You would think it would be fine to download and play an iPhone game in the car, as long as you're not the driver.
The long road can get boring. Surely a passenger can safely spend 10 minutes here and there staring at his phone instead of looking out of the window at all the scenery. It's just a little racing game. What harm can it cause? It's not as if it's real... right?
At the very least, don't let your passengers play this while driving a rental car. That's probably worse than if it were your own. Those companies are hard to deal with.