I'm currently using ArcGIS 10 Spatial Analyst's Cost Path tool to model pedestrian routes. I have sidewalks, fences, buildings and a ton of pedestrian scale features, all combined into a cost surface in which sidewalks are low cost, parks are slightly higher, crosswalks are still higher, and buildings, fences, and the like are barriers. The model works great to define optimal routes, compare routes to each other, assess the walkability of areas, and build walkingsheds.
But I have one major problem: bridges and tunnels. Because the cost surface forces all the features into a single 2D plane, I'm unable to model a bridge with a sidewalk that crosses over a street with a sidewalk. I must choose which one should get cut off. An alternative is to find all the bridges and tunnels in the City and model all combinations of under/over, but the model takes forever to run as it is! Is there a third party tool that can perform in least cost path in 3D, or am I missing a workaround of some kind?