If you go for a drive tonight, you'll see reflectors shining brightly from mailboxes. You'll see reflectorized stop signs. If bike riders are out, you'll see their pedal reflectors . All these reflectors will appear bright, and very easy to avoid.

So here's the seven million dollar question: If all these reflectors are so darn bright and easy to see, how come the bike safety nerds insist you need active lights to be seen at night?

There is a very scientific answer: reflectors work only under very specific conditions. Those conditions happen to prevail in most of the nighttime driving we do, so we get the impression that reflectors work most or all of the time. But reflectors don't work at all if those conditions aren't met, and many well-defined bicycle accident types occur in situations when we can expect reflectors to not work.

Few people understand how easy it is to wander outside the range of conditions in which reflectors will work. But it's astonishingly easy.

Why would a reflector decide to malfunction? And how could it? It doesn't have electrical components to fail, like, say, a British car.

It does, however, have other limitations. Among them:

* It can be anywhere outside the beam of a driver's headlights.

* It can be tilted at an angle ("entrance angle") that severely degrades its optical performance. (If you look at bikes parked on the campus bike rack, you'll see reflectors aimed in all sorts of dysfunctional directions.)

* The driver's eye may be outside the narrow cone of light which the reflector sends back to the light source. (The angle between the light source and the driver's eye is the "observation angle.")

* Fog can completely block the reflector when other lights remain visible. (Howzat? The farther light travels through fog, the more the light gets absorbed-and light from a reflector is making a round trip, twice as far as light from an active light source.)

* The driver may have a burned-out headlight (possibly a lethal problem if it's the left headlight-generally, the right headlight's observation angle is too big for good reflector performance). Or the headlights may be mis-aimed or covered with dirt. Or powered by a Lucas electrical system in the throes of an 8-volt brownout.

* The reflector surface can be abraded, covered with moisture or dust, or otherwise altered in a way that wrecks its optical performance.

This list is surely incomplete, but it makes a point: many factors can prevent a reflector from beaming light at the intended observer. This point is not hypothetical-our nightly accident rate shows that. Roughly once per night in this nation, a person is killed on a bicycle after dark. Many more are injured. Very often, I suspect, these accident victims have Consumer Product Safety Commission approved- and required- reflectors on their bikes. So here's my message to those who say, "These reflector requirements are safe and effective." You've lost all credibility.

To understand how reflectors can fail, you need to learn about fifty cents worth of industrial engineering. The relevant topics are: entrance angle, observation angle, headlight beams, positions on the roadway, the human propensity to make big mistakes, and "other."