Generally, for public pedestrian bridges, the railing height requirement is 42 inches. For residential walking bridge railings, the height requirement is 36 inches. These requirements may vary depending on the bridge's potential users. For example, for a low trail bridge with a drop less than 4 ft, a guard rail may not be required. Or a bridge primarily used for bicyclists may require a higher guardrail typically 54 inches. There are many factors that influence pedestrian bridge railing requirements so it is best to review your local building code requirements.