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Comment by jerrre | Hacker News Reader

Don't most of you points also apply to drunk walking?

The damage is a function of mass and speed. Obviously walking < e-bike << car

I cannot believe this is getting downvoted. The reasons cars are inherently dangerous is 99% because they are heavy and fast. Everything we build around that (traffic rules, dedicated lanes, buffer spaces, ...) is purely to deal with this fundamental physical reality.

With great power comes great responsibility. Whether you take that perspective in Joules, Watts, Newtons, kg.m/s, the conclusions are roughly the same. Drivers needs to be hundreds of times more responsible than cyclists or walkers.

To quickly do the calculations:

Walking: 5 km/h * 70kg = 97 kgm/s

e-bike: 20 km/h * 90 kg = 500 kgm/s

car: 50 km/h * 2000kg = 28,000 kgm/s

Energy is proportionate to the square of velocity. So it's:

Walking: 5 km/h * 70kg => 875 (although this is a very slow estimate for a walker) (please ignore the non-canonical units)

e-bike: 20 km/h * 90 kg => 18,000 (I think your estimate for ebike mass might be on the low side at 20kg, but whatever)

car: 50 km/h * 2000kg => 2,500,000 (and that's a fairly low speed for cars! Drunk drivers often drive faster than is wise.)

Everything else is a rounding error compared to the energy of a car.

Realistic estimates:

Walking: 6kph, 70kg => 100J (0.02% of car)

Analog bike: 18kph, 80kg => 2kJ (0.36% of car)

E-bike: 25kph, 100kg => 4.6kJ (0.82% of car)

Car: 50kph, 2000kg => 560kJ (100% of car)

Thanks. Just for the sake of fleshing out the speeding angle:

Car, 64 kph => 164% of car at 50 kph

Car, 110 kph => 480% of car at 50 kph

> Walking: 5 km/h * 70kg => 875 (although this is a very slow estimate for a walker)

In the context of this thread (a walker who’s drunk), I don’t think it’s very slow.

Also, https://en.wikipedia.org/wiki/Preferred_walking_speed:

“The preferred walking speed is the speed at which humans or animals choose to walk. Many people tend to walk at about 1.42 metres per second (5.1 km/h; 3.2 mph; 4.7 ft/s).”

https://www.sciencedirect.com/science/article/pii/S209575641...:

“The results show teenagers walk at an average speed of 1.45 m/s, young adults walk at an average speed of 1.55 m/s, middle age pedestrians walk at a speed of 1.45 m/s, older pedestrians walk at speed of 1.09 m/s, and elderly or physically disabled pedestrians walk at a speed of 1.04 m/s.”

5km/hour is about 1.4m/s; the fastest of these speeds is 5.6 km/hour.

Initial kinetic energy is not the right physical quantity to look at. Most of that kinetic energy will remain in the car/bike/…, i.e. it doesn't tell you much about how much energy will get transferred to the victim – it merely gives you a bound from above.

More details: https://news.ycombinator.com/item?id=35233887

Wouldn’t joules be more indicative of damage caused?

No. The kinetic energy of the car/bike/… doesn't tell you anything because you don't know how much energy gets transferred to the victim until you have applied momentum conservation to the elastic/inelastic problem. So, the right approach would be (in this order): Calculate momenta, calculate how much momentum gets transferred to the victim via momentum conservation, deduce the resulting change (increase) in kinetic energy of the victim. This kinetic energy will be converted into heat (= damage/injuries) in one way or another, so it's the relevant physical quantity for our considerations.

Finally, in case of an inelastic problem (likely with cars, not so likely with bikes or people), you also need to consider the energy loss during momentum transfer. Once again, this energy will become heat (= do damage), so it adds to the aforementioned increase in kinetic energy when we're interested in how much damage will be done.

Just as there is the law of conservation of energy, there is also the law of conservation of momentum. Both explain it equally well IMO.

How can mv and mv² explain the damage "equally well", when one is linear and the other is quadratic with respect to velocity?

The point is the extreme magnitude of difference between a car and a bike/person. This shows it just fine.

Momentum is always conserved. Energy is partially conserved. If it squishes, it will be more like MV, if it bounces, it's more like 1/2 mv^2.

This is awesome, thank you (and ditto for this whole chain of replies).

it's worth noting that most collisions don't happen at full speed (and combination of velocity matters a lot).

The argument above was that you can still hurt a bus driver psychologically by having them kill you. So it doesn't really matter that an ebike is heavier than walking.

The argument above was that you can still hurt a bus driver psychologically by having them kill you. So it doesn't really matter that an ebike is heavier.

Literally just being drunk enough can end up in you being dead or maimed.

From a harm reduction perspective, Drunk Driving -> Drunk Bicycling feels like it reduces the capacity for damage roughly proportionally to Drunk Bicycling -> Drunk Walking. At a critical level, the speeds you can comfortably achieve are reduced at each step, thereby increasing the amount of reaction time available to avoid an incident, reducing the ramifications of an error, and reducing the amount of damage your body has the capacity to do (by nature of the amount of kinetic energy you are attempting to control).

one major difference is that someone who is incredibly drunk can still drive a car at 100kmph, but they can't balance well enough to ride their bike.

Not sure if this is an example of anti-fragility, but it made me think of that. there's def something diff about bikes for this. Get too drunk as a cyclist and you remove yourself from the situation in relatively safe way, just by not being able to stay in control :)

You have to be really plastered to not ride a bike any more. Like a lot, a lot.

That's true, but most fatal car crashes happen for people who are very drunk https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/... reports that in the US, of the 2019 crashes of drivers who had a BAC above .01, 68% had BACs of .15 or higher (and about half were .20 or higher). That's a group that would have a pretty hard time riding a bike at high enough speed to do much damage. None of this is to say that biking while drunk is a good idea. It probably at least triples your likelyhood of being hit and killed by a car. Defensive riding is one of the things I would expect to be significantly impaired by relatively little alcohol, and it's probably dark out which makes everything riskier.

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