Judged in terms of established precedents, Forza Horizon simply has no right to be this good. Handing off the keys to a prestige franchise to an all-new development studio was a massive gamble in itself, but the risk-taking doesn't end there. Playground Games didn't take the obvious safe route and simply iterate on the existing formula using the established engine: instead the fledgling developer took that technology, retained its core attributes and then expanded upon them, taking the whole franchise in a new direction.
From a gameplay perspective, the most significant deviation from previous Forza titles is the departure from the series standard circuit racing, with the new action now transplanted into Colorado. It's an enticing proposition: take the most exciting driving machines money can buy and take them out onto the open road - go anywhere you want, do anything you please. Here, Playground Games faced its first challenge - Forza 4 already pushes the Xbox 360 very close to its limits: the massive engine upgrade envisaged would inevitably require compromise. The solution the studio came up with is the kind that Forza fans are likely to view with suspicion - the series' signature 60Hz update was halved, bringing Horizon more into line with the majority of console games.
It's understandable, of course. Moving to an open world represents some fundamental technical challenges. In a traditional track-based set-up, the developers know that players will effectively be moving in just one direction and can optimise track layout accordingly for best performance - that's a luxury you don't have in rendering an open world. On top of that, there's a significant memory issue too: both Gran Turismo 5 and Forza Motorsport 4 can load almost all the assets required into the existing space available before the race begins.