The Boston Globe has published an article titled “Survival Guide for Running and Biking in the Dark.” This picture appeared with the article: you need it to understand my comments.
The article addresses issues of retroreflector placement and motion, while not discussing when retroreflectors can work, or cannot work at all. The bicyclists in the illustration do not have the headlight and red rear-facing retroreflector or taillight required by law.
Retroreflectors shine light back toward the source, so they look bright to a driver whose headlights are aimed at them. Retroreflectors can be very useful when that condition is met — usually, for a driver approaching from the rear — but they do not substitute for lights. The illustration perpetuates the illusion that they do, showing retroreflectors shining brightly from a viewpoint where they would be completely dark.
The bicyclists in the illustration are already directly in front of a car, and would need headlights to have been visible in time for the driver to yield right of way. The runners approaching the intersection are outside the car’s headlight beams, and their retroreflectors would shine weakly if at all for the driver of the car.
A driver entering from side street, pulling out of a parking spot, backing out of driveway — or a pedestrian stepping into the bicyclist’s path — does not have headlights aimed at a bicyclist’s reflectors. The bicyclist must have a headlight to be visible under these conditions.
The statement in the article in particular, “[d]on’t put all your faith in lights: Flashing lights are better than nothing but not sufficient by themselves” implies that reflectors are more important than lights, and promotes flashing lights. The USA, unlike for example Germany, has no government regulation of bicycle headlamp beam patterns. Many bicycle headlights these days are blindingly bright. A flashing headlight cannot reliably light the cyclists’ way, is hard to track, and risks triggering seizure disorders in people susceptible to them.
For more detail on nighttime equipment, please see the discussion of reflectors and of bicycle headlights which I have posted elsewhere. Massachusetts law on nighttime equipment for bicyclists is here.