Certainly, it gives a cyclist a split second if someone opens a door suddenly. That's not what it's really for - nothing can prevent that. For people opening a

Certainly, it gives a cyclist a split second if someone opens a door suddenly. That's not what it's really for - nothing can prevent that. For people opening a door sensibly (most people), it gives a unambiguous warning that a door is ajar ahead maybe 5 or 10 seconds before the door opens more. In the dark, where reflectors work, that's a lot more than you might get if you don't notice the door is ajar.