In a typical city scenario, a car has KE of 1/2 (2000 kg) (50 kph)^2 ~= 2e5 Joules. A bicyclist has 1/2 (100 kg) (30 kph)^2 ~= 3.5e3 Joules. Almost 100x less capable of inflicting damage on a typical pedestrian. Add onto this that bikes are way more maneuverable, less collision surface area, are totally exposed to the same forces exerted on the pedestrian with which they may collide... your comparison with cars is intellectually dishonest because you are comparing a human on top of a 10-20kg machine with a human inside a 2000kg metal box with an incredible array of safety features for the driver and hardly any for pedestrians outside.

You need very little kinetic energy to make a cyclist topple and fall. The fall itself can lead to injury and death, lying on a street in the midst of all the traffic is even more dangerous.

Any touching of car and cyclist is very, very dangerous. That's why I brought up the example of cyclists passing cars with practically no space between them. They feel it's safe, but it's incredibly dangerous.

A 100kg cyclist + bicycle vs a 1500kg car (+ driver)

If I'm filtering through slow traffic at 10mph and clip the side of a car, I'll have a minor tumble - scuffed handlebars, grazed palms, dented pride.

If a motorist clips me at 40mph, I'm almost certain to suffer serious injuries. I might be hit on the back of the head by a wing mirror, I might be dragged under the wheels, I might be launched over the hood, I might be bounced into some street furniture.

Some of what you have said is correct but let’s make this clear: cycling across a pedestrian crossing at walking pace posses vastly less threat to human life than a car doing that would.

Fundamental physics, momentum=mv

Bicycle plus rider: 100kg

Car: 1500kg

That’s a 15x

For a comparison,

Human: 80kg

Bicycle + rider: 100kg

That’s 1.25x

If we assume that risk is proportional to momentum, then that’s an 125% increase of threat from a bicycle vs a human (running at the same speed) and 1500% increase of threat from a car vs a bicycle.

So to compare cars to bicycles doesn’t work, they pose a vastly reduced threat to human life va cars and in fact, cars pose more of a threat to cyclists than they do to pedestrians (I.e more cyclist die in car accidents that pedestrians).

> engineers have spent a lot of time trying to minimize the amount of energy a human absorbs during a car collision. It's not perfect, but it's superior to the lack of protection offered when bicycling.

This is aimed at protecting the occupants of the vehicle, but some of the more recent design trends (thicker a frame pillars, higher center of gravity) make collisions even more dangerous for pedestrians and cyclists. I’m not saying those features of cars are a bad thing, but I feel like compensating for the extra risk to pedestrians and cyclists has not gotten as much engineering attention as it should. It’s tough because it’s a broader problem that has to do with how we design roads, and the attitudes people have when they get behind the wheel

The amount of energy imparted upon impact to a pedestrian is mostly a factor of momentum and the shape of the vehicle.

One of the most common injuries for cyclists is the ubiquitous broken collar bone. This often happens to folks when they're coming to a stop, or even when they're motionless, and they lose balance and tip over. The unavoidable reaction is to put one's arm out to mitigate the fall. If the arm doesn't bend at the elbow, energy gets transmitted straight to the collarbone and may cause a fracture.

Another common injury is concussion. These are, of course, worse at higher speeds because they usually occur when the cyclist goes over the handlebars. This can happen so fast that there is no time to protect the face-- hence the term "face plant". But a head injury can also happen when tipping over at zero speed.

Not totally disagreeing about the speed aspect, just the usage of the kinetic energy factor to imply how much worse the accident "could be". Velodrome cyclists regularly wipe out at 40+ mph. Serious injuries and death can occur, but that is highly dependent on the specifics of the crash. The overwhelming majority of the time, higher speed means nastier road-rash.

2) Cars pose a risk to cyclists, even if cars are going the same speed, which is a fictional scenario, as many motorists don't go the same speed as cyclists and instead overtake them dangerously ASAP.

3) The risk posed by cycles to pedestrians is far less than the danger posed by autos to cycles.

But in the absence of more relevant and recent studies, I prefer to trust my own experience. I feel safer on a bike when I go faster because I'm moving at a slower speed relative to the cars more likely to hit me, and I can get out of the way of others more quickly.

Also, as someone who is both a cyclist and a driver, there’s a bit less urgency and frustration to pass a 15-20 km/h bicycle when you’re only allowed to do 30 anyway than when the limit is 50.