They definitely use the maps. In order to accurately localize the car, a detailed map can be used as part of the likelihood function for location to answer "how likely is it that the vehicle is at position (x,y) given the most recent measurements?" If you don't have an accurate map, it's very difficult to come up with a likelihood function that is accurate enough to keep the car in its own lane.
Keeping the car in its lane is overrated. Any human who drives in snow knows the rules. If there are other cars around, center yourself with respect to them. If there are not, lanes are irrelevant. Even many narrow side streets in snowless areas force you to improvise in a similar way to handle oncoming cars.
Keeping the car in its lane amounts to keeping it from hitting anything. In any case - lane markers or not - controlling the car on shared roadways requires localizing it in terms of its environment. A high-fidelity map of the environment improves the localization accuracy that can be achieved from noisy sensors. If the map is more accurate, you can tolerate more noise from the sensors.
Most residential streets in Pittsburgh, as in a great deal of the rest of the world, do not have lane markings, though many are two-way and have parked cars on one or both sides of the road. Pittsburgh provides an additional difficulty, that many of these roads are too narrow for what is asked of them.
One of the tricks of driving in an older city with on-street parking is weaving in and out of unused spots to make way for oncoming vehicles that have no means to move over themselves—or won't, for fear of clipping a side-view mirror, or because they can't bring themselves to care about how little space they've left for others. Sometimes you have to stop and wait for one car to make its way through a tight stretch before going through yourself, rather like pulling up to a one-lane bridge that has been generated at runtime.
The trouble is that the lanes have to be imagined with respect to the current conditions, bearing in mind what other drivers are likely to expect of you; knowledge of the road is not enough.
This is a social problem as well as a technical one, and I will be interested to see the solutions.
Lane markings are good if they're there. Often they get obliterated by construction, wear, weather, precipitation. Sometimes there are several sets, rewritten after construction and only partly erased. Humans can have a hard time following them.
