[reorder9695]

In my experience as someone who rides a bike with hydraulic disc brakes and drives frequently, they really don't stop very quickly. I pretty much always am more confident I can stop my car faster. The bike may have a lot less mass but it also has a lot less traction and it's much easier to lock up a wheel and skid instead of stopping quickly, especially in poor conditions/on road paint. I do cycle fairly fast, but this is on a mountain bike on the road, certainly not fast compared to a fit person on a road bike.

I will give you that they're much more maneuverabile and accidents can often be avoided by putting the bike into a space in the road that a car couldn't go into.

[pandaman]

Maneuverability of a bike suffers from the same traction issue. At the speed when you can't plant your foot to pivot around bikes need to go much slower than cars in the same corners to not skid out. Check out descends on any road race, bikes are braking to 20-30 mph in sone corners, which cars pass at full speed. Not to mention that a car can skid fairly easily on either axle, while on a bike very few can get out of the rear wheel skid and the front wheel skid is the game over.

[mmooss]

> The bike may have a lot less mass but it also has a lot less traction

I've long wondered about that tradeoff: I've heard some (non-physics-aware) people say that heavier cars/trucks stop faster because they have more traction, but obviously there's a big tradeoff in that equation. As a guess, on a moving wheeled vehicle, 1 kg adds forward energy proportional to velocity, and downward energy constant and independent of velocity? And maximizing that tradeoff would seem to be an engineering goal in order to minimize muscle/gas expenditure. (I'm sure it's well-known but I'm too lazy to look up the details.)

That ignores road/tire and air resistance which are proportional to velocity. And I suppose a key question is the degree to which traction depends on vehicle weight: maybe it depends more on tire and road characteristics, for example. Certainly ice or some kind of low-grip tire would be a big factor.

And that ignores locking up the wheel, as you pointed out.

[pandaman]

You could just get on a bike and test for yourself your theory. People who ride bikes tell you they are not good at stopping quickly, perhaps you should ask yourself how you became so knowledgeable without riding?