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Comment by imatworkyo | Hacker News Reader

I believe speeding is very much a laughing matter and overblown as a justification for tickets and revenue (not evn considering traffic cameras). Speed limits are arbitrary low and cause traffic and accidents due to the resulting congestion. There has been studies stating that (on the highway) a speeding driver is usually safer than someone driving under the speed limit.

Also to address the stopping for children element. You seem to think stopping distances carry the full force of the starting speed. If an obstacle is ahead and im going 5 mph faster than the speed limit. Even if I hit the obstruction, its not like im going 30mph when I hit it. If I could have stopped at 25, but going 30 caused me to hit something I would only collide at 2-5 mph at best.

The difference in stopping distance between 25 and 30 feet must be a few feet. Lets not act like you would hit and kill the kid cause you were driving 30mph when you started stopping.

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Momentum is an unintuitive thing. See, for example http://www.brake.org.uk/events/15-facts-a-resources/facts/12...:

A vehicle travelling at 20mph (32km/h) would stop in time to avoid a child running out three car-lengths in front. The same vehicle travelling at 25mph (40km/h) would not be able to stop in time, and would hit the child at 18mph (29km/h). This is roughly the same impact as a child falling from an upstairs window. The diagram below illustrates the impact at various speeds. The greater the impact speed, the greater the chance of death. A pedestrian hit at 30mph has a very significant one in five chance of being killed. This rises significantly to a one in three chance if they are hit at 35mph

Or the charts here:

https://one.nhtsa.gov/nhtsa/Safety1nNum3ers/august2015/S1N_S...

That's assuming a 1.5s reaction time. Surely that's quite slow and accounts for the bulk of the difference.

1.5sec?!? You've got to be senile and on your phone to pull that off.

The reaction times quoted at the link associated with the main quote are actually: a reaction time of 0.67 seconds, which assumes the driver is alert, concentrating and not impaired. Driving when tired, distracted or impaired significantly increases reaction times, so the thinking distances above should be regarded as minimums.

The NHTSA assumes 1.5 seconds because: A typical reaction time to perceive a threat such as a deer or a child running into the road is about 3/4 second, and then you add another 3/4 second to decide to act and move your foot to the brake pedal. I agree it's unintuitively long, but I also know that reaction times have always been slower that I think they are when they've been tested...

The braking distance isn't linear with speed. It goes with the kinetic energy of the vehicle which is proportional to speed squared.

A vehicle traveling at 30 mph has about 40% more kinetic energy than the same vehicle traveling at 25 mph.

It will also close more of the available braking distance during the reaction period (only 20%, but that's a lot).

So the collision speed for the higher initial speed will be greater than the difference in the initial speeds, not lesser.

That said, doing other things to improve pedestrian safety are likely to be more effective than sweating the posted speed limit.

In my home town, speed limits are lowered during winter, because that lowers emissions (from diesel cars, as well as asphalt dust), and that literally makes fewer people die from bad air.

In this case, it has absolutely nothing to do with road safety. But if you drive over the posted limit, you are killing slightly more (fractional) people than if you keep to the limit.

> There has been studies stating that (on the highway) a speeding driver is usually safer than someone driving under the speed limit.

I doubt that would be true if no one was speeding. I also doubt that self driving cars will be speeding (considering liability) so it would be interesting to se how the result of such studies change once self driving vehicles are commonplace.

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