In the USA, it's typically not an issue to drive across state lines or to cross the border to Canada. However, you'll most likely pay an extra insurance fee if you want to travel to Mexico. Similar rules apply to Europe, as you'll need to cover a cross-border fee if you want to cross from one country to another. Failing to do so will most likely invalidate your car hire insurance. Crossing borders in Africa and Asia is generally more difficult and additional border documentation is necessary. We recommend always informing the hiring agency about your planned itinerary.