Hacker News Reader

Comment by lutusp | Hacker News Reader

A related topic -- I wonder how many young drivers realize that, reaction time aside, it takes four times the distance to stop a car at 80 MPH as at 40 MPH. Four times the distance, not twice, as many people think.

The reason? A moving mass has a kinetic energy equal to 1/2 mv^2 -- note the squared velocity term. I think this is something that young drivers should be taught. Older drivers learn this by experience and acquired instinct, but young drivers, especially those who don't learn physics or mathematics in school, may learn it the hard way.

Reference: http://arachnoid.com/conservation_of_energy/#Kinetic_Energy

View on HN

If physicists ruled the world, speed limit signs would be replaced with momentum limit signs.

Cute, but momentum and kinetic energy aren't comparable.

Momentum: p = mv

Kinetic Energy: E = 1/2 mv^2

IOW E is the first integral of p with respect to velocity.

Your suggestion has something important going for it, though -- more massive vehicles would have to slow down.

Given one, the other is implied though. Only the limit will have to be set accordingly.

You're wrong. The relationship is different, the graph of the speed limit vs velocity would also be different, for the same masses/cars.

So Smart cars and motorcycles should be allowed to drive at 100+ mph?

Momentum (or kinetic energy) as modified by human reaction times, which in turn are a function of the road design and other local conditions. That implies signs which are not only tailored to their location, but can automatically adjust based on weather and human activity patterns. Streetlights come the closest to approximating this, but I'm not aware of streetlights that react to weather.

(Road design is a fascinating field; the intersection of physics, materials science, automobile engineering, and human psychology. It's possible to design a road to allow the average person to drive a modern midsize sedan in relative safety given good weather conditions, but you have to make the road a closed system (controlled access) and become zealous about maintaining it and the cars that use it. The Autobahn, in other words.)

This would make some sense if all vehicles had the same braking force. Though, even then, I think you'd want a kinetic energy limit. (A stopping distance limit would probably be the most direct.)

This is totally fair, and a solid observation. I think it's a little remiss to square blame on the shoulders of the young though. A lot of accidents are caused by poor reactions, tiredness and distraction. Something the older generations can be equally guilty of. (I say this as a person who could be in no way considered 'young'.)

> I think it's a little remiss to square blame on the shoulders of the young though. A lot of accidents are caused by poor reactions, tiredness and distraction.

Fair enough, but don't forget inexperience, an important factor, one that disproportionately affects young drivers.

It's best to think of a mesa of driving skill: The slope on the left side is where experience is gained, the plateau is someone's prime driving decades (hopefully!), and the slope on the right side is when inevitable physical decay of the brain and everything else takes its toll.

The trick is to make the ramp-up as uneventful as possible and to arrest the decline before it's serious. Sadly, adults who've driven for decades are commonly loathe to ever give it up, and for good reason in our car-centric society.

Also, I think the whole concept of field drug testing is misguided because it's using a bad proxy when a direct test, in the form of field reaction time testing, wouldn't be any harder to accomplish. Reaction time testing doesn't test judgment skills, but neither does testing for BAC, necessarily, and focusing on BAC ignores fatigue, which is a known killer. (And we allow stupid people to drive anyway.)

The difference is that he chose to be needlessly reckless. Had the accident happened while he was driving the speed limit, he wouldn't be facing murder charges

This is taught around day one of the mandatory theory course kids take before being allowed to even try for their licenses here. And it's on the theory tests that also need to be passed.

I seem to remember that a minimum interval of two seconds was the rule for following another vehicle, but the whole car lengths thing always threw me...

Are we talking Lincoln Continentals, or Volkswagon Beetles?

> I seem to remember that a minimum interval of two seconds was the rule for following another vehicle ...

That's a reasonable rule of thumb, but it doesn't work in all cases -- it makes too many assumptions. It only takes driver reaction time into account, and it serves to partly explain why accidents in fog tend to involve many cars and high levels of injury and destruction.

The reason? The two second rule assumes the car in front of you has the same stopping distance characteristics as your car, once the brakes are applied. But if the car in front of yours abruptly stops by hitting an immovable object (instead of skidding to a stop) and you are two seconds behind him, you will surely collide with him also, and the car behind you will collide with you, ad infinitum. People who drive at normal speeds in fog apparently don't realize this.

So a new proposed rule -- if you can't see the car in front of the car in front of you, pull over and catch up on your reading. I doubt this rule will take hold.

We are actually taught stopping distances (inc. reaction times) in the UK, for our theory tests.

I typically find that when I'm being tail-gated it's someone in their mid-life, I don't know if there is a problem with young people knowing about stopping distances.

I do know there is a problem with young people like me driving recklessly and causing crashes, I just think that there's a possibility that that comes from, well, recklessness. They know the facts and choose to ignore them.

> We are actually taught stopping distances (inc. reaction times) in the UK, for our theory tests.

That's admirable, but I think it would be better to know why those distances are true, i.e. the velocity-squared rule. The idea is very simple, certainly simpler than memorizing a bunch of distances or even noticing the overall relationship between speed and distance.

> They know the facts and choose to ignore them.

There's a substantial literature about how the teenage brain is wired differently than the brains of those both older and younger. Giving them a car only exacerbates the problem. The irony is that a 10-year-old, and a 25-year-old, would both be likely safer drivers than a 15-year-old.

It would be interesting to see a graph of driver safety versus age -- ascending safety as age progresses from zero to about 12 (hypothetically, of course), then a really big dip in safety centered on 16, then another ascent, a long level spell between 25 and 60, then a tailing off after age 60, finally a terrible and irreversible decline after about 75.

I do think that would be really interesting to see.

As a start, apparently 25% of road deaths are among 15-19 year-olds (src: http://www.racfoundation.org/assets/rac_foundation/content/d...).

We are usually taught about the velocity squared rule! I think I recall one of the problems on my theory test being based on it. It'd be hard to memorise stopping distances, although I think we do have to memorise approximate reaction time distances.

> We are usually taught about the velocity squared rule!

To me this is a sobering reminder of how much of real life is denied to American students in order to honor intellectual mediocrity, which is so widespread here that it might as well be a secular religion.

I wonder how many drivers realise that a modern high end car with good brakes will stop in a fraction of the time of a 20 year old Escort with barely legal tyres?

I wonder how many drivers realise that reaction times are not equal between drivers, and that attempting to include it within any estimations is futile because whilst I fiddle with my radio, or check my exquisitely coiffed hair in the rear view mirror, my reaction time is roughly that of my 95 year old grandmother.

I could go on?

The only truly accurate speed limit is one that which accounts for everything. Visibility, road conditions, brakes, tyres, alertness, time of day, type of road, skill of driver and probably 15 other things I haven't thought of in this little rant. Once this is accepted one can appreciate the most fundamental problem with speed limits - just because it says 60 on the sign doesn't mean that is a safe speed at which to drive.

Fixed speed limits are the only mechanic we have to try and keep people safe. A simple momentum equation tells 5% of the story.

> I wonder how many drivers realise that a modern high end car with good brakes will stop in a fraction of the time of a 20 year old Escort with barely legal tyres?

Guess what? That is false. Car braking doesn't work the way you seem to think. Assuming the brakes take hold and the wheels aren't turning, a small, light car with four skinny tires, and a very large truck with 18 large tires, stop in the same distance.

When police investigate an accident, they have one braking-distance list that assumes level, dry pavement, but doesn't care about vehicle weight, tire size, number of tires, or anything else -- all those factors cancel out.

Wet pavement, pavement not level, yes, those are factors. Car size and weight, wheel count and tire area, not relevant.

I wonder how many young drivers care?

Reply on news.ycombinator.com