Your report on its own may not be enough to get a warning placed on the map, but if enough people make the same report - Apple calls it having 'a high level of confidence' - then Apple or Google will probably take action and add an update.
Apple Maps lets you file a report regardless of whether you're in navigation mode - presumably so you can flag incidents that you're walking past or that you can see outside of your window. However, the process is different in each case.
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