus testing had revealed that frequent dead stops were one of the most frustrating things for players to deal with. As a result we thinned out the traffic in the levels, made more obstacles breakable and generally removed elements that interrupted gameplay. Additionally we pushed hard to optimize the art assets and find technical solutions that would allow us to keep the environment richly populated while maintaining a steady framerate. This sort of problem solving is an excellent example of close coordination required between art, design and engineering when working on a complex project of this nature.

In the end, Overdrive has emerged as one of our most successful environments, combining dense, challenging gameplay with spectacular visuals, excellent pacing and a hip retro vibe.