Life Alert Emergency System
Another low-tech idea based on a product that already exists.
Building on the low-technology rape whistle vain of thought, I wonder if we could adapt Life Alert's "Help, I've Fallen and I Can't Get Up" button. I know we generally associate that technology with the elderly living alone, but I think it could actually work for this kind of scenario as well.
This solution would be small and low-tech (no requirement to have a cell phone or app-enabled device), easily worn around a woman's neck or accessible in a jacket pocket. A woman could quickly hit a button if they feel uncomfortable and would get immediate access to a 24/7 hotline. Just knowing that reaching help is one button away could help someone feel safer and more confident while travelling alone in an urban area.