When you setup your new region, you need to chose the region where the route will be located. If you want a US route, chose USA. This should put the cars on the road for you.
There are some road assets that don't have cars however, but that's a good thing too. I try to use them where I want a road into a parking lot and not have the cars disappear into mid-air.
Feel free to post any questions you may have. There's a slew of users worldwide that can help.
