If you're going to test self-driving cars, it's probably safer to do it on a road without real traffic. The University of Michigan has gone to great lengths to do just that.
They've created their own city. Autonomous vehicles are a thing of our future, so to bring that future a little closer to reality they've built a 32 acre city with a population of zero.
It even has simulated 2-dimensional buildings. They call it M-City. The engineers need to find out how these cars will respond to real life situations, like stopping at a stoplight, merging lanes on highway, or going over roads of different surfaces. Other things are more difficult to predict, like temporary construction and road signs that are covered with graffiti.
The benefit of M-City is that they can create any hazard they want without hurting anyone or tying up real traffic areas. They are hoping to test worst case scenarios before they pop up on a major highway.
Electric car company Tesla has recently added several autonomous features in their models.