I'm sure dj can explain the specifics, but what makes the race great, is the support of going right through the cities. However, as with any city, some people aren't runners and don't see how a marathon benefits a community and they don't want to be bothered for a day.
Traffic 'has' to remain open on one side of the road. The course has been reversed so there isn't as much congestion amongst the runners and walkers at the end like previous years. However, the finish is a thorn among some residents that want to be able to drive around town easily. These residents are big fish in a small community and they have places to be, if you know what I mean… ::)