It depends on the OSM-based routing system, which there are many of, but I know for certain the best way to specify pedestrian access and ensure proper navigation is to map all walkways along the route and to apply tags like foot=yes, foot=no, foot=discouraged to specify pedestrian access. This is specification is even more important now since highway=trunk, which used to almost always refer to an expressway or boulevard, can now refer to any sort of segment of highway part of a major commuter or intercity route (i.e., a route as important as a motorway that may not meet motorway criteria for various reasons), including surface streets. With this change in direction for the tag, the road's color no longer solely reflects the highway's physical characteristics.