Demos for big games are largely a thing of the past. They've somewhat been replaced with 'betas' for online games, but usualy, it is rare we get to play a game in the comfort of our own home before release anymore.
Forza Horizon 3 is going a different way though. The title has now released an Xbox One demo meaning you can spend a fairly decent amount of time driving around Australia today. It is fairly sizeable too at 18 GB. It is said you will get access to several cars, including dune buggies for off roading, on top of being able to try several modes out.
So have at it. No reason not to get a taste of driving around the open world early.