If you are going to ride your bike year around, chances are that sooner you are going to want or need to ride your bike beyond sunset. To do this, you absolutely need at least two lights on your bike.
There is nothing more important that the proper selection of lights because this is the primary way of cars seeing you.
Now, many people will say that cars seeing you is a secondary importance, since you'll need a light for seeing the street.
Once you start biking in the dark, for the most part, in both the city and the county, lights are not necessarily to see the road. When you are on a bike, you naturally go slower than a car, your eyes adjust to the local lighting, and often times street lights will be more than enough light for you to bike. The primary purpose of bicycle lights is to be seen. Because of this, it is easy enough to think that you really don't need a bright light. You need just a light to "be seen."
However, we will examine our human perceptions. In reality, a light "good enough to be seen" is really a light "good enough to get hit by."
To understand why a bright light is so necessary, we must take a detour into the world of vision.
The most important thing, for a car to see you, is to understand how far you are away.
The #1 reason for cars hitting bicycles is either a car pulling out from a side street or a car pulling across a street. Both of these are problems of the driver not adequately seeing or judging distance to the bicyclist.
In vision, why we get hit is often a problem with depth perception. Everybody knows that we see in 3D. The reason that humans see in 3D is that they have stereopsis. If you have two cameras or two eyes, which act just like cameras, there is a parallax between your two cameras or eyes.
Parallex simply means that one eye will have a sightly different image from the other eye due to the fact that they are set apart in our heads. Most of us enjoyed, as a child, closing one eye then the other eye and watching the image just just back and forth. When I was a child, and bored in church, I could make the pastor jump around a bit with this trick, even through we weren't pentecostal. (Yes, that is a joke.)
Because our eyes are set apart, each eye sees something slightly different. If our brain wasn't morphing the data from our eyes, we would be seeing two images from either eye like two images on the TV screen that didn't line up.
However, our brain magically puts those two images together, and the slight differences in the images register at different depths in our vision. When you start to think about how our brain works, it is a miracle. What should be a problem with our vision design turns into a benefit. If we had to try and create it, I don't think that it would be easy. However, God using his skills to create something clever.
By this gift, we can judge if an object is 100 ft away or 1000 feet away automatically. However, some people don't have two eyes. For instance, my sister lost one of her eyes at a very young age, and her stereopsis does not exist. However, she can drive, although her depth perception simply does not exist.
If we lose an eye, how do we judge depth?
Again, the magic of the brain is that a secondary system cuts in. The brain, with just one eye, can calculate distance by an object's size. If you see a stop sign in the distance, your brain says "Stop Signs are 7 feet high, and they have a 18 inch sign." If the stop sign is a small object in your field of vision, it simply says "well it must be far away."
However, this depends on the mind having a previous reference frame for size for any given object. What happens if you stumble across a stop sign that is really small? Without other objects in you field of vision, you'd simply misjudge the stop sign and say "that stop sign is really far away."
Now, we are all familiar with this type of work. In movies, small miniatures are made to deceive us. This works very well, as long as the miniature is detailed, because we only use one camera (or eye) to judge distance.
So how does this impact night vision? Our eyes have an iris, which lets in light. When it is bright, the iris shrinks making our pupils (the black thing in the center of your eye) small. When the pupils are small, it is easier to judge distance because it helps to focus the image. However, at night, the iris gets bigger to let in more light, and it becomes harder to judge distance via stereopsis. Just like my sister, the brain switches to the secondary system of judging distance. By previously stored reference frames.
Most car headlights are similar in brightness at a given distance. So if a light looks dim, it must be far away. If a light is bright, it is close.
The other thing that helps the driver to judge distance is the distance between the front two headlights of roughly 6 or so feet. So the problem becomes obvious.
1. Bicycles don't have two headlights with a spacing of 6 feet
2. Many bicyclists have dull lights that will look like a car really far away
Even if you are close to a car, the driver will have part of his or her brain saying "don't worry, that dull light is really far away." Compounding this problem, you only have 1 light vs the 2 required to help judge the distance on the car.
So if becomes very obvious why you shouldn't have a dull headlight. You will be really challenging the driver of cars to figure out how far away you are.
This is beyond just theory. I have been nighttime biking for many years. The difference in how cars react to a bright and dull light is absolutely amazing. The brighter the light, the less that I would be cut off. To the opposite, if you can't judge depth well, you can't judge speed. So guess what happens when you have a really bright light, and a car sees you coming?
It waits. And waits. And waits. It waits because the driver can't tell how fast you are closing on it. They only see a bright light, and they expect it to be closing quickly.
As a rule, cars always wait much longer for me at night than day time. However, if you have a dull light, they will not wait at all, and often cut you off. It is clear as "day and night" for the difference.
In the next section, we will examine the different technologies of headlights on the market today.