See and be seen

During my morning commute in the pre-dawn darkness I encountered a guy on black road bike, without lights, without reflective elements, dressed in dark clothes. I only saw his outline against the light of a street light that reflected off the wet road. Considering how the light drizzle on cars wind shield refracts oncoming light and thereby break up what little light there is reflected off a person such as this, his behaviour is borderline suicidal.

As a cyclist safety in traffic highly depends on being visible.

How you are visible to others mostly depends on two things:

The contrast of your clothing to your environment

Camouflage works by blending your outer layer with your immediate environment. By matching your colours and patterns to your surrounding, it becomes harder for others to notice and detect you.
So when there is little to no light and you dress in dark clothes to match that environment you are basically camouflaged.

The light that is reflected off of you

The way our eyes work, with fading light colours lose their saturation and can only be differentiated by their lightness on a gray scale. Darker colours absorb more light than lighter ones. That means in an environment with poor illumination they are even harder to detect.

Cyclist dressed completely black are not visible at all. With luck you can detect them by them obscuring light sources behind them.

Blue and red are both colours that absorb a lot of light or rather reflect less. Our eyes are tuned to green and yellow and can detect those at twice the distance of blue. White clothing absorbs little light and stands out even more easily.

The interesting part are reflective elements. Even with mostly dark colours a bit of reflective piping or reflectors on your bike will easily double the range at when you can be detected.
Lights will multiply that distance again. While lights have an effective range where their light is useful to for you to see, it can still be picked up at several hundred meters.

Now bear in mind that these ranges for detecting a cyclist or pedestrian are without the impact of weather. Precipitation or fog can severely reduce them.

light bias

Another big factor is that our eyes adjust to the brightness of light. Just think of yourself entering a dark room, when coming from a brightly light room. It’ll take your eyes several seconds to adjust, so you stand still until you can make out your surrounding. Just then a friend of yours comes into the room with a flash light. They can see everything clearly that the light of their torch illuminates, yet they don’t notice you until you step into the light and scare the bejesus out of them.

The same is true in traffic. Your eyes may adjust to the dark conditions and you might see fairly well. So well indeed that the driver of a car with additional light surely must notice you or so you think.
For said driver this is not the case. Their eyes are tuned to the light intensity of their car’s headlights and everything outside is even harder to notice. Additionally their attention will be focused to the road. So when you cross the street in front of their car, they might not be able to stop in time.

braking distance

The distance for a car to come to a full stop is comprised of two components:

1. The distance the car travels until the driver hits the brake. This takes roundabout 1s.
2. The distance the car travels while slowing to a full stop.

Both of these distances depend on the initial travelling speed of the car.

1. distance d1 [m] = speed [kph] / 3.6
2. distance d2 [m] = speed [kph]/10 x speed [kph]/10

example:
A car travelling at 30kph.
d1= 30 / 3.6 = 8.3m
d2= 30/10 x 30/10 = 9m
so the braking distance is 17.3m

Comparing these numbers with the detection ranges above, you’ll notice that a car travelling at 50kph wouldn’t be able to stop in time to avoid hitting even a yellow dressed person. As weather affects visibility it can also affect the distance for slowing down. Different road surfaces may also affect the braking distance.

Do the smart thing

Dress yourself to be as visible as possible. If your bike kit is black as pitch add a hi-vis-vest/strap or some reflective armbands. Equip your bike with reflectors to the rear and front and for the spokes as well.

Light technology has come a long way.
Nowadays there is no reason to NOT have lights on your bike.

A set of minimal LED-lights can be had for under 10EUR. They may not add much light for you to see better, but their light is visible from a long way off. Make sure to get lights that can be mounted to your bike. Holding a torch light in your hand may impede your braking and bike handling.
For around 50EUR you can get a decent light set with good burn time and enough light to navigate an unknown path in darkness. As with everything the sky is the limit. You can buy veritable flood lights with hours worth of burn time – they will burn a big whole in your wallet though.

Of ccourse dynamo powered lights are also a viable option, as long as they are in good repair and are being used. Hubs with integrated generators give less resistance, are more reliable (e. g. No slipping on wet tyre side wall) and have/cause less wear. Thought they have an anoying pulsating light on low speeds, which may cause eye strain or other discomfort to some.