Edge Online has another fantastic feature, discussing the creation process behind GTA. From the article:
"One of the defining moments of the series happened in early 1996, not long after Baird joined the team. 'At that time, we were still a series of small missions,' he says. "We had a long, long brainstorming session where we picked up on an idea for one long level containing multiple missions proposed by one of the level designers, Paul Farley. We took this and expanded it into the open-ended structure that the game shipped with. This meant a big expansion of the scripting and improvements to game systems — they had to handle the game running over multiple missions instead of the short structure.'"