Hacker News Reader

Comment by zibby8 | Hacker News Reader

I think people are letting their personal biases (e.g. SUV=bad) get the better of them. With the rise of EVs, even small vehicles will weigh a lot. For instance, the curb weight of a Tesla Model 3 is between 3,648 and 4,250 lbs, which is about the same as a Toyota Highlander.

View on HN

SUV=bad isn’t just a personal bias. They’re more likely to kill a pedestrian because of their height. Weight isn’t the whole story.

To be fair, the model 3 is middle of the road in weight compared to all of the cars in its class, i.e. high performance sports sedan. This is quite commendable given the extra weight for a 75kwh battery pack and has lots of room for improvement as the battery tech gets better.

Bizarre that a Tesla Model 3 is called a small vehicle

Ok... a Chevy Bolt has a curb weight of 3,600 pounds as well. That's comparable in size to a Honda Fit at 163", but the Fit weighs 1,000 pounds less.

The Toyota Camry and entry-level model 3 are both about 3500lbs these days. Electric cars look likely to keep dropping in weight as the designs improve (ie. battery pack becomes more structural and less of just an additional load)

That is also terrible and we shouldn't let it happen.

rationally the speed limits should be lowered accordingly

I agree that momentum should be the prime consideration. Especially as it relates to heavy trucks. Those guys shouldn’t be allowed to go over 55.

Is momentum what actually matters?

A Ford F-150 at 45 MPH has less momentum than a typical locomotive has at 1 MPH, but I'd sure rather be hit by the latter.

Velocity is squared so yes that part matters more. Your extreme example is an edge case, sure, but not realistic. But overall, momentum is the force applied.

That doesn’t sound right. A 1mph train hitting you won’t kill you, it’ll just push you along. The sudden acceleration of all or parts of your body is important.

Stand there for a few seconds or minutes. It’s what glaciers did. They travelled far slower than 1mph and took out mountains.

I don’t make the physics, I just live with them.

Right but you assume the person would be stupid enough to plant their feet and get slowly dragged under the train. Reaction time drops sharply as MPH increases.

Unlike mountains, people are not nailed to the ground waiting to be carved out, they move when they get pushed by a slow but massive object. Only a tiny amount of the train's momentum will get transferred to the person, roughly 1mph x mass of the person worth.

p = mv, so in momentum velocity is simply linear. It's in KE = 1/2 * m * v^2 that has the square (kinetic energy).

Why momentum and not energy? I'd say the latter. It is energy that deforms your body. So, reduce the speed by 15%, reduce energy by 28% (2*15% to first order, or 1-(1-15%)^2 exactly).

Or, to put it differently, if the vehicle has twice the mass, speed should be reduced by 30%.

Part of the reason I like driving on the Autostrada in Italy is that trucks are speed-restricted. Their speed limit is posted on the back of the truck itself so it's easy for anybody to see if the truck is speeding, and I'm led to believe that the penalty for speeding trucks is pretty severe. This may also be true in other countries; I don't know. It's not the case in the US and I wish it were.

Seattle changed a bunch of speed limits to 25MPH but the problem is that the roads were designed for faster speeds so now instead of everyone predictably going similar faster speeds, you get an unpredictable mix of some people going the speed limit and some people going closer to the old speed limits.

In reality I think you need to redesign roads for lower speeds if you truly want a large percentage of drivers to follow the limits.

I guess you're referring to traffic calming? I'm not sure what Seattle has done but you don't necessarily need to totally redesign streets for that. Things like speed bumps or islands can be added so reduce the speed that drivers can travel on the road. Have they done anything like that?

I was always wondering if speed bumps significantly reduce the lifespan of cars.

It'll never happen because speed limits aren't. Nobody actually follows the speed limit, people on average drive at speeds that match the safety conditions of the roads. It's not a coincidence that that speed roughly matches the speed limit. If the speed people actually drive differs from the limit and cops start actually start enforcing it the signs will eventually be set to that speed - 5 after people complain to their city council (or if they cant because fed money the cops will just be directed to not enforce it).

The only real instance where speed limits differ form "the speed drivers feel safe driving" is school zones. But outside these situations speed limits are a waste of time because the roads are designed for specific speeds and people are naturally really good at feeling that speed. They exist because cops need some objective justification for pulling over people who drive faster than the norm.

This is true, and the reason why most roads should be heavily narrowed to have their design speed match their speed limit.

Speed limits get automatically complied with when it feels unsafe for the driver to exceed them, and all places where automobiles are permitted should be adapted to that reality.

In contrast, your Highlander has a higher centre of gravity, and kids can be standing next to it without you being able to see them.

Reply on news.ycombinator.com