Pedestrians are not allowed to walk on _____ unless there is a sidewalk more than 3 feet wide and signs permitting pedestrian access.

The correct choice pertains to toll bridges or highway crossings, where pedestrian access is typically restricted for safety reasons. These areas are designed primarily for vehicle traffic at higher speeds, and the lack of designated pedestrian areas increases the risk of accidents. While some crossings may allow pedestrian access, it is conditioned on the presence of suitable sidewalks and clear signage indicating that it is permitted.

In contrast, the other scenarios generally accommodate pedestrians more readily. For example, residential streets often have less traffic and may include sidewalks, so pedestrians can safely navigate these areas. Similarly, shopping mall parking lots are typically designed with patrons in mind, often providing walkways and pedestrian crosswalks. Rural unmarked roads, though they can be risky, do not have outright prohibitions on pedestrian access and might allow for walking where conditions permit. Thus, the unique regulations regarding toll bridges and highway crossings make this the correct answer.